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CHAPTER - I
CHARGE - TRANSFER PROCESS AND EDA COMPLEX FORMATION
f . f . INTRODUCTION
Eversince the formulation of a theory for the intermolecular
charge-transfer (CT) transitions in the electron donor-acceptor
complexes by Mulliken [I], the topic received a great attention from
the chemists and a lot of work in this field has since been done. The
theory, as such, now stands well tested. Most of the Mulliken's
predictions have been found to be generally true. Beginning with the
first investigation of benzene - iodine system by Benesi-Hildebrand
[ 2 ] , there has been considerable interest in the spectral and
thermodynamical properties of CT complexes [3-61.
Electron donor-acceptor (EDA) complexes, also known as
charge-transfer (CT) complexes, are organic molecular complexes
formed between two molecules, which possess regions differing
considerably in electron density. One molecule acts as an electron
donor and the other as an electron acceptor. Their formation is usually
accompanied by the appearance of a new absorption spectrum that is
not simply an addition of the spectra due to the donor and acceptor
alone. The EDA complexes are formed almost instantaneously and
exist in a reversible equilibrium with the donor and the acceptor.
Donor Acceptor Complex
The equilibrium or formation constant K, is also known as the
association constant of the complex. Usually, a new spectral
absorption band appears in the near UV-visible region of the
spectrum indicating the presence of the new species, the EDA
complex. To the naked eye, this often manifests itself as a distinct
colour change, which takes place on mixing the donor and the
acceptor solutions. They are generally characterized by a 1:l
stoichiometry.
EDA complexes play a role in all realms of science. They are
gaining importance as potential high efficiency, second-order non-
linear optical materials [7]. The importance of EDA complexes in
biological systems have been suggested by Pullman [8], Szjent-
Gyorgyi [9] and others. It has been established that many antibiotics
and other drugs exert their influence by initial formation of EDA
complexes with the amino acids of body proteins [lo]. Charge-
transfer interactions of proteins, amino acids and amines in polar and
non-polar solvents were studied by Slifkin [ll-131. It has also been
suggested that CT interactions play a role in respiratory processes
[14], hormone activity [15-171 and the carcinogenicity of some
compounds [I 8,191. A study of EDA complexes has permitted the
indirect determination of ionisation potentials of less volatile aromatic
compounds, which would not have been possible with other available
techniques [20]. EDA complexes play important role in the fields of
analytical chemistry [21] and organic semi-conductors [22]. It was
found that the electrical conductivity of the complex formed between
tetracyanoquinomethane and tetrathiofulvalene is comparable to that
of copper [23]. EDA complexes also appear as intermediates in
various reactions.
1.2. THEORY OF EDA COMPLEXES
The nature of the bonding between the donor and the acceptor
moieties in these complexes was quite puzzling to start with. A
number of reviews and monographs have appeared on the subject of
electron donor-acceptor complexes [6,25,26]. A number of theories
were proposed, including those involving a normal covalent bond, an
ionic interaction and various types of dipole-dipole and dipole-
induced dipole interactions. When x-ray diffraction results on crystals
of the complexes which can be prepared in the pure solid state,
showed that the donor and the acceptor are usually separated by
distances of over 30 pm, which are much larger than the usual
covalent bond lengths of about 15 pm, and are only slightly less than
van der Waals distances, it became apparent that the force of
interaction between the donor and the acceptor moieties in the
complex is of longer range than the usual binding forces.
With the development of the valence bond theory, attempts
were made by Brackman [27] and others to interpret the charge-
transfer complex as a resonance hybrid of "no bonded" and bonded
canonical structures. The theory of bonding in these complexes were
finally put on a firm footing by Mulliken using the molecular orbital
treatment in a classical series of papers, now available as a lecture and
reprint volume [ 6 ] , where in he proposed that while electrostatic
interactions are mainly responsible for linking the molecules in the
ground state, charge-transfer bonding is the dominant mechanism in
the excited state. He attributed the new spectral bands as arising
essentially due to charge-transfer forces. He later agreed with the
view of Briegleb [4] that the stability of certain n-n complexes may
be due to predominant electrostatic forces and hence preferred the
name electron donor-acceptor complexes over the designation charge-
transfer complexes which he had initially employed.
The wave-function of the complex consists of contributions
from the wave-function of this charge-transfer dative state as well as
the state in which there is no bond between the two. For weak
complexes (i.e., for which K < 1) , the contribution of the dative state
to the ground state wave-hnction of the complex is small. There is an
excited state, the charge-transfer state. For the loose complex, the
ground state is mostly no-bond, while the excited state is mainly
dative. Excitation therefore essentially amounts to the transfer of an
electron from donor D to the acceptor C. Theory shows that the
spectroscopic absorption will be of high intensity. The energy level
diagram is in fact much more complicated, involving a number of
locally excited states of D and of C, and a number of charge-transfer
states. Mulliken classified donors such as the arenes which donate
their n- electrons, as x-donors, and molecules lacking n-electrons but
possessing a lone pair of electrons, such as the aliphatic amines, as
n-donors. Aniline can act both as a x-donor because of the n-electrons
in the phenyl ring, and as an n-donor through the lone pair of
electrons on the nitrogen atom.
1.3. RECENT TRENDS IN THE THEORY
Mulliken's theory considers the complex as a hybrid resonating
between a non-polar structure and a polar one, resulting from the
transfer of one electron. In Dewar's theory [28] the transfer is
supposed to take place between the highest occupied orbital of the
donor (HOMO) and the lowest unoccupied one of the acceptor
(LUMO). More recently Flurry [29] studied the EDA complexes as
new molecules which are a linear combinations of the HOMO and
LUMO from the donor to the acceptor respectively. Using
perturbation theory Murrel et al. [30] calculated the energy as the
total of the contributions from different types of interactions like Van
der Waals, electrostatic etc. Chesnet and Moseley [3 11 consider these
complexes as a 'super molecule' made up of donor and acceptor
molecules.
According to Nagy [32], Mulliken's theory exaggerates the
importance of electronic transfer neglecting other types of
interactions. On the other hand Murrel's theory gives good results for
stabilization energies and equilibrium distances but does not account
for the new band that appears as a characteristic of these complexes.
The CND0/2 calculations on the geometry and stabilization energies
of certain selected EDA complexes by Yanez et al. [33] showed that
the 'super molecules' approach gives fairly consistent values for the
stabilization energies. The variation of the dipole moment of the
complex indicated that the electron transfer is important in stabilizing
these complexes.
Chiu [34] developed an unified molecular charge transfer
theory which embraced all ranges of molecular interactions. In the
limit of strong electronic effect (uv and visible region) it reduces to
Mulliken's theory of CT molecular complexes and in the limit of
dominating vibrational effect (IR and F-IR region), it reduces to
intervalence CT theory.
f .4. EXPERIMENTAL METHODS TO STUDY EDA COMPLEXES
Mulliken's charge transfer theory predicts the existence of an
intense absorption spectrum corresponding to the transition YN+YE
and its total absolute intensity can be approximately computed. If YN
is nearly pure no-bond character and YE nearly pure ionic character,
the spectra associated with the transition is called the intermolecular
charge transfer spectrum. Light absorption causes an electron jump
from the ground state to the excited state. Large amount of data is
available in literature regarding the spectral characteristics of CT
complexes. In a majority of systems the characteristic CT band
appears in the uv-visible region and can be easily characterised.
However, frequently intramolecular and charge transfer spectra may
overlap or sometimes also interfere quantum mechanically. In such
cases it may not be possible to identify the CT spectra uniquely.
One other important method is the study of the dipole moment
changes on formation of EDA complexes. Since an electron jumps to
excited state the dipolar character of the niolecule undergoes a
change, hence the measurement of dipole moment may prove to be a
diagnostic tool in the interpretation of the EDA complexes.
Several other physico-chemical methods are also available for
the study of EDA complexes [ 5 ] . Several authors have observed
small changes in the chemical shift in the NMR spectra of mixtures in
which the acceptor-donor type interactions occur. A simple
interpretation of the direction of the shift due to complexation was
given by Pople et al. [ 3 5 ] . It was shown by Hanna and Aashbogh
[36] that in a donor acceptor interaction to form 1 :1 complex the
chemical shift of the acceptor protons is related to the strenght of the
donor acceptor interaction. This technique was used very
successfully by Foster and Fyfe [37] in the stydy of nitrobenzene
complexes with several aromatic compounds. Wehrnan and Popov
[38] studied the donor ability of tetrazoles using this technique.
Similar studies were made by Muralikrishna and Krishnamurty [39]
to study the interaction between piperidine and TCNQ. Dielectric
polarization studies were successfully used to determine the
formation constants of several EDA complexes 140-421. X-ray
crystallography can be used for the study of EDA complexes. In
general the intermolecular distance between the partners of an EDA
complex where only non-normal inter-crystalline forces operate have
slightly larger intermolecular distances. Thus Wall work [43] has
suggested that inter atomic distances of less than 3.4 A are
characteristic of charge transfer bonded molecules.
The Infrared spectroscopic studies of EDA complexes are rare.
It was sho~vn that quinone complexes are characterised by the shift in
carbonyl group frequency [44]. In the extreme case where charge
transfer is more or less complete in the ground state, one can observe
the infrared spectrum of the ionised donor and the acceptor [45].
Mulliken's theory suggested that EDA complex should be
paramagnetic due to displacement of the charge from the donor to the
acceptor. This has been proved experimentally. But no extensive
work has been done.
Direct measurement of the ionization potential (ID) of aromatic
molecules have been made for relatively few substances. Price
et al.[46] have determined the ID of some donors by direct photo-
ionization experiments. Watanable [47] has measured ID for a range
of substituted benzenes and for some aromatic hydrocarbons by the
same method. The electron impact method has also been used to
obtain the values for a few aromatic hydrocarbons. However, values
obtained for ID by different workers from the electron impact method
are often not in agreement with one another. Baker, May & Turner
[48 ] have determined the ID of substituted benzenes by photoelectron
spectroscopy. An alternative method for obtaining ID is from
measurements on the CT absorption maxima.
The comparison of the position of absorption bands with the ID
of the donor was found by Mc.Connel1, Ham & Platt [49] to be
facilitated by a relationship (eqn.2) that is linear for I D values between
7 and 12 ev. n
where ET is the charge transition energy, C1 and C2 are perhaps
essentially constant for a series of complexes.
It is evident that there exists a quadratic dependence on the ID
and the actual theoretical curve should be a parabola and not a
straight line. Mulliken and Person [6] have shown that the
experimentally observed linear relationship are not so conflicting with
the above equation as long as the ID of the donor does not in general
fall much below 7 ev, with the consequence that the segment of the
parabola described by the eqn.(3) over which the results have been
applied approximates to a straight line. In general, the equation
hvCT = a ID + b . . . (3)
has been used, where 'a' and 'b' are constants for a given acceptor.
The value of 'a' in the above equation has no direct significance since
the line itself is only an approximation. Foster [ 5 ] has shown that the
constant 'b' contains terms such as othe electron affinity of the
acceptor and coulombic interaction terms between the nobond and
dative states. 'a' and 'b' are assumed to be constants for a series of
similar donors reacting with a given acceptor in the same solvent.
1.6. EQU1LIBNUh.I CONSTANT OF EDA COMPLEXES
Theoretically, K can range from zero (implying no complex
formation) to infinity (implying complete conversion of
stoichiometric quantities of the donor and the acceptor into the
complex). Generally, complexes with formation constants less than
unity are classified as weak, while for strong complexes D l . In
many situations, these complexes are non-isolable in the pure state,
thus ruling out the possibility of the direct measurement of their
molar extinction coefficients at the charge-transfer maximum and,
therefore, the direct evaluation of their formation constants. In 1949,
however, Benesi and Hildebrand [2] reported a graphical method for
the simultaneous evaluation of the formation constant, K, and the
molar extinction coefficient, E, of 1:l EDA complexes formed
between some aromatic hydrocarbons (acting as electron donors) and
iodine (an electron acceptor) in inert solvents like carbon tetrachloride
and heptane:
ArH -t Iz f ArH.1,
By measuring the absorbance, Z, at the charge-transfer
maximum, of a series of solutions in which the initial concentrations
of both the donor and the acceptor were varied (while holding the
concentration of the donor very much greater than that of the
acceptor, i.e., D>>C) and using the relationship
(where I is the path length or the cell size), they were able to estimate
both K and E for each donor-acceptor system fkom the linear plot of
C/Z versus 11D.
The following years saw a spate of similar publications by a
number of ~iorkers employing a variety of donors and acceptors.
Even as the limitations of the Benesi-Hildebrand approach became
apparent, a number of modifications and improvements, including
other methods of plotting the spectrophotometric data on the basis of
linear equations similar to Benesi-Hildebrand's, were suggested.
1.6.a. DETERMINATION OF EQUILIBRIUM CONSTANT BY
METHODS OTHER THAN UV-VIS. SPECTROMETRY
The reason for the preference of workers to the
spectrophotometric method of evaluation of formation constants of
charge-transfer complexes lies in the fact that spectrophotometers of
fairly high accuracy have been available for decades, as a standard
instrument in most laboratories. The spectrophotometer, especially
the computerized one, is easy to operate and from the absorbance
readings, K and E can be evaluated quickly, on an electronic
calculator in the linear regression (LR) mode using a Benesi-
Hildebrand type of equation.
Besides the uv-visible spectrophotometric technique, many
other methods of evaluating the formation constants of charge-
transfer complexes, have been developed over the years:
1. Infra-red spectroscopy, using the same principles as uv-visible
spectrophotometry;
2. Nuclear magnetic resonance spectroscopy; from the change in
the chemical shifts leading to a relationship similar to Benesi-
Hildebrand's;
3. Distribution methods, partitioning one of the components of
the complex between two immiscible liquids;
4. Polarography, by measuring the shift in the half-wave
potential as the donor is added to the solution of the acceptor;
5. Calorimetry, which allows simultaneous measurements of the
formation constant as well as the heat of formation of the complex;
6. Dielectric constant measurements for determining the
formation constants of hydrogen-bonded complexes;
7. In the case of reactive charge-transfer complexes, from the
rates of their reaction following their (nearly instantaneous) formation
in equilibrium; and,
8. Relaxation methods.
The principles of these methods have been reviewed by Foster
[24]. Many of them suffer from infirmities similar to those
encountered with the spectrophotometric method.
1.6.b. SPECTROPHOTOMETRIC DETERMINATION OF 'K' AND
'E' BY BENESI-HILDEBRAND METHOD (1949)
Iodine has long been known to dissolve in different solvents to
produce various colours, ranging from violet in carbon tetrachloride
to brown in alcohols, Early colorimetric investigations in 1909 by
Hildebrand and Glascock (cited in [Z]) indicated the formation of a
1 : 1 compound in equilibrium between iodine and ethanol dissolved
together in carbon tetrachloride. Attempts to calculate the formation
constant of the complex from the intensity of the colour were
hampered by the impossibility of the direct determination of its molar
extinction coefficient. A workable solution was first devised by
Benesi and Hildebrand [2] in 1949. They carried out a
spectrophotometric investigation of the interaction of iodine acting as
acceptor with the aromatic compounds, benzotrifluoride (C6H5CF3),
benzene, toluene, o- and p-xylenes and mesitylene (acting as donors)
in carbon tetrachloride and in heptane at room temperature (22'~). On
mixing the donor and the acceptor, dissolved separately in the inert
solvent, there was a colour change and the spectrum of the mixture
showed a new absorption band with maximum at a wavelength of
about 500 nm, a region in which neither the donor nor the acceptor
has any significant absorption. They rationalized this as being due to
the formation in equilibrium of a 1 : 1 complex between the donor and
the acceptor:
K
Arene -t I2 t + Arene.12
(Donor) (Acceptor) (Complex)
If x is the equilibrium concentration of the complex, D and C
are the initial concentrations of the donor and the acceptor, then
Since D>>C it follows that D>>x and the tern (D-x) in the
denominator of the above equation can be replaced by D to obtain
whence KDC-KDx = x
Dividing throughout by KD x
or, -=- ] + 1 x KD
Since the observed absorbance Z is due to the complex alone,
Z= ExE . . . (8)
where 1 is path length of the light in the cell, and E the molar
extinction coefficient of the complex; whence, from eqn. (4.)
CZ 1 1 1 -=- .- + - Z K E D E
which is the Benesi-Hildebrand equation. This is of the type
Y = B X + A . . . (9)
A plot of Y, i.e., C11Z versus X (i.e., I D ) must be linear with the
slope, B = 1/KE, and intercept, A = 1/E. Benesi and Hildebrand
obtained good linear plots for all the systems. From the intercept, they
obtained an estimate of E and substituting it in eqn. (4) they obtained
estimates of K.
In employing eqn. (5 ) , Benesi and Hildebrand, as well as
Keefer and Andrews, who published their findings on the interactions
of these donors with bromine [SO] and iodine monochloride [5 11 soon
after Benesi and Hildebrand [2], chose to represent x in the numerator
in moles per litre, (C -x) also in moles per litre, but in the term (D-x)
they represented both D and x in mole fractions. The final result is
that D in eqn. (4) remains to be represented in mole fractions of the
donor in the mixture, so that K is the formation constant of the
complex in per mole fraction units. In fact, the equation which, by
some authors, is referred to as the Keefer-Andrews equation is
identical with the Benesi-Hildebrand eqn. (4).
Most workers, however, prefer to represent the concentration of
all the three species, viz., the donor, the acceptor and complex in
molarities, that is mole per litre, so that the value of K obtained by
them is in the more conventional litre per mole units. These two differ
in magnitude, being related to each other in dilute solutions (in which
ideal behaviour can be assumed to hold good) as follows:
where d is the density of the solvent and M, its molecular weight.
The Benesi-Hildebrand eqn. (4) is valid for all scales of
concentration, but the magnitude of K evaluated will depend on the
scale employed. In this thesis, the formation constant is based on the
molarity scale, except where specifically stated otherwise.
The Benesi-Hildebrand equation reduces to the form
C 1 1 1 -=- .-f - Z K E D E
when all absorbance measurements are made in 1 cm. cells. It must be
noted that the function Y comprises of two variables, C and Z, the
concentration of the acceptor and the absorbance, respectively. C, as
well as, D, were varied in their work, which should be the preferred
design of the experiments. But many subsequent workers preferred to
keep C constant while changing the concentration of the donor, D, in
a series of mixtures. This leads to yet another form of the Benesi-
Hildebrand equation
1 1 1 1 - ---.-+- Z KEC D CE
A plot of 1/Z versus 1/D should be linear with the slope,
B =1/KEC and intercept, A = 1/EC, where C is the constant acceptor
concentration. This design has the advantage of making the
preparation of the series of mixtures a facile task, since two stock
solutions, one of the donors and another of the acceptor, alone need to
be prepared.
Some further remarks on the Benesi-Hildebrand equation
and its application are in order. Nearly all workers have employed
the condition D>>A in their studies because most of the electron
acceptors (though not iodine) have low solubilities in the solvents
employed. The first equation that is written for the formation constant
of the complex, viz.,
is symmetric with respect to D and C, i.e., the value of K is unaltered
if D and C are interchanged. The Benesi-Hildebrand eqn. (1) is valid
also under condition of C>> D, but in the form
t D 1 1 1 -=- .- + - Z K E C E
K and E have been determined in some cases from
spectrophotometric data obtained when the initial concentration of the
acceptor is much higher than that of donor (C>>D) and consistent
values have been obtained [52]. But Ross and Labes [53] found that
in the case of N, N-dimethylaniline interacting with 173,5-
trinitrobenzene in chloroform, such conditions lead to estimates of K
and E very different from those evaluated with D>>C. This they
attributed to the tendency for formation of 1 :2 and 2: 1 complexes in
this system.
The Benesi-Hildebrand procedure was graphical: they
plotted the chosen functions on a graph sheet, drew what was visually
the straight line of best fit and then proceeded to measure the
intercept to determine E. Similar procedures were followed by other
workers who employed similar linear equations to estimate K and E
(q. v.). However, after the advent of electronic calculators, linear
regression came to be increasingly employed to determine the slope
and the intercept of the line of best fit, whose "goodness fit" is
characterized by R, the correlation coefficient.
1.6.c. ASSUMPTIONS INHERENT IN THE
BENESI-HILDEBRAND MODEL
The Benesi-Hildebrand equation, and indeed the various linear
and non-linear equations to be reviewed below, and which are
employed for the purpose of evaluating the formation constant and
the molar extinction coefficient of EDA complexes rest on a number
of assumptions; any departure of the actual system to a significant
extent from any of these assumptions will lead to model failure with
the results that the estimates of K and E become unreliable.
1. The foremost assumption is that a 1 : 1 complex alone is formed,
and higher order complexes are absent, which needs to be confirmed.
For instance, Lofti and Roberts [54] confirmed this in the case of the
interaction between trimethylsilyltriptycene and tetracyanoethylene
by verifying that the Job plot had a maximum at 1:l stoichiometry
and showed no detectable asymmetry. Howevere, Foster [6] has
suggested that the Job's plot is not always capable of distinguishing
between instances of different stoichiometry. Dimicoli and HCl&ne
[55] have used it to show the presence of 2: 1 as well as 1 : 1 complexes
in a system studied by them. But not all workers have taken this
precaution of establishing the stoichiometry of the complex prior to
proceeding to evaluate K and E.
2. It is assumed that the solutions are ideal in behaviour, so
that activity coefficients of all the species can be assumed to be unity
and the concentration equilibrium constant evaluated by the method is
the same as the true or thermodynamic equilibrium constant (based on
activities of the species). This implies that there is no variation of K
with the concentration. But it is open to question whether, when
concentrations of reactants as high as 0.5 M or 1 M are used, some
departure fiom ideality would not occur.
3. It is assumed that the complex obeys the Beer-Lambert law.
This is a vital assumption, since any non-compliance with the law
would lead to a failure of the method. The molecules act as absorption
centres for the radiation and for the law to be valid, they should act
independently of each other. This restriction, that they should not
interfere with each other causes Beer's law to be a limiting law
applicable in dilute solution of concentrations less than 0.01M (of the
absorbing species). Failures of the law in ho~nogeneous systems, e.g.,
solutions, are unknown [56]. However, if the absorbing species tends
to associate or dissociate, deviations do occur. At concentrations
above 0.01M, deviations fiom Beer's law occur due to the change in
the refractive index of the solution as the concentration is changed.
4. Implicit in the concept of the EDA complex formation as
purely due to an interaction between a donor molecule and an
acceptor, is an assumption that the environments, including the
solvent molecules, have very little role to play. Hence the need to use
'inert' solvents. Carbon tetrachloride, along with heptane and hexane,
have long been solvents of choice. However, Anderson and Prausnitz
1571 in 1963 found by spectrophotometry that carbon tetrachloride
itself interacts with arenes forming weak complexes. For instance,
with benzene, the complex has a K value of about 0.009 L0.004 L 1
mole at 2 5 ' ~ . Evans [58] found that the interaction of iodine with
heptane in perfluoroheptane, which is perhaps one of the most 'inert'
of solvents, gives a good linear Benesi-Hildebrand plot, the straight
line passing almost exactly through the origin, i.e., with a near zero-
intercept. He interpreted it to indicate the formation of a very weak,
possibly of a collisional type, charge-transfer interaction between
iodine and heptane. The myth of 'inert' solvents has been exploded
long ago: no solvent can remain an innocent spectator in an
interacting system. How far the interaction between the solvent
(always present in a great excess) and the donor or the acceptor
affects the determination of the equilibrium constant of complex
formation between the donor and the acceptor is not easy to quantify.
But many workers tend to attribute anomalous values of K, at least in
part, to such solvent-reactant interactions.
5. One anomaly often encountered is that, for a series of related
donors interacting with a particular acceptor to form weak complexes,
one expects that, as the electron-donating capacity of the donor
increases and K increases, the transition moment should increase
resulting in an increase in E as well. Many cases of the opposite trend
have been found, including those for the interaction of iodine with
benzene and mesitylene in Benesi and Hildebrand's original work [ 2 ] .
Orgel and Mulliken [59], in order to account for these contradictory
trends, advanced the idea that, besides definite, discrete 1:l charge-
transfer complexes, absorption in the band may also arise whenever a
donor and acceptor are sufficiently close to one another during
collisions without forming a complex. These 'contact' charge-transfer
pairs, they estimated, may account for roughly three-fourths of the
charge-transfer absorption in the case of the benzene-iodine system,
with the actual complexes themselves contributing only one-fourth.
Carter et al. [60], however, differed and instead proposed a model in
which solvation by the solvent of the donor, the acceptor and the
complex all play a role and if, competition between complexing and
solvation is taken into account, there is no need to introduce two
kinds of charge-transfer absorption, one due to the complex and the
other due to 'contact pairs'; the behaviour of weak complexes can
then be fitted into the same theory as that of strong complexes. They
concluded that the Benesi-Hildebrand treatment, in the case of weak
complexes, leads to underestimates of K and overestimates of E.
Emslie et al. [61] soon countered the concept of solvation as a major
factor by pointing out that for the same, given complex, the values of
E evaluated by the Benesi-Hildebrand type of equations is fairly
constant in a series of solvents of widely varying solvating power
(while the formation constants vary). They instead suggested that the
observed anomalies may result from the complex not obeying Beer's
law as the concentration of the species present in excess, which is
usually the donor, is increased. They showed that even small
differences in E can lead to large differences in the calculated values
of K and E, but the product KE will remain unaltered.
Person [62] opines that the distinction between true complexes
and contact pairs appears to revolve around the charge-transfer
stabilization energy (which is greater than the ordinary van der Waals
attraction energy: if the stabilization energy is greater than the
translational kinetic energy of kT, the interaction must be considered
to be a complex; if the energy of formation is less than kT, then the
interaction is better described as a 'contact'. These questions, he
concludes, are difficult, if not impossible, to answer.
Orgel and Mulliken [59] have discussed the complications
arising from the formation of several geometrically and/or
electronically different 1: 1 complexes: they showed that if two or
more isomeric 1 : 1 complexes are formed, the spectrophotometric
method measures a total equilibrium constant, K (measured) = C Ki,
where Ki is the formation constant for the ith complex and a weighted
molar extinction coefficient, E (measured) = Ki CEi/K, where Ei is the
molar extinction coefficient of the i'" complex.
1.6.d. OTHER LINEAR EQUATIONS AVAILABLE FOR THE
EVALUATION OF 'K' AND 'E'
( 1 ) The Foster-Harnmick-Wardley Equation (1952):
Hammick was one of the first to foresee, as early as in 1936,
the possible use of absorbances to determine formation constants of
charge-transfer complexes [63], but it fell to Benesi and Hildebrand to
evolve a practical method. Foster, Hammick and Wardley [64] were
concerned with the general case of the formation of higher order
complexes, such as D,C and DC,, Considering the formation of D,C,
if D is the initial concentration of the donor. C that of the acceptor,
and x the concentration of the complex in equilibrium, then
If D>> C, since x < C, effectively, D - x = D; then
z x = - (for 1 cm path length) E
L or KCE - K Z =- D
... (17)
Putting Y = Z/D" and X = Z, this becomes a linear relationship
between Y and X, if C is kept constant in all the mixtures. A plot of
Z/D" versus Z should be linear with the slope, B = - K and intercept
A = KCE. The linear relationship obtains only when the proper choice
of n is made. The data are plotted for different values of n (=I, 2, 3,
etc ) and that value of n which gives a straight line is inferred to give
the stoichiometry of the complex. From the slope and intercept of this
line K and E are calculated. It may be noted that the slope of the
FHW plot is always negative. Using this method, Foster et al. found
that the acceptor 1,3,5- trinitrobenzene forms a 1: 1 complex with
diphenylamine but a 1:4 complex with dimethylamine. The
composition of the complexes were confinned by the method of
continuous variations due to Vosburgh and Cooper [65] .
If the complex is known to have 1:l stoichiometry then the
FHW equation (1 5 ) reduces to
z - = -KZ+KCE D
...( 18)
for constant acceptor concentration.
( 2 ) The Scott Equation (1956):
The Scott equation [66] ,
is readily obtained from the Benesi-Hildebrand equation (2). The
Scott plot of ZCDIZ versus D should be linear with the slope,
B = 1/E and intercept, A = 1/KE. E is given by the reciprocal of the
slope while it is obtained from the reciprocal of the intercept in the
Benesi-Hildebrand method. The roles of the slope and the Intercept
are thus interchanged in the two plots. In both treatments, E is
evaluated first and then K from the product KE. The Scott treatment
obviates the difficulties when the data are poor, as often happens in
the case of weak complexes: the intercept of the Benesi-Hildebrand
plot may be nearly zero (implying a nearly infinite molar extinction
coefficient for the complex), or, in worse cases, it may be negative (as
has been observed in some cases) leading to negative values of both
K and E, which would be absurd. In the Scott plot E is estimated from
the slope and turns out to be almost always positive (since the slope is
almost always positive, unless the data are atrociously bad) leading to
a positive estimate of K.
Scott pointed out that a great advantage of his method is that
one extrapolates through region of increasing dilution to the intercept,
and that, with precise experimental measurements, one can also
determine the initial slope at high dilution; moreover, if the points do
not define an exact straight line, this method gives a more reasonable
weighting to the different measurements. Apart from Benesi-
Hildebrand's, Scott's is the most widely employed procedure for the
treatment of spectrophotometric data of 1 : 1 charge-transfer
complexes. For a path length of 1 cm., the Scott equation becomes
CD 1 1 --- = -.D +- Z E KE
. . .(20)
and if C, the concentration of the acceptor, is kept constant in all the
mixtures of the series,
D 1 1 - =-.D+- Z EC KEC
A plot of D/Z versus D should be linear with the slope, B = 1/EC and
intercept, A =l/KEC .
( 3 ) The Seal-Sil-Mukherjee Equations (1982):
Seal, Sil and Mukherjee [67] rearranged the Benesi-Hildebrand
equation into the following two forms for absorption in a 1 cm cell.
Form I : 1 z
Z = --.-+EC . . .(22) K D
and Form I1 : D 1 D=EC-- - - Z K
Both the equations do not contain the product KE. A plot of Z
versus Z/D as well as a plot of D versus D/Z will be linear. For the
first plot, the slope, B =-lK and the intercept A = EC; for the second,
B = EC and A = -1lK. These two equations, the authors claim,
overcame the inherent defect present in the earlier linear equations, in
that a complete separation of K and E has been achieved: each can be
determined exclusively from the slope or the intercept. This, they
claim, should lead to a more accurate evaluation of K and E from a
given set of data than would be possible with the Benesi-Hildebrand,
Scott or Foster-Hamrnick-Wardley plots. Seal et al. employed
literature data on a number of systems to determine the values of K
and E as estimated on the basis of their two equations, but these do
not differ significantly from those reported by the original workers.
( 4 ) The El-Haty Equation (1991):
When the initial concentrations of the donor and the acceptor
are comparable, El-Haty [68] derived the following linear
relationship.
and used it to investigate solvent effects on charge-transfer complex
formation: a plot of (C+D) versus CDIZ should be linear with the
slope B = E and intercept, A = - 1K. This plot is, in fact, the
'inverse' of the Rose-Drago-Ayad linear plot: (C+D) is plotted along
the X-axis and CD/Z along the Y-axis in the Rose-Drago-Ayad plot,
while they are plotted along the Y -axis and the X-axis, respectively,
in the El-Haty plot. The El-Haty equation is not restricted by the
condition D >> C; also it does not contain the product KE and
achieves a separation of K and E.
( 5 ) The Rose-Drago-Ayad Equation (1994):
Rose and Drago [69] derived an absolute equation applicable
with no restriction on the relative concentrations of the interacting
species. In its most general from, it is also applicable to regions where
the charge-transfer band of the complex and the absorption band of
the acceptor overlap. It is generally applicable to 1: 1 complexes, but
similar equations can be derived for higher order complexes.
For the formation of a 1 : 1 complex,
D t C 3 DC
Ayad [70] modified the Rose-Drago equation and arrived at an
equation of the form
This leads to a plot of CD/Z versus (C+D) being linear with the slope,
B = 1/E and intercept A = IIKE.
( 6 ) The " Inverse" Benesi-Hilderband Method :
All the above workers have employed the concentration CD of
the donor as the x-variable and the observed absorbance Z due to the
complex as the y-variable in the regression. Ordinary linear
regression requires that the x-variable be free from error, while the
uncertainties lie in y-variable. The regression minimized the sum of
the squares of the deviations (of y) to obtain optimal estimates of the
slope and the intercept. It has been pointed out by Baskara raju [71]
that the present day double beam spectrophotometers incorporating
computerized devices, can measure absorbances accurately with
deviations of less than i 0.002, if the absorbance scale of the
instrument has been calibrated using accurately prepared standard
solutions of potassium chromate.This for a maximum absorbance
reading say, 1.000 for the solution with the highest initial donor
concentration, implies an error of less than 0.2%. It is shown that the
errors in the concentration of the donor and the acceptor in the
mixtures are usually much higher than this due in part to uncertainties
in their assay. So it is the concentration term which must be taken as
the y-variable, (though it is still the "independent variable") for
regression purposes. Therefore another linear equation has been
proposed [71].
This equation demands a plot of 1/D vs 112 to be linear from which
slope one can get K = - intercept and E = - C .intercept
This equation is the inverse form of the B-H equation, since x-and y-
co-ordinates have been interchanged.
Due to the presence of experimental errors these eight
equations invariably lead to different estimates of K (and E), the
difference getting larger as the levels of error gets higher. The
unreliability of the estimates of K, especially for week complexes
(defined as this for which K < I), as obtained fkom
Benesi-Hilderband type of plots was realized quite early;
Person [62] has shown that for reliable estimates of K the
concentration of donor in the most concentration solution of the series
must be greater than about O.l/K.
1.7. NON-LINEAR TECHNIQUES OF DETERMINATION OF 'K'
Non-linear regression methods broadly fall into two categories:
those which employ derivatives and those which do not require the
computation of derivatives. Both use a cycle of computations which
is repeated a number of times (iterations) until the estimates of the
parameters reach a pre-designed level of precision. Methods which
employ derivatives or gradients are generally quicker to converge to
these regression estimates than iterative methods which do not
employ gradients. All, however, require initial, rough inputs of the
parameters that are sought to be optimized. Usually these are
obtained from a linear plot, such as Benesi-Hildebrand's or Scott's.
The limitations of the Benesi-Hildebrand type of linear
equations to estimate the formation constant of complexes have long
been apparent and therefore non-linear regression methods have also
been attempted to arrive at more reliable estimates of K from
absorbance data. Since they employ iterative procedures, they are
well suited for programming on computers. Among the early non-
linear methods are those due to Wentworth et a1 (1967) [72], and
Rosseinsky and Kellawy (1969)[73], both of whom have used the
Deming routine [74], which requires the computation of partial
derivatives of a trial function. Subsequently, Farrell and Ng6 [75]
used the Rosenbrock [76] routine which does not use derivatives but
is slower to converge.
( 1 ) The Wentworth / Rosseinsky-Kellawy Method :
Without imposing restrictions on the relative initial concentrations
of the donor and the acceptor, the following equation, when solved
for x, yield the concentration of the 1:l complex present in the
equilibrium.
Then for absorbance in a 1-cm cell
Equation (27) is non-linear in C and D. This is an iterative
procedure which, with approximate initial input of K and E, rapidly
converges to the non-linear regression estimates. Rosseinsky and
Kellawy [73] using independently the same routine, showed that the
absorbance data of many complexes, reported by earlier workers,
gave non-linear regression estimates of K and E significantly different
from those calculated using linear plots. The fact remains that non
linear regression must be superior, since the dependence of Z and C
and D essentially non-linear. A few workers have since adopted
Wentworth's [72] techniques for evaluation of K and E.
( 2 ) Deming Method :
Much earlier, in 1938, Deming [74] had evolved an iterative
method for the minimization of sum of squares of deviations of a
variable which is non-linearly dependent on two or more variables.
This method is actually an adaptation of the well-known Newton-
Raphson iterative procedure for finding the minimum of a function in
numerical analysis. It employs a first-order Taylor expansion to
obtain a linear function of the parameters. It is an elegant routine and
generally converges smoothly to the optimized estimates of the
parameters in a few iterations if the data are not very erratic and the
initial inputs of the parameters are fairly close to their true values.
Otherwise, no convergence may result. The method also estimates
the standard errors in the computed parameters.
Baskara Raju [71] contended that the errors are more likely to
be present on the concentration terms rather than in the absorbance
readings, and he derived from first principles the expression
EZ+KEC z- KZ* D = ... (28)
K E ~ C - KEZ
in which the initial concentration of the donor, D is expressed as a
function of the other two variables, C and 2. If the experimental
conditions are such that the relative error is higher in the donor
concentration that in the acceptor concentrations or in the absorbance
units (as can arise in the case of highly pure acceptor like iodine,
employing good spectrophotometer), it is the sum of squares of the
derivations in CD which must be minimized rather than those in Z
Baskara raju has adopted the Deming [74] routine for evaluation of K
(and E) from spectrophotometric data using equation.(28). These
estimates of K and E will vary from those obtained by the
WentwortMRosseinsky Kellawy method since the objective function
being minimized are different.
( 3 ) Nelder - Mead Method :
The Nelder-Mead downhill simplex function minimization
routine was first formulated for the asymmetric simplex by Spendley
et a1 [77], but was improved for the more general simplex by Nelder
and Mead [78] in 1965. It is a direct method of function
minimization which does not require the computation of derivavatives
as in the Deming routine. It is slower to converge than the gradient
methods, but has a remarkable elegance and simplicity about it. In
the present case, it can be given in the form
C = EZ + KEDZ - KZ*
K E ~ D -KEZ
If CA is constant in the series of solutions, the Demings routine
is inappropriate since it is meant for the regression of one variable on
another. The appropriate procedure here is to employ a function
minimization technique such as Rosenbrock's. Baskara Raju et
a1. [79] have preferred to employ the Nelder-Mead downhill simplex
routine. These estimates will differ from those obtained in the
WenworthiRosseinsky Kellawy method using eqn (27) and also from
those obtained by the Deming method using eqn (28). Like
Rosenbrock's and unlike the WenworthRossinsky Kellawy and
Deming's method the Nelder Mead method routine does not give
estimates of the standard errors in K and E.
8 STRUCTURE - RELATED INTERACTIONS AND
CORRELATION ANALYSIS
Structural variation in the vicinity of the reaction centre of a
compound results in an almost continuous variation on its
electrophilic or nucleophilic character. This capacity may be used as
a delicate probe into the effects which electron perturbation produces
upon reaction affinity and from which the electronic demands of the
reaction may be inferred.
Correlation analysis involves relating empiricallly the
reactivities of a series of compounds in which the structure of the
substrate is varied by the introduction of substituents, while the
solvent and the reagent remain constant. A linear relationship,
involving the logarithm of rate coefficient (k) or equilibrium constant
(K), and referred to as a linear free-energy relationship (LFER), is
generally observed and is interpreted using the slope and intercept of
the simple linear regression. The polar effect of a substituent
comprises all the processes whereby the substituent may modify the
electrostatic forces operating at a reaction centre, relative to the
standard substituent, which is usually the hydrogen atom. The most
familiar of these is the Hammett relationship [SO] which takes the
form
~ O ~ K = I O ~ K O + o p . . . (30)
where KO is the equilibrium constant of unsubstituted or parent
compound. The substituent constant o, measures the polar effect
(relative to hydrogen) of the substituent in a given position, meta or
para, and is independent of the nature of the reaction. The reaction
constant p is a measure of the extent of susceptibility of the reaction
to polar effects.
1.9. SCOPE OF THE PRESENT WORK
Nogami et a1 [81] studied the interaction of aniline with
chloranil and the formation of the product was rationalized via EDA
and o complexes as intermediates. In the present investigations, we
have tried to understand the effect of substitution on the donor aniline
molecule through the formation of EDA complexes with chloranil as
the acceptor.
CI 0 0
Aniline Chloranil EDA Complex
(or substituted (or its bromo aniline ) analogue, bromanil)
To get an insight into the changes brought about on the electron
affinity by the presence of different groups on the quinoid skeleton,
the spectrophotometric study has been extended to another acceptor
bromanil. The shift in CT transition due to substitution, the changes
on the ionization energy (ID) of the differently substituted donor
molecules, the changes in the thermodynamic parameter K and the
Gibbs free energy of formation AGO for chloranil and bromanil
systems are reported. The different spectroscopic and thermodynamic
parameters are correlated. A comparison is made between systems
performed with the two acceptors.
CHAPTER - I1
MATERIALS AND METHODS
2.1. PURIFICATION OF MATERIALS
All the materials used for the study were purified by following the
standard procedures available in the literature [82]. The purified
materials were all used immediately after the final step of purification
without any storage except in the case of chloranil and bromanil, which
were sufficiently stable.
2.1.a. ACCEPTORS :
1 p-Chloranil : Commercially available synthesis grade sample
('Loba Chemie') had 97% assay. It was recrystallised from AR. glacial
acetic acid and dried in air. Its purity was confirmed by determining the
melting point (290' C). The bright yellow coloured pure crystalline
sample was stored in an amber coloured sample tube.
2. p-Bromanil : The compound was prepared by the method of
Torrey and Hunter [83].
Hydroquinone (10 g) was suspended in glacial acetic acid and
bromine (90 g) was added. The solution was allowed to stand overnight.
An equal volume of water and concentrated nitric acid were added and
heated in a water bath. The bromanil formed was recrystallized from
acetic acid until its m.p. agreed with the literature value (299 - 300'~).
2.l.b. DONORS:
1. Aniline : The analar grade aniline (Fischer sample) was dried by
keeping in contact with potassium hydroxide pellets for several hours. It
was then distilled at 184" and the middle portion of the fraction was
collected.
2. m-Chloro Aniline: The procured reagent grade CDH sample of
m-chloro aniline was dried over potassium hydroxide pellets and
distilled to get pure sample boiling at 230' C.
3. p-Chloro Aniline: The commercially available crystalline (m.pt.
72' C) 'Burgoyne', LR sample of p-chloro aniline was purified by
distillation at 232' C (its boiling point) with hot water circulation in the
condenser. Care was taken to avoid solidification of the material in the
condenser. The oily distillate on cooling gave colourless pure crystals.
4. p-Bromo Aniline : It was prepared by hydrolysis of p-bromo
acetanilide which had been obtained by the bromination of analar grade
acetanilide (BDH) by following the prescribed procedure [82]. The
compound was then recrystallised from ethanol and the purity was
checked by finding its melting point (66 C).
5. p-Iodo Aniline : Pure aniline was used for the synthesis of p-iodo
aniline by iodination following the procedure available in the literature
[82]. The crude product was dried in air and refluxed with light
petroleum (60 - 80 C boiling fraction) in a flat bottomed flask fitted
with a double surface condenser over water bath maintained at 75-80' C.
After about 15 minutes, the contents are emptied into a beaker set in a
freezing mixture of ice and salt and stirred constantly. The crystallized,
almost colourless needles of p-iodo aniline was filtered under suction,
dried in air. The purity was confirmed by finding its melting point
(63 C).
6. m-Toluidine : The Fluka, pract. sample was first dried by treating
with potassium hydroxide pellets for several hours and then distilled at
204 O C just before use.
7 . p-Toluidine : The synthesis grade (Merck, zur synthesi) crystalline
sample was purified by distillation at 200 O C with hot water in the
condenser, taking special care to avoid crystallization in the condenser
itself. The middle portion of the distillate on cooling yields colourless
pure crystals of p-toluidine (m.pt. 44 O C).
8. p-Anisidine: The synthesis grade sample (S.D's) with an assay of
98% was further purified by recrystallization from benzene, employing
activated carbon to remove traces of coloured impurities. The crystals
obtained were used after checking the purity by determing the melting
point (57.2 O C).
9. N-Methyl Aniline: The commercial (L.R) BDH sample was
purified following the standard procedure [82]. Requisite quantities of
pure commercial N-methyl aniline, concentrated hydrochloric acid and
crushed ice are taken in a 500 ml beaker and the sodium nitrite solution
was added little by little with stirring, taking care not to increase the
temp above 10 O C. Stirring was continued for another hour and the
separated oily layer was washed once with water and dried over
anhydrous magnesium sulphate. It is then distilled at 196 O C to collect
the pure sample of N-methyl aniline.
10. N,N-Dimethyl Aniline: Commercial BDH sample of N,N-
dimethyl aniline and acetic anhydride are heated under reflux for about
3 hours and then cooled. The solution was distilled and the pure
colourless liquid boiling at 193 - 194 O C was collected.
11. Diphenyl amine: The analar BDH sample was found to be very
pure and was used as such after checking its melting point (54 O C).
2 .1 .~ . SOLVENT:
Benzene : The analytical reagent grade, thiophen-fiee benzene
(Qualigen's sample) was procured. It was shaken well with water in
order to remove any traces of acid and the benzene layer was separated.
It was then dried by first treating with anhydrous calcium chloride,
filtered and then placed over sodium wire . The solvent was then
refluxed for about one hour, distilled and the middle portion of the
distillate at its boiling point (80 " C) was collected.
2.2. ANALYTICAL TECHNIQUE
Spectrophotometric studies are carried out on a Shimadzu UV-
Vis. Spectrophotometer (UV-160). It is a micro computer controlled
double beam recording instrument (Kyoto, Japan). The instrument
combines a monochromator, keyboard, CRT and Graphic Printer. It has
a scanning speed upto 2400 nm per minute. It scans from 200 - 1100
nm and permits various spectral processings such as
expansion~compression of spectra, peak-pick, derivatives, smoothing,
data storage and arithmetic calculations between spectra. It incorporates
many standard calculation programmes such as data determination at a
fixed wavelength, automatic quantitative analysis by 2/3 - wavelength
calculation or derivative values, kinetic measurements and
multicomponent analysis. The instrument was kept in an AJC room and
the measurements were done at 298 K.
The required quantities of the pure acceptor and donor substances
are weighed accurately in calibrated standard measuring flasks and made
up to the mark with benzene solvent to get 0.0015 M and 1.5 M stock
solutions of the acceptor and donor respectively. 2 ml of the acceptor
solution and 1 ml variation (with pure solvent) of donor solution are
mixed to get a total constant volume of 3 ml of the mixture. A uniform
concentration (0.001 M) of the acceptor and nine different
concentrations of the donor solutions varied uniformly in the range 0.1
to 0.5 M were used for all the systems except with p-anisidine for which
half of the stock and final concentrations were used.
Matched pair of quartz cells of 1 cm path length and 4 ml capacity
were used. The freshly prepared donor and acceptor solutions were
mixed in the cell itself very quickly with the help of a calibrated 2 ml
syringe in which acceptor solution was taken. A trial spectrum was
recorded in the 'spectrum mode' to fix the wavelength of maximum
absorption in the CT region i.e., the charge transfer maxima (ACT). The
absorbance values at the 3LCT were recorded in the 'photometric mode'
within 3 seconds of mixing in all the cases, taking the pure acceptor
solutions of final concentration as the reference. The absorbance values
of the pure donor solutions of varying concentrations used were recorded
against pure solvent as the reference. These values were subtracted from
the respective absorbance values for the mixture, in order to get the
absorbance due the complex alone. The increase in the absorbance with
increasing concentration of the donor was noted. The 3LCT values were
found to be sensitive to substitution.
To confirm the 1 : 1 stoichiometry of the complexes formed, Job's
continuous variation method [84] was carried out . Solutions of
equimolar concentration (0.045 M)of the donor and acceptor were
prepared and the concentration of the fractional molarities of the solution
were varied uniformly so that the overall molar concentration was kept
constant. The absorbance values at the C.T maxima were recorded and
the contribution due to the complex alone is found out by subtrac,ting the
absorbances of the pure donor and acceptor solutions of respective
concentration 1851. The complex absorbance values were plotted against
the mole fraction of the donor to arrive at the result.
The formation constant (K) and molar extinction coefficient (E)
values for different systems were computed by eight linear and three
non-linear methods as described in Ch.1, using the program in QBASIC
given in the Annexure I.
2.3, CONDITIONS ADOPTED TO ARRIVE AT THE RELIABLE
ESTIMATES OF 'K'
As the spectroscopic method of evaluation of K is liable for wrong
estimation by even very small errors present in the absorbance readings
or concentrations of donorlacceptor solutions, much care was taken to
get the most accurate values of absorbance for every concentration in all
the systems. This was achieved by strictly following the conditions
given below during our entire experimental work.
1. Best grades of chemicals and reagents available were used after
subjecting to rigorous purification, following the standard procedures.
2. Calibrated weights, microburettes and volumetric flasks were used.
3. The donor substances and the solvents were purified freshly before
starting every experiment. Storage for more than 24 hours was
strictly avoided.
4. Before using the purified solvent, it was ascertained that peroxides
were totally absent by testing with the Reisenfeld-Lebafsky reagent
(2% potassium iodide in water buffered to a pH of 7 with 5% sodium
hydrogen carbonate): the non-development of a yellow colour, (or a
blue colour on subsequent addition of starch solution) indicating that
iodine is not released, vouches for the absence of peroxides and
hydroperoxides.
5. Mixing of the donor and acceptor solutions in the cell itself, kept
inside the spectrophotometer, using a calibrated syringe so that the
readings were taken quickly after mixing (in just three seconds).
6. Absorbance readings were taken in the 'photometric' mode so that
quick scanning became possible.
7. The absorbance due to the complex alone is found out by subtracting
the absorbances of the pure donor and acceptor solutions of
respective concentration and the net absorbance value was used for
the calculation of K and E as proposed by Anunziata et al. [85].
8. A minimum of nine different donor concentrations were used for
collection of data.
9. Experiments were repeated several times using newly prepared
solutions until constant absorbance values were obtained.
10.The absorbances were measured at more than one wavelength near
the charge-transfer maxima, in order to confirm the reliability of the
K values obtained.
1 1 .The absorbance versus concentration data had been analysed by eight
linear and three non-linear methods of evaluation of K. The
correlation coefficient (R) in the case of linear regression methods
and the (minimized) sum of deviations squares (S) in the case of non-
linear methods were determined.
12.Temperature change during the experiments was kept minimal by
using an effective air-conditioner.
CHAPTER - 111
RESULTS AND DISCUSSION
3.1. RESULTS
3.1.1 Studies with ring substituted aryl amine complexes:
The molecular complexes formed between electron donors of low
ionisation potential and acceptors of high electron affinity have their CT
absorption at relatively longer wavelengths, often well separated &om
the absorption bands of the components themselves. It is well known
that p-chloranil is a strong n: acid and forms EDA complexes with a
variety of Lewis bases. The strong acidic properties of p-chloranil are
due to the higher electron affinity conferred by the electron withdrawing
effect of the extended 7c orbital of the quinone and the planarity of its
structure. The aliphatic amines and the substituted anilines are the ones
which are often used as electron donors.
Foster and coworkers [86,87] examined chloranil complexes
formed with various methylbenzenes in cyclohexane and
carbontetrachloride media. Plots of CT band wavelength versus free
energy of complex formation were found to be linear. Foster [88] has
also plotted ionisation potential Vs charge transfer band wavelength for
some chloranil complexes in carbon tetrachloride. The agreement was
fair. Slifkin [12] has investigated the CT band arising at 600 - 800 nm
region representing complexes betweeen chloranil and donors
triethylamine, diethylamine and ethylamine. He attributed a charge
transfer band to a n -+ .n* transition from the lone pair of nitrogen in the
amino group to an empty chloranil orbital. Datta et al. [89] and Sarkar
et al. [90] have investigated the interaction of p-chloranil with a series of
phenols. It was found that all these EDA complexes (except with
p-quinol) were stable. In contrast, o-chloranil forms unstable complexes
with all phenols [91].
Arylamines, by virtue of their aromatic .n-electron system and the
arnine group, offer the possibility of functioning either as n- and lor as
n-electron donors towards a given electron acceptor, giving 1:l
complexes in the former two cases or a 1:2 complexes if both sites
function at the same time. These possibilities do not appear to have been
thoroughly considered hitherto.
In the present study, the following EDA complex formation
systems were investigated with p-chloranil (CA) and p-bromanil (BA) as
the acceptors, different substituted anilines as donors and benzene as the
solvent. Generally, quinones are known as good electron acceptors.
Hammond [92] has shown that the electron afficinity of p-benzoquinone
changes with the nature of the substituent at the quininoid nucleus.
Since a study of the electronic spectra of such molecules and the spectra
of the EDA complexes formed by them will provide a wealth of
information regarding the nature of inter molecular interactions. We
have chosen chloranil (tetrachloro-p-benzoquinone) and bromanil
(tetrachloro-p-benzoquinone) as acceptors. We report our results and
conclusions on the nature of interaction of these EDA complexes by
performing several correlations between the spectral and thermodynamic
parameters. We tried for the establishment of a linear free energy
relationship between the formation constant of these complexes and the
substituent constant o of the donor amines.
/ Sl.No / Sys tern j ~ 1 . ~ 0 / System 1 1 1. CA + Aniline 1 9. 1 BA t Aniline I
Intense blue to violet colours developed immediately on mixing
solutions of chloranil (or bromanil) and the arylamines, ascribable to the
formation of molecular complexes of the EDA type. The absorption
spectra of the complexes were well separated from the absorption
spectra of either of the components. Figures 3.1 - 3.8 shows the overlay
spectra of three resolved bands in the solvent benzene, one arising from
the local excitation of donor (D), the other from the local excitation of
the acceptor (C) and the third one from the CT excitation of the EDA
complex. The stoichiometry of the complexes have been verified by
Job's [84] method and the results collected for two representative
systems are presented in Table 3.1.
The 1 : 1 stoichiometry of the complexes formed by chloranil and
bromanil with all the amines was hrther confirmed by the perfectly
linear B.H plots and non-linear FHW-2 plots (cf. eqn.(l7) with n =2)
obtained in all the cases.
The increase in CT absorbance values with increase in
concentration of the donor has been recorded and the results are
summarized in Table 3.2 to 3.17. The last column of the tabulated
results gives the correlation coefficient (R) in the case of linear
regression methods and the (minimized) sum of deviations squared (S)
in the case of non-linear methods. The wavelength versus absorbance
spectrum of the donor-acceptor complex for all the concentrations are
obtained as overlay picture directly from the spectrophotometer, of
which a few are illustrated in Figs.3.9 to 3.12.
The pK, values of various ring substituted aromatic amines under
study are collected from the literature [93] and are presented in Tables
3.18 and 3.19. The given Hammett [80] a constant values are those
considered by Mc Daniel and Brown [94] for all the substituted anilines
except p-iodoaniline, the value for which is taken from van Bekkum,
Verkade and Wepster [95] .
To determine the ionization potential of the donor as per the
equation (31, hvCT = a ID -t b, Fanell and Newton [96] have used the
values 0.80 and -4.2 for 'a' and 'b' respectively for chloranil system.
Srinivasan and Uma Maheshwari [97] have found out these values to be
0.8058 and -4.2094 in their studies with chloranil acceptor. We have
used these values of 'a' and 'b' and calculated the ionization potential
values of the donors (Tables 3.18 and 3.19).
The following procedure is adopted to arrive at the values of K by
different methods suggested in chapter-I through a computer program
written in QBASIC [71]. The program inputs include the number, N, of
datum points which may vary from experiment to experiment. The
donor concentrations in each mixture need not be evenly spaced but it is
a matter of experimental convenience to space them in that manner. The
donor concentrations D (I) and the corresponding absorbances Z (I) are
entered as data towards the end of the program. It calculates the eight
linear and the three non-linear estimates of K and E and displays the
summarized results on the screen with detailed intermediate results as
output in the file "PRO DAY.
The values of K obtained by eight linear and three non-linear
methods for all the systems performed are given in Table 3.2 to 3.17. As
per the findings of Bhaskara Raju, Srinivasan and Sivaramakrishnan
[79], the Nelder-Mead routine returns the best estimates of K and is
more reliable than those returned by the other linear and non-linear
methods, especially for systems in which acceptors like chloranil and
bromanil are used. Hence the values of K obtained by Nelder Mead
method alone are collected and presented in the summarized form in
Tables 3.18 and 3.19 and are used for correlation purposes. The
observed molar extinction coefficient (E) values and the calculated
Gibb's free energy of formation (AGO) obtained from the relationship
AGO = - RT In K ... (31)
(where R is the gas constant and T is the temperature in absolute units)
are all collected and presented in Tables 3.18 and 3.19.
3.1.2 Studies with N-substituted aryl amine complexes :
One of the important properties of N-substituted aryl amine bases
is their ability to form EDA complexes with acceptors like chloranil and
bromanil. To understand the effect of structure on the reactivity of the
charge-transfer reactions, one can think of changes in the reaction center
brought about by substitution in the ring. Similarly one can think of a
change in the reaction center by substitution in the N-atom of aniline
substrate molecule. Hence for such an investigation, we took chloranil I
bromanil acceptors in benzene solvent and varied the donors by N-
substitution to constitute primary, secondary and tertiary amines and the
following systems were investigated:
! 1 ATO. I SYS tern NO. 1 ~ y s tenz I
1 17. CA + N-Methyl aniline 1 20. / BA + N-Methyl aniline I I I
I i i 1 18. CA + N-Phenyl aniline , 21. a BA + N-Phenyl aniline I i I i
The absorbance versus concentration values are reported in Tables
3.20 to 3.25. The values for aniline in these tables are taken from Tables
3.2 and 3.1 0. The results of the observed spectral parameters LC-, ET
and ID, the thermodynamic parameters K (Nelder Mead) and AGO
values, along with the literature [93] values of pK, are collected and
presented in the summarized tables 3.26 and 3.27.
3.2, DISCUSSION
Spectral measurements have been carried out under identical
conditions at a single temperature for a closely related series of ring and
N-substituted aryl amine donors with chloranil and bromanil acceptor
EDA complexes. Hence it is reasonable to assume that the same
mechanism is operating throughout these reactions. With the results
obtained from our measurements, we have performed the following
analysis of the various parameters to arrive at the nature of
intermolecular interactions of the EDA complexes studied and to probe
into the effects of electronic perturbations produced by the substituents
on the reaction center.
The stoichiornetry of the EDA complexes have been confirmed by
the Job's method as illustrated for one system each for chloranil and
bromanil in Figures 3.13 and 3.14, in which the results presented in
Table 3.1 are plotted. It is found that the absorbance reaches a
maximum when the mole fraction is in the same ratio as that of the
stoichiometry, namely 1 : 1. The linearity of the B.H plots (as illustrated
for the two systems in Figures 3.15 & 3.16) and the non-linear F.H.W
plots obtained for higher order complexes @lot of Z I Dn Vs Z with
n > 1) further confirms the absence of higher order complexes. The
illustrative liner F.H.W plot (n = 1) for the 1:1 complex formed by
aniline with chloranil in benzene solvent is shown in Fig. 3.17 and that
with bromanil is shown in Fig. 3.18.
3.2.1. Influence of substituents on the spectral parameters:
The early studies of Foster and co-workers 186-881 on chloranil
complexes with methyl benzenes, nitro benzene, iodine and iodine
monochloride showed a linear relationship between CT bands and the
free energy of complex formation in non-linear solvents. Gore and
Wheals [98] and Mukherjee and Chandra [99] have studied the
interaction of chloranil with aniline and its methyl derivatives,
Foster and Hanson [I001 studied the interactions of chloranil with
indoles and reported a bathochromic shift with increase in methylation
of indoles. Generally in chloranil complexes, the absorption below 400
nm is ascribed to n+n* transition and absorptions above that
wavelength is attributed to n+n* transition [ I 0 1 - 1031. The influence of
the substituents in the present study has been revealed by the observed
spectral changes. Relative to the parent H in the series, the substituents
3-CI, 4-1, 4-Br and 4-C1 shift the wavelength of the charge transition to
the lower side while the substituents 3-Me, 4-Me and 4-Me0 shift the
charge transfer maxima to the higher wavelength side. Though the CT
maxima are lower in bromanil system, a perusal and Tables 3.18 and
3.19 shows the same trend as that of the chloranil system. For the arnine
X-C6&-NH2, the substituent group X, if electron withdrawing, produces
a blue shift while an electron donating X produces a red shift.. '
complex)
The measure of change in energy of the CT transition reaction
hvCT = ET are indicating the higher energy demand by electron
withdrawing groups and lower energy demand by electron donating
groups. The summarized results indicate the higher energy demand of
the bromanil complexes confirming the lesser electron affinity of the
acceptor compared to that of chloranil as observed by Hammond [92].
Chowdry and Basu [I041 reported that the CT interaction energies
and pKa values do not appear to correlate. Nag Chaudhury and Basu
[I051 considers the pKa value as a measure of the n-electron ionisation
energy. Figures 3.19 & 3.20 and the data in Table 3.18 and 3.19 show
the experimental values of CT transition (ET) energies of these EDA
complexes of chloranil and bromanil compare satisfactorily with the
reported pKa values.
The calculated values of ionisation energy of the donors of these
electron-transfer reactions (making use of eqn.(3)) as reported in Tables
3.18 and 3.19, show a clear trend of the electronic perturbation of the
substituent on the reaction center. It is evident from a comparison of
either ET or ID of m-substitued amines with similar para substituted
amines that the halogen and alkyl substituents at meta position require
more energy for this electron transfer reaction than at the para position.
Halogen substituents in general require more energy than alkyl
substituents since for halogen, opposing resonance and inductive effects
operate at p-position. At m-position the +R effect is weak and +I effect
is dominant for alkyl groups whereas -I effect is dominant for halogen
substituents. The order of ID fbr various amines are:
ID (ev) of 8.26 8.18 8.15 8.14 8.13 8.03 7.90 7.83
X-C6H4-hi2
Since the set of donors are identical for the two acceptor systems,
and in the absence of the literature values of a and b for bromanil
system, the ID values of donors are plotted against ET values of bromanil
system (Fig. 3.21; r = 0.998). From the slope and intercept of the
straight line obtained, the values of 'a' and 'b' for bromanil are found to
be 0.9047 and - 4.8560 respectively.
It can thus be concluded from the results obtained that the position
of the C.T absorption band varies with the ID of donor.
3.2.2. Influence of substituents on the magnitude of K & its
correlation with ET and ID:
Though different methods have been adopted for the
determination of K and E and all the estimates are reported, due to the
inadequacies present in all the linear methods of evaluation, the non-
linear methods are best suited for evaluation [71] and the Nelder Mead
method has been chosen. For the discussion of the results, the Nelder
Mead method estimate of K has been employed as it has been assessed
that, since both chloranil and bromanil have assays of about 98%, the
Ievel of errors in the acceptor concentration is much higher than that in
the donor concentration or in the absorbance readings due to
instrumental error.
It is seen from our results that the observed trends in the stability
constants of the series of amines remain the same in all the methods of
evaluation. It is possible to conclude that the changes in the values of K
are due to the changes in the electronic perturbations brought about by
the substituent's inductive effect at the meta position and mesomeric
effect at the para position. It will be meaningful to examine changes in
K,,, (= Kx / KH) for the series of anilines X-C6f&-NH2 with structural
changes in X and interpret the results
complex) K r e ~ (BA 0.401 0.484 0.585 0.641 1.000 1.400 1.830 3.673
complex)
The above order indicates that a very strong complex is formed by
p-methoxy aniline (3.5 times stronger than aniline) while a very weak
complex (3.34 times weaker than aniline) is formed by m-chloro aniline
for chloranil complexes. The same trend is observed in bromanil
complex systems also.
Bhattacharya and Basu [106], Chowdhury and Basu [104],
Dwivedi and Banga [I071 have reported from their works on EDA
complexes that the complex stability (K) order satisfies the condition
that the lower the ionization potential, higher will be the stability. From
our works, a linear variation is observed between the ionization energy
of the donors and the stability constant of the complexes formed as
shown below (rounded off to three decimals), the same being illustrated
in Fig. 3.24 and 3.25.
(M-') 0.374 0.481 0.544 0.598 1.250 1.607 2.056 4.331 CA complex
(M-l) 0.308 0.373 0.450 0.493 0.770 1.077 1.408 2.826 BA complex
One does infer that the reaction is facilitated by substituents which
are electron releasing and is retarded by electron withdrawing groups at
the reaction center. As observed and reported by several workers [107-
1091, fiom the data given in Table 3.18 and 3.19 it is apparent that there
is a good correlation between the measured CT energies and the
evaluated equilibrium constants in the logarithmic form as (I+ log K)
for both chloranil and bromanil systems (r = 0.95 and 0.97 respectively).
3.2.3 Establishment of LFER with Hammett's substituent constant
and evaluated equiiibrium constant & other spectral parameters :
In any discussion of substituent effects on reaction rates and
equilibria, three components must be borne in mind; the substituent or
"source" of the perturbation, X, the reaction site or "detector" of the
perturbation, Y and the molecular frame work (core) through which the
effect is transmitted. This is the benzene ring in the original Hamrnett
systems. The two basically distinct electronic effects namely inductive
and mesomeric effects that may be generated by a substituent on a
reaction site are responsible for the changes in the rate of a chemical
reaction or the position of chemical equilibrium. Model systems in
which reaction center and substituent are separated by an aromatic ring
as core, eg. the Hammett system, permit a mixture of resonance and
inductive interaction to be .transmitted, the former only weakly from a
meta- position.
The influence of polar substituents on reactivity concern the
potential energies of the reacting systems. Changes in log K are a good
measure of potential energy effects and it provides some explanation for
the success of the Hammett equation. The most common application of
the Harnmett equation is in connection with mechanistic studies. Many
authors have sought to show that the influence of meta- or para-
substituents in a given reaction of an aromatic system supports a
postulated mechanism or atleast does not disprove it. Relevant evidence
may be obtained variously from the p value, from the kind of o values
needed in the correlation, or from the linearity or non-linearity of a
particular Hammett plot.
A number of workers have examined the effect of substituents on
the donor-acceptor interactions of a related group of donors with an
acceptor. The equilibrium constants and the enthalpies of formation in
such system have been correlated with the substituent constant. Foster
and Goldstein [ I 101 in their work on spectroscopic studies of aryl
ketones- iodine complexes report that the first ionization potential of
some para substituted acetophenones show a correlation with the
Hammett constant for the substituent group and concluded the ionisation
potential refer to the removal of an electron fiom the carbonyl oxygen
rather than from the n- aromatic system. While the same two authors
[ I l l ] in their work on aryl ketones - TCNE complexes suggest, based
on the IR studies, that the complex formed should be of the n - 7c type.
Since our investigations too reveal a very good correlation between ID
and o (r = -0.95) and since the acceptors chloranil and bromanil in our
study are 7c acceptors like TCNE, we may conclude that the EDA
complex formed between aryl amines and the two acceptors may be both
n-n as well as n-n type which is supported by the earlier work [112].
The data in Tables 3.18 and 3.19 have been cast into a Harnmett
Plot of (1 + log K) vs o. The slope of the Hammett plot is p. Fig. 3.26
shows an excellent linear correlation (r=0.99) between the two
parameters with a slope of - 1.550 for chloranil system and Fig. 3.27
shows a straight line ( ~ 0 . 9 8 ) with a slope of - 1.3653 for bromanil
system. Thus the reaction constant p obtained for the two systems from
the slope of the straight lines is in accord with the theory of CT complex
reactions. The sign and magnitude of p indicates the spontaneity of the
reaction and a measure of the extent of electron demand at the reaction
center.
The p value obtained is in conformity with the work done by
Aloisi et al. [113] and by Srinivasan and Sindhu Jayarajan [ I 141. We
know that the parameter p measures the ability of the core to transmit the
electronic effects. The magnitude of the p gives a measure of the degree
to which the reaction is responding to substituents. Since this is on a
logarithmic scale, a change in p of 1, (i.e, 10' = 10) indicates a ten fold
change in rate / equilibrium. The negative sign indicates that the
reaction is facilitated by electron donation. From the value of p for the
EDA complexation reaction and fEom the perusal of Tables 3.18 and
3.19, we infer that for a change of substituent from m-chloro to
p-methoxy (Ao = 0.641) the reaction is sensitive to the substituents by 101.55 x 0.641 = 9.85 (or - lo), there is about 10 fold increase in the
equilibrium for chloranil system. The value gets reduced to 7.5 times
increase (10 1.3653 x 0.641 = 7.50) in bromanil systems. Thus the fitting of
an LFER in these reactions implies unchanging mechanism in spite of
changes being made to reaction conditions viz., changing the
substituents on the substrate.
When we utilise the Harnmett's [so] substituent constant (0) listed
in Tables 3.18 and 3.19 to correlate with the spectral property, namely
ET or ID of both chloranil and bromanil complexes, we find that a linear
free-energy relationship is established (Fig. 3.22 & 3.23; r = 0.95 & 0.96
for CA and BA systems respectively).
The positive and negative values of Gibb's free energy of
formation (AGO) (Tables 3.18 and 3.19) derived from the K values of
these donor-acceptor complexes indicate the retardation and
enhancement of these reactions by the substituents, electron donating
groups acccelerating the reaction and electron withdrawing groups
decelarating the same. A good correlation exists between AGO and ET
(r = 0.95 for CA system and 0.97 for BA system ; Fig. 3.28 & 3.29) in
both cases. As per the findings of Medina et al. [I 151, not only does the
correlation exist, but is also linear as is seen in Figs. 3.30 & 3 1, where
AGO values are plotted against pKa values of the bases (r = 0.98 and
0.97 respectively)
3.2.4 Comparison of the parameters of chloranil and
bromanil complexes:
Foster and Thomson [I 161, in their works on EDA complexes
reports that a plot of ET of tetracyano benzene complexes verses ET of
chloranil complexes of differently substituted same series of donor is
linear and reports that this relationship is generally observed for energies
of inter-molecular CT transitions. Venu Vanalingam and Subba Ratnam
[117] have reported a linear relationship between ET (1) and ET (2) for a
given donor with two different acceptors where (1) refers to the new
acceptor and ( 2 ) , the standard acceptor (chloranil), provided the standard
acceptor chosen has the same basic skeleton. Our studies of the effect of
changes in the substitution on the basic skeleton of p-benzoquinone is
very interesting. Since the substitutions are symmetrical, similarities
exist between the observed phenomena. Figure 3.32 shows a linear plot
(r = 0.998) between the observed ET values of chloranil and bromanil
complexes of ring substituted donors.
Figure 3.33 shows a linear relationship between the log - log plots
of formation constants of the two series of ring substituted donors with
chloranil and bromanil EDA complexes with a regression coefficient
value of 0.996. Establishment of the linear relationship is a clear
evidence for the changes in the structure producing proportional changes
in the AGO values and indicates an unchanging mechanism in spite of
changes brought out in the p-benzoquinone skeleton of the acceptor
moiety.
3.2.5 Correlation studies on N-substituted aniline complexes:
In the case of N-substituted aniline systems, it is found that the ACT
values of the EDA complexes gradually change as aniline (530 nm),
N-methyl (598 nm), N-phenyl(640 nrn), and N,N-dimethyl aniline (648
nm). The observed red shift on going from a primary amine to
s e c o n d a ~ amine and then to the tertiary amine is expected to be so
based on the electron donating strength of ArNR2 > ArhHAr > ArNHR
> ArhX2 predicted by the inductive and mesomeric effect of the alkyl
and aryl groups.
One of the most significant observations made in this investigation
is the smallest value of K obtained for N-phenyi aniline in its complex
formation with either chloranil or bromanil which may be attributed to
the polar and steric effects on the two bulky phenyl groups.
The factors that determine the stability of the EDA complexes
formed are : (a) Steric factor and (b) Electronic factor. Steric factor
include Van der waal's repulsive interaction (steric hindrance), strain
from bond angles, bond lengths, electrostatic interactions etc. Electronic
effect manifest from groups which can release electron to the reaction
site or withdraw electron from reaction site and it determines the
feasibility of the reaction. The release of electrons to the reaction site is
followed by a favourable equilibrium for the forward reaction and vice
versa.
The basic strength of the N-substituted anilines as reflected in their
pKa values (Table 3.26) is in the order: N-phenyl < aniline < N-methyl <
N,N-dimethyl. As the two phenyl groups attached to the nitrogen in
N-phenyl aniline can withdraw the lone pair electrons of the nitrogen by
the mesomeric effect, this arnine is a very weak base (weaker than
aniline). This property of N-phenyl aniline is reflected in the stability
constant values for its complexes with chioranil and bromanil.
A perusal of Tables 3.26 and 3.27 shows an observable anomaly in
the estimated values of K which follows the order:
DONOR N-Phenyl- Aniline N,N-Dimethyl- N-Methyl-
aniline aniline aniline
(M-') 0.7405 1.2499 (CA complex)
1.6623 1.9352
0.4516 0.7695 (BA complex)
0.9429 1.1084
As reported by Smith [118] and Tafi [119], the electron donor
strength of the N-substituted anilines decrease in the order ArNR2 >
ArNHR > ArNH2 predicted from inductive effect is expected to change
when bulky substituent groups exert steric hindrance in the formation of
EDA complex. The magnitude of the interaction is determined by the
factors, according to the works of Medina et al. [115], (a) one on the
electronic type and (b) the other being steric in nature.
Nogami et al. [81] considers that the values of the stability
constant reflect the donor strength of the arnine towards chloranil and
concluded that the same steric effects are present in the EDA complexes.
Hence the reason for the lower K values of the tertiary amines than that
of the secondary arnines may be purely due to the over whelming steric
factors. The stability order observed is N-phenyl < H < N-methy1 >
N,N-dimethyl. Positive AG" values have been obtained for diphenyl
arnine where as the other three amines gave negative AGO values in
chloranil systems. This trend is observed in bromanil complexes too,
though with a lesser magnitude, exhibiting a profound similarity
between the EDA complexes of these two acceptors.
CONCLUSION :
Several important conclusions may be drawn from these studies:
1. The results indicate that substitution in general affects the
electronic structure and charge distribution in the reaction center and that
stronger complexes are formed by electron-donating substituents and
weaker complexes are formed by electron-withdrawing substituents in
the substrate molecules.
2. Though several linear methods have been used for the
determination of stability constants, non-linear methods developed
recently (in particular, that of Nelder Mead Scheme) can be successfully
employed for obtaining the best estimates of stability constant, K.
3. Stability constants of the complexes with bromanil are lower
than those of chloranil.
4. The spectral shift and intensity changes observed are due to the
influence of the substituents.
5. Both elctronic and steric factors play a major role in stabilizing
the K-substituted arylarnine complexes.
6. It is seen that a good linearity exists between the Hamrnett
substituent constant (o) values and the log K values of our ring
substituted amine systems.
7. The magnitude of reaction constant p obtained for the two
(chloranil and bromanil) series suggest the extent to which the reaction is
responding to the substituents and its negative sign indicates that the
reaction is facilitated by electron donation.
8. A profound similarity has been found and demonstrated
between the EDA complexes formed by the two acceptors.
9. In order to understand the molecular properties of EDA
complex systems, one should take into account the electrostatic as well
as charge-transfer interactions.
10. The diversity of the sign and the magnitude of the overall
substituent effect is the result of different electronic perturbations caused
and cancellations between opposing effects.