the effect of solvents on continuous absorption spectrum...

234

Nernst 1888 Z. phys. Chem. 612.— 1904 Z. phys. Chem. 47, 52.

Noyes and Whitney 1897 Z. phys. Chem. 23, 689. Roseburgh and Lash Miller 1910 Phys. . 14, 816. Sacher 1901 Z. anorg. Chem. 28, 385.Sand 1901 Phil. Mag. 1, 45.Volmer and Wick 1935 Z. phys. Chem. A, 172, 429.Wirtz 1938 Z. Elektrochem. 44, 303.

J, N . Agar and F. P . Bowden

The effect of solvents on the continuous absorption spectrum of bromine

B y R. G. A ic k in , N. S. B ayliss and A. L. G. R ees

Chemistry Department, University of Melbourne

(Communicated by F. G. Donnan, F.R.S.— Received 26 July 1938)

I ntroduction

The following reasons prompted us to select bromine as the subject for a study of the effect of the solvent on the absorption spectrum of a dissolved substance:

(1) Bromine has a simple molecule, and its absorption spectrum in the gaseous state has been interpreted fairly satisfactorily (Acton, Aickin and Bayliss 1936; Aickin and Bayliss 1938; Darbyshire 1937; Mulliken 1936).

(2) I t has continuous absorption in a convenient spectral region (5000 to below 2200 A), where many common solvents are transparent. There are advantages in working with a continuous spectrum, since it is possible to compare absorption coefficients under varying conditions withput the difficulties that are associated with the blurring out of band structure (Rabinowitch and Wood 1936).

(3) The continuous absorption of bromine is complex, being composed of transitions to several excited states (Acton, Aickin and Bayliss 1936; Aickin and Bayliss 1938). This makes possible a direct comparison of the effect of a solvent on different electronic transitions.

I t was fortunate that while this work was in progress, the work of Porret (1937) was published, enabling a further comparison to be made with liquid bromine.

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This paper presents an account of the effect on the absorption spectrum of bromine, a t concentrations between 0-00015 and 0-0010 mol./l., of the following solvents: 2N sulphuric acid, cyclohexane, chloroform, carbon tetrachloride, benzene, chlorobenzene and toluene. The main effect in the case of 2N sulphuric acid will be that of the w ater; an acid solution of this particular concentration was chosen in order to suppress the hydrolysis of bromine to less than one part in 1000 (Liebhafsky 1934). Other solvents were tested, such as carbon bisulphide, several alcohols and bromoform. Carbon bisulphide was rejected because it absorbs light strongly throughout a considerable part of our chosen spectral region, while in the case of the alcohols, even tertiary butyl alcohol reacted too rapidly with bromine. Bromoform gave trouble owing to a fairly rapid photo-decomposition that resulted in the liberation of bromine, and it also was discarded.

The effect of solvents on spectrum of bromine 235

E x p e r im e n t a l

Our materials were all carefully purified. The refractive indices recorded below were determined by a Zeiss-Pulfrich refractometer, and are compared with the values given by Weissberger and Proskauer (1935), denoted below by W.P.

Bromine. The sample was A.R. bromine that had been previously distilled over a mixture of pure calcium bromide and calcium bromate by Professor E. J. Hartung, to whom we are indebted for the sample. I t was subsequently distilled in vacuo several times, the head and tail fractions being discarded. The tail fraction contained less than one part of iodine in 5 million of bromine.

Cyclohexane. Since the most likely impurities are unsaturated hydrocarbons, particularly cyclohexene, which have a much lower freezing point than cyclohexane, the product was purified by fractional freezing, followed by drying over fused calcium chloride, n = 1-42606 (20° C) (W.P. 1-42636).

Chloroform. B.D.H. A.R. chloroform was repeatedly washed with distilled water and then dried over calcium chloride. I t was then fractionated, collected over calcium chloride, and kept in the dark until used. According to Twyman and Allsopp (1934), chloroform prepared in this way becomes useless for spectrographic work after 12 hr. We therefore distilled this solvent on the day on which it was to be used, n = 1-44808 (15° C) (W.P. 1-44858).

Carbon tetrachloride. Merck’s guaranteed reagent was fractionated, and stored, over potassium hydroxide, n = 1-45966 (20° C) (W.P. 1-46040).



236

Benzene. Merck’s reagent was guaranteed free from all sulphur compounds. I t was dried over freshly cut sodium for several days before use. n = 1-50388 (15° C) (W.P. 1-50440).

Chlorobenzene. A good commercial sample of chlorobenzene was fractionated and dried over calcium chloride. The sample had a constant boiling point of 132° C.

Toluene. In order to remove sulphur compounds from the commercial sample, it was shaken with two portions of concentrated sulphuric acid for 30 hr. each, followed by a shaking with distilled water for 12 hr. After repeated washing with distilled water, it was dried over calcium chloride, fractionated, and the product dried over sodium. I t did not appreciably decolorize bromine during the time of an exposure. 1-49833 (15° C) (W.P. 1-49985).

M e t h o d

Our method of photographic spectrophotometry consisted, in outline, of passing a beam of suitable continuous radiation through a cell containing the liquid under examination, and then on to the slit of the Hilger all-metal medium spectrograph. On each plate (10 x 4 in.) we obtained about eight exposures, all with the same time of exposure. One of them was the absorption spectrum of the bromine solution. For the others, the cell was emptied, refilled with the pure solvent, and the fight beam was weakened by known amounts by interposing calibrated wire-gauze screens, which were mechanically shaken eccentrically in a plane normal to the path of the fight. In addition to the above exposures, a spectrum of a brass or iron arc was photographed at each side of each plate, in order to provide wavelength calibrations. After development, photometer traces were taken across each plate between corresponding fines of the arc spectra.

The transmission of the bromine solution at each chosen wave-length was thus compared directly with the transmissions of the calibrated screens, and the actual values for the solution were determined by graphical interpolation between the known screen values. After the concentration of each solution was obtained, by shaking with aqueous potassium iodide and titra tion against standard sodium thiosulphate, the absorption coefficients were calculated from the relation log10(/0/7) = ecd, where e is the absorption coefficient, c is the concentration of bromine in mol./l., d is the length of the absorbing column in cm., and 70 and I are the incident and transmitted intensities respectively. In different experiments, our bromine concentrations ranged from 0-00015 to 0-0010 mol./l.

The absorption due to the solvent itself was corrected for by taking the

R. G. Aickin, N. S. Bayliss and A. L. G. Rees



calibration spectra through the pure solvent, contained in the cell that had been used to contain the solution. The cell holder was so designed th a t it was easy to remove the cell and replace it accurately in the same position with respect to the optical path.

As sources of continuous radiation we used a 30-watt car headlamp or a water-cooled hydrogen tube, according to the spectral region. In each case, the applied voltage was hand regulated for constancy during a run. The absorption cell was of silica, had plane windows, and was 5 cm. long (Hilger “ Presil” , type D). In the experiments that were performed to fix the positions of the ultra-violet maxima, and in which we did not determine absorption coefficients, the solutions were contained in an all-silica Baly cell by Hilger. Exposure times were controlled electrically by means of a synchronous electric clock, and exposures of any integral multiple of 30 sec. could be repeated with an error not exceeding 0*2 sec. Both quartz and glass optical systems were used with the spectrograph according to the region of the spectrum. Previous work had shown us that stray light in the spectrograph introduced errors owing to the reception by “ unexposed” portions of the plate of radiation sufficient partly to overcome the inertia of the emulsion. In the present experiments, we guarded against this error by constructing a diaphragm that fitted into the camera of the spectrograph, and which shielded all but the strip of plate, about 0*5 cm. wide, on which the spectrum was being photographed. The calibration of the wire- gauze screens, and the construction of the photoelectric photometer, have been described previously (Acton, Aickin and Bayliss 1936).

Our technique included the precautions that are necessary in photographic photometry (Harrison 1929). Blackened diaphragms placed at intervals along the optical path minimized the amount of stray light reaching the slit of the spectrograph. Kodak process, Universal and Ordinary plates were used. In no case did the emulsion contain a dye which after development might provide a background of uneven transmissivity. A metol- hydroquinone developer was used, and the plates were brushed lightly to ensure even development. After hardening and washing, they were swabbed with distilled water and slowly dried.


R e s u l t s

Our absorption coefficients, defined as above, are given in Table I. The absorption curves of bromine in the various solvents are shown in figs. 1-3. In each case, a comparison is drawn with the absorption of gaseous bromine, which is represented by the broken curves derived from the results of



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Waveno.

cm . - 1

1888919042191521940019581196951992820165203242054920782209712117121494217952202222314226432283423111233852365223930241252435824603248522518625436258872630126767271422746827754283412876529053294672995130350305353078331161314863176931979323113263433927333533376034040343123467634914353963569135963

R. G. Aickin, N. S. Rayliss and A. L. G. Rees

T a b l e I

Water15-8 33-519-5 34-222-6 39-925 1 44-729-4 52031-2 56 137-8 61-540-3 67-644-8 72-550-5 83-555-9 86-462-0 10467-2 11078-3 —861 12793- 7 137

103 150111 164116 178122 194128 —133 212136 —139 —

147 207153 201156 189164 158157 —152 138144 86-7130 —120 71-2111 63-690-3 59 178-4 37-7700 38-654-3 34 1411 51-4300 61-724-5 62-221-8 85-718-6 10219- 5 11320- 1 130160 14619-6 15823 0 18226-9 20633-3 23340 1 27744 1 29652 1 32261-5 36372-6 45785-1 (418)94- 4 —

105

42-8 40-948-6 33-852-5 69-460-8 78-972-8 82-5740 85-382-0 96-992-5 10898-0 122

103 131108 142115 153119 166127 190133 209139 218148 232156 248163 255174 265176 278181 299182 —

193 309

203 302200 294185 280169 —

152 260129 239105 22788-5 23170-9 24658-6 292

— 32842-5 63338-5 73937-9 104832-1 143333-5 169033-5 —

32-8 —

33-9 —

37-6 —

40-3 —

46-2 —

39-9 —

39-7 —

43 0 —

43-4 — ■

49-7 —

54-9 —

54-7 —

51-5 —

56-9 —

58-2 —

59-4 —

67-8 —

77-1 75-779- 9 78080- 7 90-091-0 105-997-4 111

105 120114 131122 146134 151141 161150 169159 183176 196184 215199 228206 234216 255229 272231 285244 301250 326258 336258 351261 358263 —262 —

___ OK4.

260 353246 364227 390208 448184 —200 708213 859222 910312 —418 —509677 —897

Absorption coefficients of bromine dissolved in

CarbonCyclo- Chloro- tetra-hexane form chloride

34-837- 1 46-4 55-7 62-2

69-1 80-6 87-2 94-2

100 108 111 119 128 133 141 154 162171 180 189 192 203

206 196 184172 148 125 10090-6 78-5 71-7 49-5 44-9 40-5 44-238- 6 36-2

52-765-278-593-5

115143168201

Chloro-Benzene benzene Toluene




Acton, Aickin and Bayliss (1936) below 30,000 cm.-1. Above this frequency, the absorption of gaseous bromine is shown diagrammatically, since it is too weak to be plotted accurately on our chosen scale of e (Aickin and Bayliss

W a v e le n g th (A)

5000 4500 4000 3500 3000

jS 200

20000 30000W ave num ber (cm .-1)

F ig . 1. B roken curve, gaseous brom ine; O brom ine dissolved in cyclohexane; Q brom ine dissolved in 2N sulphuric acid.

F ig . 2. B roken curve, gaseous brom ine; O brom ine dissolved in carbon te trach lo ride ; Q brom ine dissolved in chloroform .

1938). In fig. 3 the upper broken curve is a relevant part of the absorption curve of liquid bromine, which has been sketched in from the data of Porret (1937).

Previous work on the absorption spectrum of bromine in solution includes that of Bovis (1924, etc.), who studied the solvents water, carbon



240

tetrachloride, chloroform, ethanol and carbon bisulphide, and of Gillam and Morton (1929), who studied the absorption of halogens dissolved in carbon tetrachloride. Gillam and Morton present their results only in the form of

W a v e le n g th (A )


5000 4500 4000 3500

.2 200

J 150

3000020000W ave n u m b er (cm .-1)

F ig . 3. U pper b roken curve, liquid b rom ine; lower b roken curve, gaseous b rom ine ;O brom ine dissolved in benzene; Q in chlorobenzene; H in to luene.

curves; but they appear to be in reasonably good agreement with ours. The results of Bovis are quoted in terms of optical densities, which on recalculation give absorption coefficients that are mostly lower than ours.

D is c u s s io n

In general, the effect of environment on the absorption of a substance may appear as a bodily displacement of the absorption, or as a change in the absorption coefficient (i.e. of the transition probability), or as a combination of both. In the case of bromine, it is necessary to consider these possibilities with respect to each of the several components of the absorption. The two main components in the absorption of gaseous bromine, A (Amax = 4150 A)



241

and B (Amax = 4950 A), are quite distinct in the broken curves of figs. 1-3. In addition to these, there is the absorption throughout the quartz ultraviolet, e rising from a minimum of 0-2 a t 3036 A to about 3 at 2165 A. This absorption is probably complex, and may contain weak bands (Aickin and Bayliss 1938).

The ultra-violet absorption. The most obvious feature of figs. 1-3 is the tremendous increase in the intensity of the ultra-violet absorption of bromine in solution, as compared with the gaseous state. The most marked effect occurs with the aromatic solvents, while liquid bromine occupies a place intermediate between these and the non-aromatic solvents. Another feature common to all the solutions in this region is fhe occurrence of deviations from Beer’s law, e increasing slightly with the concentration. These deviations are considerable in the case of carbon tetrachloride solutions, where the irregularity of the absorption curve above 30,000 cm.-1 indicates the possible presence of diffuse band absorption in this region. A similar effect was noticed by Aickin and Bayliss in gaseous bromine at rather higher frequencies. Such deviations from Beer’s law set a limit to the accuracy of absorption coefficients in this region, and probably explain the differing results of various authors.

In several solvents, it was possible to find a maximum in the ultra-violet absorption, when by using thin films of solution (about 1 mm.) in the Baly tube we were able to reduce the absorption due to the solvent itself. In each case the maximum due to the dissolved bromine was quite distinct from the solvent cut-off, and the positions of the maxima are given in Table II.

The effect of solvents on spectrum of bromine

T a b l e II. U l t r a -v io l e t m a x im a i n b r o m in e s o l u t io n s

A b so rp tio n m a x im u m in A d u e to

S o lv en t B ro m in e S o lv en tC y c lo h ex an e 2500 2150C h lo ro fo rm 2725 2230B enzene 2925 2600C hlo robenzene 2960 2650T o lu en e 2975 2750

Bovis (1927) observed a maximum in chloroform solutions at 2700 A, but we were unable to find the maxima that he records at 2580 A in both water and carbon tetrachloride solutions.

I t does not seem possible that these maxima could be due to compounds formed by chemical reaction between bromine and the solvent. In the case of the aromatic solvents, for instance, the absorption of the various bromo-



242

benzenes and bromotoluenes is recorded as beginning at about 2800 A and extending to shorter wave-lengths (Purvis 1915 ; Ley and Engelhardt 1910). There is no maximum in the ultra-violet absorption of liquid bromine as far as 2500 A, or probably even as far as 2200 A (Porret 1937)- In gaseous bromine, the maximum may have been reached at 2200 A (Aickin and Bayliss 1938), although the measurements do not extend quite far enough to be certain. However, both Porret and the latter authors agree in considering the ultra-violet absorption of bromine to be complex, and in support of this, unpublished measurements by Bayliss and Rees show a definite maximum at about 2700 A when bromine is mixed with carbon dioxide. This coincides with the maximum that is obtained in chloroform solution. The maxima in other solvents are as yet unexplained, unless they are the same maximum which has been displaced by the solvent effect. Such large displacements seem unlikely in view of the small displacements that are suffered by the A and B maxima in the same solvents.

The ultra-violet absorption of iodine is known to be greatly affected by solvents, and here too, deviations from Beer’s law have been observed (Groh and Papp 1930).

The visible absorption continuum (Amax = 4150 A). The effect of solvents on the visible continuous absorption of bromine is to produce a marked increase in the absorption intensity accompanied by a small or zero shift of the maximum towards shorter wave-lengths, except in the case of water, where there is a large displacement of the maximum 1750 cm.-1), butonly a small increase in the intensity.

The total solvent effect is the sum of the effects that are exerted separately on the A and B components of the continuum. From.our results, it is not possible to obtain the effect on the B continuum with any accuracy. In gaseous bromine, Acton, Aickin and Bayliss (1936) obtained the absorption curve of the B continuum in the following way. They assumed that when plotted against wave numbers as abscissae, the absorption curve of the A continuum was symmetrical about its maximum, and that the contribution of the B component was practically zero a t the maximum of A . The absorption coefficients on the high-frequency side of the A maximum were then used to determine the contribution of A to the absorption on the low- frequency side, and the remainder of the absorption was attributed to the B component. This procedure cannot be applied to the absorption in solution, since in the important region of the high-frequency side of the A component, there is a considerable but unknown contribution from the ultraviolet absorption, making it impossible to disentangle the B component with any accuracy.




243

I t is evident, however, from figs. 1-3 tha t the displacement of the B maximum in the non-aqueous solvents is small, if not zero. The intensity also appears to have been affected less than in the case of the A component. In cyclohexane, the intensity of the B components is actually less than in the gaseous state.

In the case of the A component, the increase in intensity is about 30 % in chloroform and carbon tetrachloride, and about 45% in cyclohexane. At first sight, the effect of the aromatic solvents seems to be much greater; but in these cases the absorption at the A maximum receives a large contribution from the enormously enhanced ultra-violet absorption. Taking this into account, the net effect in the aromatic solvents is of about the same magnitude as in the other non-aqueous solvents, and hi no case is the main absorption enhanced to the same extent as in liquid bromine.

In cyclohexane, the A maximum is not displaced relative to the gaseous state. In the other non-aqueous solvents, there are small displacements to higher frequencies, the greatest shift being 550 cm.-1 in carbon tetrachloride. Scheibe (1937) has interpreted such shifts in terms of displacements of the potential energy curves of the molecular states that are concerned in the absorption. If his explanation applies in the present case, any such displacements are small.

However, we believe that another explanation must be considered, in view of the possible complexity of the A continuum (Mulliken 1936). If the A continuum consists of two components of about equal intensity, and which are not exactly superimposed, and if the effect of the solvent is to increase the intensity of the high-frequency component more than the other, then the absorption maximum will appear to shift to higher frequencies. Such an explanation is supported by the absorption curves in figs. 2 and 3, particularly in the solvents carbon tetrachloride, chloroform and benzene. The displacement of the maximum has been accompanied by a definite sharpening, and in our detailed results in Table I there is definite evidence of the appearance of a shoulder just on the low-frequency side of the maximum.

The interpretation of the various phenomena suffers from the uncertainty that still exists in the theory of the bromine continua. Mulliken (1936) has discussed the following alternative assignments of various electronic transitions to 'the A and B components of the absorption:

(1) A : mixture of 1ITu<-1B f and zn o+u<-xZ f .

Coupling: Q — s.

The effect of solvents on spectrum of bromine



244 R. G. Aickin, N. S. Bayliss and A. L. G. Rees

(2) A : XI 1 U XZ+.B : mixture of zn o+u<-xZ+ and .Coupling: approaching A — Z.

He regards (1) as more probable on general grounds, the main evidence against it being that of the Franck-Condon principle. The transition to 1ITU is thought to lie at slightly higher frequencies than that to 3/7q+m. Explanation (1) is in accord with our interpretation of the displacement of the A maximum in non-aqueous solvents. The effect of solvents on the intensities of A and B is interpreted as showing that the transition probabilities are affected in the decreasing order XI7U<-XZ+, zn Xu<-xB g .

(2) receives its greatest support from the Franck-Condon principle (Darbyshire 1937; Bayliss 1937). The A maximum is considered to be single, and the displacement of the maximum must be due to displacement of potential energy curves as discussed by Scheibe. Our results would then indicate that solvents have a greater influence on the XZ+ transitionthan on the 3IIU XZ+ transitions. But (2) corresponds to assuming a coupling that approaches A — Z ,where inter-system transitions are forbidden. Under these circumstances, one would expect the “ forbidden” transitions 3IIU+-XZ+ to be more affected by the perturbing influence of solvents than xIIu*r-xZ+, which is the reverse of the experimental observation. The phenomena in solution thus seem to favour (1) as the correct interpretation' of the continua. Although the Franck-Condon evidence in favour of (2) is strong, it is however based on the assumption that the Morse curve of the 3/70+M state of bromine is correct up to high vibrational quantum numbers.

The large displacement of the absorption in aqueous solution must be interpreted according to Scheibe (1937). I t will be noticed (fig. 1) that the absorption as a whole has been broadened, and that the individuality of the B component has been lost. The A component has been increased little in intensity, and it is difficult to say whether the intensity of B has been decreased, or whether it has been merged in A by virtue of a greater displacement towards higher frequencies. Aqueous solutions of bromine recall the phenomena that have been observed in aqueous and alcoholic solutions of iodine, where a large displacement of the maximum towards the ultraviolet is accompanied by an actual decrease in the intensity (Getman 1928; Groh and Papp 1930).



S u m m a r y

The visible and ultra-violet absorption spectrum of bromine has been investigated in the following solvents: water (2N sulphuric acid), cyclohexane, chloroform, carbon tetrachloride, benzene, chlorobenzene, toluene.

The weak ultra-violet absorption of gaseous bromine is enormously enhanced in solution, particularly by the aromatic solvents. The smallest effect occurs in carbon tetrachloride, where considerable deviations from Beer’s law were found. Absorption maxima in the ultra-violet were found at 2500 A in cyclohexane, at 2725 A in chloroform, and in the neighbourhood of 2950 A in the aromatic solvents.

In the region of the visible continuum, the non-aqueous solvents cause a marked increase in absorption intensity combined with a zero or small shift of the maximum towards the ultra-violet. Water causes a large displacement (1750 cm.-1) of the maximum to higher frequencies; but only a very small increase in intensity. If the A component of the visible continuum is complex, the displacement of the maximum in non-aqueous solvents can be interpreted as due to a greaters olvent effect on 177M<-127+ than on 3TI0+U

The transition zI I lu<-1Efis less affected still. Difficulty is found in interpreting the observations if the B continuum is complex and A simple.


R e f e r e n c e s

A cto n , A . P ., A ick in , R . G. a n d B ay liss , N . S. 1936 Chem. 474.A ick in , R . G. a n d B ay liss , N . S. 1938 In th e P ress .B ay liss , N . S. 1937 Proc. Roy. Soc. A , 158, 551.B ov is, P . 1924 C.R. Acad. Sci.,Paris, 178, 1964.— 1927 C.R. Acad. Sci., P aris, 185, 57.— 1928 A n n . P hys., Paris, 10, 232.

D a rb y sh ire , O. 1937 Proc. Roy. Soc. A , 159, 93.G e tm a n , F . H . 1928 J . Am er. Chem. Soc. 50, 2883.G illam , A . E . a n d M o rto n , R . A . 1929 Proc. Roy. Soc. A , 124, 604.G roh , J . a n d P a p p , S. 1930 Z. phys. Chem. A , 149, 153.H a rr iso n , G. R . 1929 J . Opt. Soc. Am er. 19, 267.L ey , H . a n d E n g e lh a rd t, K . 1910 Z. phys. Chem. 74, 1.L ie b h a fsk y , H . 1934 J • Am er. Chem. Soc. 56, 1500.M ulliken , R . S. 1936 J.Chem.. Phys. 4, 620.P o r re t , D . 1937 Proc. Roy. Soc. A, 162, 414.P u rv is , J . E . 1911 J . Chem. Soc. 99, 1699.— 1915 J.Chem. Soc. 107, 496.

R ab in o w itch , E . a n d W ood , W . C. 1936 Trans. Faraday Soc. 32, 540.Scheibe, G. 1937 Angew.Chem. 50, 212.T w y m an , F . a n d A llsopp , C. B. 1934 “ S p e c tro p h o to m e try .” H ilger.W eissberger, A. a n d P ro sk a u e r , E . 1935 ‘‘ P h y sica l C o n s tan ts o f O rgan ic S o lv e n ts .”

O xford .

Vol. CLXIX A. 16



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