dong-sun lee / cat-lab / swucat2010-fall version chapter 24 spectrochemical methods
TRANSCRIPT
Dong-Sun Lee / CAT-Lab / SWU http://mail.swu.ac.kr/~cat 2010 -Fall version
Chapter 24
Spectrochemical methods
This composite image of sunspot group was collected with the Dunn solar telescope at the Sacramento Peak Observatory in New Mexico on Mar. 29, 2001.
The lower portion, consisting of four frames, was collected at a wavelength of 293.4 nm.
The upper portion was collected at 430.4 nm.
The lower image represents calcium ion concentration, with the intensity of color proportional to the amount of calcium ion in the sunspot. The upper image shows the presence of the CH molecule.
Richard P. Feynman (1918~1988) was one of the most well-known and renowned scientists of the 20th century. He was awarded the Nobel Prize in Physics in 1965.
Spectrophotometry
Spectroscopy :
the science that deals with the interaction of electromagnetic radiation with matter.
Spectrometry :
a more restrictive term,
denotes the quantitative measurement of the intensity of electromagnetic radiation at one or more wavelengths with photoelectric detector.
Spectrum (pl. spectra) :
a display of the intensity of radiation emitted,
absorbed, or scattered by a sample versus a
quantity related to photon energy(E), such as
wave length() or frequency().wave length(, nm)
or frequency(, cm–1).
Intensity
Spectrum
Absorption spectra of Fe(III)-salicylic acid complex.
max=524nm
5 ppm
10
15
20
25
30
CAT-Lab/SWU
UV-visible absorption spectra of cefazolin antibiotics.
Plane-polarized electromagnetic radiation showing the electric field, and the direction of propagation.
Electric field component of plane-polarized electromagnetic radiation.
Properties of light :
Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light
Duality ;
1) Wave theory ------ Huygens
= c
wavelength (cm/cycle) × frequency (cycles/sec) = velocity (cm/sec)
where wavelength, , is the length per unit cycle.
Frequency, , is the number of cycles per unit time.
C = 2.99792458 × 108 m/s is speed of light
2) Particle (energy packets ; photon) theory --- Newton
E = h = hc /
where E is the energy in joules (J)
h is Plancks constant (6.62608 × 10 – 34 J s)
1 erg = 10 –7 J
1 eV = 1.6021 × 10 – 19 J
Wave number, , is the number of cycles per unit length, cm.
= 1 / = cm – 1 (reciprocal centimeter ; Kayser)
= / c
= E / hc
Ex. 400 nm x eV ?
E = h = hc / 6.63 10 – 34 J s 3.00 108 m s – 1
=
400 10 – 9 m 1.6 10 – 19 J/eV
= 3.1 eV
Change in wavelength as radiation passes from air into a dense glass and back to air. Note that the wavelength shortens by nearly 200 nm, or more than 30%, as it passes into glass; a reverse change occurs as the radiation again enters air.
Regions of EM spectrum
Designation Wavelength Energy or Transition range wave number
Cosmic ray
-ray
X-ray
Vacuum UV
near UV
Visible
Near IR
Middle IR
Far IR
Microwave
Radio wave
10 – 12 m
10 – 11 m >2.5 105 eV
10 – 8 m 124 eV
180 10 – 9 m 7 eV
380 10 – 9 m 3.3 eV
780 10 – 9 m 1.6 eV
2500 10 – 9 m 4000 cm – 1
50 10 – 6 m 200 cm – 1
10 – 3 m 10 cm – 1
0.3 m
Nuclear
K,L shell electron
Middle shell
Valence electron
Molecular electron
Molecular vibration
Molecular vibration
Molecular rotation
Molecular rotation Electron, & nuclear spin
The visible spectrum
Wavelength Color absorbed Color observed (nm) (complement)
380-420 Violet Green-yellow
420-440 Violet-blue Yellow
440-470 Blue Orange
470-500 Blue-green Red
500-520 Green Purple
520-550 Yellow-green Violet
550-580 Yellow Violet-blue
580-620 Orange Blue
620-680 Red Blue-green
680-780 Purple Green
ROYG RIV
Red, Orange, Yellow, Green, Blue, Indigo, Violet
The electromagnetic spectrum showing the colors of the visible spectrum.
Types of interaction between radiation and matter
1. Reflection & scattering 2. Refraction & dispersion 3. Absorption & transition 4. Luminescence & emission
Emission orchemiluminescence
Sample Sample
Refraction
ReflectionA B
Sample
Scattering andphotoluminescence
Absorption alongradiation beam Transmission
C
Types of interaction between radiation and matter.
Several spectroscopic phenomena
1) depend on transition between energy states of particular chemical species
E* higher energy (excited state) E applied energy E o lowest energy (ground state)
2) depend on the changes in the optical properties of EM radiation that occur when it interacts with the sample or analyte or on photon-induced changes in chemical form (e.g. ionization or photochemical reactions)
Emission or Absorption Photoluminescence chemiluminescence A B C
Antistokes Stokes Combination of nonradiative transition transition and radiative deactivation D E F
Common types of optical transition.
non-radiativeprocess
Radiativeprocess
nonradiative
Absorption methods.
Photoluminescence methods.
Emission or chemiluminescence processes.
Absorption of EM radiation
Sun
Eye Visual center
Source Monochromator Cuvet Detector
P0 P
b
C
Incidentlight
P P – dP
dbb
b = 0 b = b
Emerging light
Molar concentration [C]
Absorption of EM radiation
Color of a solution. White light from a lamp or the sun strikes the solution of Fe(SCN)2+. The fairly broad absorption spectrum shows a maximum absorbance in the 460 to 500 nm range. The complementary red color is transmitted.
Attenuation of a beam of radiation by an absorbing solution.
Reflection and scattering losses with a solution contained in a typical glass cell.
Absorption methods. Radiation of incident power 0 can be absorbed by the analyte producing a beam of diminished transmitted power (a) if the frequency of the incident beam, 2 corresponds to energy difference, E1 or E2 (b). The spectrum is shown in (c).
Sample Incidentradiation 0
Transmitted radiation
(a)
2
1
0
E2 = h2 = hc/2
E1 = h1 = hc/1
(b)A
2 10
(c)
Lambert Beer’s law
Transmittance T = P / P0
%T = (P / P0) 100
Absorbance (A, O.D., E, As) A = log T = log P/ P0
Lambert’s law
Lambert and Bouger found that the intensity of the transmitted energy decrease exponentially as the depth (b ; path length of the beam through the sample) increases.
dP = k P db
dP/P = k db
dP/P = k db
ln P/P0 = k b
log P/P0 = (k/2.303) b
A = log P/P0 = (/2.303) b
T A
Path length Path length
Effect of path length on transmittance and absorbance of light.
Beer’s law
Beer in 1852 found that concentration (C) is a reciprocal exponential function of transmittance and absorbance is directly proportional to the concentration.
dP = P dC
dP/P = dC
dP/P = dC
ln P/P0 = C
log P/P0 = (/2.303) C A = log P/P0 = (/2.303) C
Lambert - Beer’s law A = bC where is molar absorptivity
Effect of concentration of analyte on transmittance and absorbance of light.
A
[C][C]
log T
Limitation Beer’s law
1. Concentration deviation ; A = log T = log P/P0 = bC (Eq 1) (0.434 / T) dT = b dC (Eq 2)
Eq 2 ÷ Eq 1
(0.434 / T) dT log T dC / C = ÷ b b = (0.434 / T log T) dT C/C = (0.434 / T log T) T
A
[C]
4
2
1
C/C
Twyman Lothian curve
T = 36.8 % A = 0.434
normal working range15%T(0.824A)~80%T(0.097A)
2. Refractive index deviation A = bC [ n / (n2 + 2)2] where n is refractive index
3. Instrumental deviation ; difficult to select single wavelength beam
max
The effect of polychromatic radiation on Beer’s law.
Choosing wavelength and monochromator band width.Increasing the monochromator bandwidth broadens the bands and decreases the apparent absorbance.
Absorbance error introduced by different levels of stray light.
Deviation from Beer’s law caused by various levels of stray light.
Chemical deviation from Beer’s law for unbuffered solution of the Indicator HIn.
4. Chemical deviation ; dissociation or reaction with solvent ex. Acidic form intermediate form basic form
5. Solvent deviation T = tsolution / tsolvent
6. Temperature ; narrower spectrum band at below 50C
7. Pressure ; gas phase sample
Errors in spectrophotometric measurements due to instrumental electrical noise and cell positioning imprecision.
Typical visible absorption spectra of 1,2,4,5-tetrazine in different solvent.
Absorption spectra of KMnO4
CAT-Lab/SWU
Partial energy level diagram for sodium, showing the transitions resulting from absorbtion at 590, 330, and 285 nm
Energy level diagram showing some
of the energy changes that occur during absorption of IR, VIS, UV radiation by a molecular species.
Electronic transitions
The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition which can be considered;
1.Transitions involving p, s, and n electrons 2.Transitions involving charge-transfer electrons 3.Transitions involving d and f electrons (not covered in this Unit)
When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each other. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level.
Types of the electronic transition
Transition Wavelength (nm) log Examples
* < 200 >3 Saturated hydrocarbon
n * 160~260 2~3 Alkenes, alkynes, aromatics
E * 200~500 ~ 4 H2O,CH3OH, CH3Cl CH3NH2
n * 250-600 1~2 Carbonyl, nitro, nitrate, carboxyl
(note) forbidden transition ; * , *
James D. Ingle, Jr., Stanley R. Crouch, Spectrochemical Analysis, Prentice-Hall, NJ,1988, p. 335.
*
*
n
ELUMO
HOMO
Unoccupied levels(antibonding)
bonding
non-bonding
Frontierorbital
Occupied level
Characteristics of electronic transitions.
Absorbing species containing p, s, and n electrons
Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy. The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band.
Possible electronic transitions of , , and n electrons are;
* Transitions
An electron in a bonding s orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo * transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to * transitions are not seen in typical UV-Vis. spectra (200 - 700 nm)
n * Transitions
Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n * transitions. These transitions usually need less energy than * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n * peaks in the UV region is small.
n * and * Transitions
Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the p electrons.
Molar absorbtivities from n * transitions are relatively low, and range from 10 to100 L mol-1 cm-1 . * transitions normally give molar absorbtivities between 1000 and 10,000 L mol-1 cm-1 .
The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species. Peaks resulting from n * transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital. Often (but not always), the reverse (i.e. red shift) is seen for * transitions. This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced - resulting in a small red shift. This effect also influences n * transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.
Charge - Transfer Absorption
Many inorganic species show charge-transfer absorption and are called charge-transfer complexes. For a complex to demonstrate charge-transfer behaviour, one of its components must have electron donating properties and another component must be able to accept electrons. Absorption of radiation then involves the transfer of an electron from the donor to an orbital associated with the acceptor.
Molar absorbtivities from charge-transfer absorption are large (greater that 10,000 L mol-1 cm-1).
Bonding in formaldehyde.
Energy level diagram for formaldehyde.
Energy level diagram for formaldehyde.
* * * 2p 2p sp2 n sp2 1s 2 H CH C C=O O atoms fragment atom fragment atom C 1s2 2s2 2p2 O 1s2 2s2 2p4
General guideline to the use of UV data
(nm) Number of band Intensity(log ) Transition
<270 Single band 2~4 n * Amines, alcohols, ethers, thiols <2 n * C N 250~360 Single band & 1~2 n * 200~250 no major absorption C=O, C=N, N=N, NO2, COOR, COOH, CONH >200 Two bands 3~4 Aromatic system >210 Bands 4 , -unsaturated ketone or diene, polyene (cf. Woodward-Fieser rule or Fieser-Kuhn rule) >300 Two absorptions low n * <250 high * Simple ketones, acids, esters, amides, other and n electrones (cf. Woodward rule or Nielson rule) Visible Highly colored compounds Long chain conjugated(4~5) chromophores Polycyclic aromatic chromophores Simple nitro, azo, nitroso, -diketo, polybromo, polyiodo, quinoid
Hyperchromic effect (increase absorption intensity) max
max
Hypsochromic shift Bathochromic shift (Blue shift) (Red shift)
Hypochromic effect (Decrease absorption intensity)
Red shift and blue shift.
Increasing solvent polarity Hypsochromic shift (Blue shift)
* Increase energy level gap n
ex. C = O H O
H Conjugate effect Bathochromic effect (red shift) *
Chromophore Red shift C=O (C=C)1~n CN C=N N=O N=N C=S COOH CONH
Auxochromophore Red shift
-OH -OR -NH2 -NHR NR2
Selected electronic transitions in organic molecules
Absorption Molar maximum, absorptivity, Electronic max Compound transition (nm) (1 mol1 cm 1)
Ethane * 135 Water n * 167 7000
Methanol n * 183 500
Ethane * 165 16500
Acetone * 150 n * 188 1860
n * 279 15
Benzene * 180 60000
* 200 8000
* 255 215
Phenol * 210 6200
* 270 1400
Common solvents used in UV and their transparencies
Minimum wavelength Approximate for 10 nm cell transparency Solvent (nm) region (nm)
Water 190 180-200
Cyclohexane 195 210-400
Hexane 200 205-400
Methanol 200 205-400
Ethanol 200 210-400
Dichloromethane 220 210-400
Chloroform 240 250-400
Dioxane 190 220-400
Absorption characteristics of saturated compounds with hetero atoms (n*) transmission
Absorption maximum Molar absorptivity
Compound max (nm) (1mol-1cm-1) Solvent
Chloromethane 173 200 Hexane
Methanol 177 200 Hexane
Di-n-butyl 210 1200 Ethanol sulphide Trimethyl lamine 199 3950 HexaneMethyl iodide 259 400 HexaneDiethyl ether 188 1995 Gas phase 171 3982 Gas phase
Absorption data for conjugated alkenes ( transition)
Absorption maximum Molar absorptivity
Compound max (nm) (1mol-1cm-1) Sorvent
1,3-Butadiene 217 21000 Hexane
1,3,5-Hexatriene 253 -50000 Isooctane
263 52500 Isooctane
274 -50000 Isooctane
1,3-Cyclohexadiene 256 8000 Hexane
1,3-Cyclopentadiene 239 3400 Hexane
Absorption characteristics of individual chromophores
Absorption Molar Chromophoric maximum absorptivitygroup Formula Compound max (nm) (1mol-1cm-1) Transition Solvent
Ethylenic RCH=CHR Ethane 165 15000 * Gas phase 193 10000 * Gas phaseCarboxyl RHC=O Acetaldehyde 290 16 * Heptane
RR1C=O Acetone 188 900 * Hexane 279 5 n * HexaneAzo – N=N – Azomethane 347 4.5 n * Dioxane
Nitro – NO2 Nitromethane 271 18.6 n * EthanolNitrito – ONO Amyl nitrite 218.5 1120 * Petroleum etherSulphoxide S=O Cyclohexyl 210 1500 * Ethanol methyl sulphoxide
Absorption data for carbonyl chromophore Absorption Molar
maximum absorptivity
Compound max (nm) (1mol-1cm-1) Solvent Transition
Saturated aldehydes and ketones
Ethyl methyl ketone 279 16 Isooctane n *
Acetone 279 15 Isooctane n *
Acetaldehyde 293 11.8 Isooctane n *
Cyclopentanone 299 20 Hexane n *
Isobutyraldehyde 290 16 Hexane n *
Cyclohexanone 285 14 Hexane n *
Carbonyl-containing compounds
Acetic acid 204 41 Ethanol n *
ethyl acetate 207 69 Petroleum ether n *
Acetyl chloride 235 53 Hexane n *
Acetyl anhydride 225 47 Isooctane n *
Conjugated ketones and aldehydes
Methyl vinyl ketone 320 50.5 Ethanol n *
212.5 9294 Ethanol *
Unsaturated carboxylic acids and esters
CH2=CH –COOH 200 10000 Ethanol *
CH3–(CH =CH2)–COOH 254 25000 Ethanol *
Absorption characteristics of benzene substituted with chromophores
* transition n * transition
K-band B-band R-band
max max max Compound (nm) (1mol-1 cm-1) (nm) (1mol-1 cm-1) (nm) (1mol-1 cm-1) Solvent
Benzene 184 60000 204 7900 256 200 Ethanol
- - 255 215 - - Ethanol
Nitrobenzene 252 10000 280 1000 330 125 Hexane
Benzonitrile 224 13000 271 1000 - - Water
Benzaldehyde 244 15000 280 1500 328 20 Ethanol
Absorption of benzene substituted with auxochromes
* transition
E-band B-band
Compound max (nm) (1mol-1cm-1) max (nm) (1mol-1cm-1) Solution
Benzene 204 7900 256 200 Hexane
Chlorobenzene 210 7600 265 240 Ethanol
Phenol 210.5 6200 270 1450 Water
Aniline 230 8600 280 1430 Water
