Jan. 5, 1960
29
SPECTRA OF CHARGE-TRASSFER COMPLEXES
-
singlet was found in the pH region S 9, for both Nmethylformamide and N-methylacetamide. Our finding is thus in qualitative agreement with their results.
[CONTRIBUTION PROM THE
We are indebted to Professor John T. Edsall for his active interest in this work and for advice in the preparation of the manuscript. CAMBRIDGE, MASS.
DEPARTMENT O F CHEMISTRY,
STATE UPiIVERSITY O F IOWA]
Infrared Spectra of Charge-transfer Complexes. 111. Complexes with Bromine, Chlorine and Iodine Monochloride. Solvent Effects Involving X-H Stretching Frequencies BY WILLISB. PERSON, RONALD E. ERICKSON' A N D ROBERT E. BUCKLES RECEIVED APRIL 6, 1959 Infrared spectra of a number of charge-transfer complexes involving bromine as the acceptor have been studied ill the region of the Br-Br stretching vibration. The infrared absorption due to the Cl-C1 stretch in the benzene-CI2 complex has been reinvestigated and the intensity estimated. I n addition several more complexes of iodine monochloride have been studied. These studies are correlated in terms of the plot of ea, the added effective charge due to complex formation, against A k / k , the relative change in force constant due to complex formation; in fact the same relation is found to fit all the coinplexes studied thus far (with I Q 2ICN,* Br2, Cls and hydrogen-bond complexes3). This is interpreted as providing further support for the model of the bonding in the complex in terms of the two resonance structures: D---X-I'ff (D-X)----Y-. This resonance model can also be related successfully to the very large body of experimental data on X-€I stretching frequencies correlated by Bellamy, et aZ.21
Introduction As part of the program begun recently in this Laboratory on the investigation of the effect of charge-transfer complex formation on the spectra of the molecules involved2J and in continuation of a study of molecular complexes with halogens4 we have extended the experimental observations on the infrared spectra of bromine complexes reported earlier.6 A41thoughthe infrared spectra of halogen complexes had been studied i t was not until the work of Collin, D'Or, and Alewaeters on the infrared spectra of Br2 and Clz in benzene* that i t became apparent that the most dramatic changes in the vibrational spectra of molecules participating in charge-transfer complexes would occur in the halogen ~ i b r a t i o n . ~ This was confirmed in the study of iodine monochloride where it was observed that the spectrum of the halogen shows regular changes in half intensity width, frequency and intensity as the strength of the charge-transfer coinplex increases.2 These changes were interpreted2 in terms of the charge-transfer theory,'O and the resonance structures (1) Monsanto Predoctoral Fellow, 1957-1968. ( 2 ) W. B . Person, R. E . Humphrey, W. A. Deskin and A I. Popov, THISJ O U R N A L , 8 0 , 2049 (1958), paper I of this series. (3) W. B. Person, R. E. Humphrey and A. I . Popov, ibid., 81, 273 (1959). (4) R. E. Buckles and W. D. Womer, ibid., 80, 5055 (1958); R. E. Buckles, W. D . Womer and R. E. Erickson, Division of Organic Chemistry, ACS meeting, San Francisco, April 19,58.p. RS-h-;. ( 5 ) W. B. Person, R. E . Erickson and R. E. Buckles, J . cht711. Phys., 2'7, 1211 (1957). ( 6 ) W. Haller, G. Jura and G. C Pimentel, iDid., 22, 720 (1954), and references cited there. (7) D . L. Glusker and H . W. Thompson, J . Chem. Soc., 471 (1955). (8) (a) J. Collin and I,. D'Or, J. Chem. Phys., ZS, 397 (1955); (h) D'Or, Alewaeters and Collin. Rec. I T Q W chim., . '75, 862 (1956). (9) However, more recent studies b y Ferguson (see for example E. E. Ferguson, Speclrochim. A c t a , 10, 123 (1957), and references cited there) indicate t h a t t h e changes in the spectrum of the donor are indeed just as Ereat as in the spectrum of the acceptor. These r h a n g r s are not readily observable because of interference from the uncumplexed donor which is usually present in large excess. (10) (a) R. S. Mnllikcn, T H I S JOVRNAL, 7 4 . 811 ( l ' J 3 2 h (1)) f i i c rriv. chiin.. 7 5 , 843 (19.3).
(D-S)+. . .Yb
D . . .X-Y a
Here D is the donor molecule and X-Y is the acceptor bond. The solid lines represent covalent bonds and the dotted lines represent Coulomb and/ or van der Waals attractions. As the formation constant of the complex becomes larger, the importance of structure b in the ground state increases; this explains the changes in infrared spectra of IC1 satisfactorily.2 The qualitative similarity of these changes to the changes in the 0-H bond in hydrogen-bond formation is notable. In order to compare these similarities on a more quantitative basis, the correlations of Huggins and Pimentel" between frequency shift and intensity increase for hydrogenbonded systems were m ~ d i f i e d . ~When Ea, the increased effective charge as defined in reference 2 , was plotted against A k / k , the relative change in the force constant of the X-Y diatomic molecules, it was found that not only could the data for the hydrogen-bonded system be fitted to a straight line, but also the data for IC1 complexes and for ICN complexes (for the I-C stretch) could be fitted to the same straight line.12 In addition, the point for Brz-benzene5 seemed to fit on the curve. (11) C. hl. Huggins and G. C. Pimentel, J . P h y s . C h e m . , 60, 1615 (1956). (12) T h e simplest interpretation of e, given in references 2 and 3 is t h a t i t is t h e additional charge on the Y atom in the complex due t o the contribution from the resonance structure b. I t s definition in terms of the experimental d a t a is given in the equation
''
E = - =
eo
dQ
+
ea =
1.537 X
4BZ
Here B is t h e apparent integrated molar absorption coefficient in darks (see footnote 15) ; I" is t h e reduced mass in atomic mass units; and co is the effective charge of t h e uncomplexed molecule (in carbon tetrachloride solution). T h e relative change in force constant, A k / k , is defined b y
aklk
(ka
- kcomplea)/ka
Here ko is the force constant for the uncomplexed X - Y molecule and kcompiox is t h e force constant for this bond in the complex. T h e X-Y bond is treated throughout as a diatomic molecule, an approximation which should be reasonably correct for the cases considered her?.
W. B. PERSON,K. E. ERICKSON AND H. E. BUCKLES
Vol. s2
known volume of donor solvent (usually with a one milliliter volumetric pipet). In the cases where the donor was a solid, carbon tetrachloride solutions of known concentration were used. Concentrations of iodine monochloride were in the range from 0.05 to 0.15 M . Bromine solutions were prepared by two methods. I n most of the experiments, known volumes of pure bromine were pipetted into the donor solvent, or into a solution of the donor in carbon tetrachloride. I n the study of bromine complexes of benzene, chlorobenzene, o-dichlorobenzene and benzophenone, the concentrations of bromine were checked by removing aliquots from the solutions immediately after the spectra were run, pipetting them into potassium iodide solutions, and then titrating the resulting solution with standard sodiuni thiosulfate solution. The concentrations measured in this way agreed fairly well with those calculated from the first method. T h e concentration range of bromine was from 0.4 to I .5 M. Solutions of chlorine in benzene could be prepared with the least amount of reaction taking place when the following procedure was used. About 200 ml. of benzene was cooled t o about 5” (ice-water bath) in a dark brown glass-stoppered bottle. The room was darkened and chlorine gas, which had been passed through sulfuric acid, was bubbled into the benzene for about 20 minutes. At this point a one-ml. aliquot was withdrawn, pipetted into a prepared potassium iodide solution and titrated with standard thiosulfate. The chlorine-benzene solution was placed in the cesium bromide cell with as little illumination as possible. After the spectra Wave Number, cm-1, were determined a new aliquot of solution was withdrawn Fig. 1.-Typical spectrum of bromine (0.63 M)in benzene and the amount of chlorine again determined. Normally there was fittle difference between the two analyses. These as obtained directly from spectrometer. The curve labeled solutions were very concentrated (about 3 AT). I is the absorbance of the solution of bromine in benzene; Spectrometer.-The spectra were obtained on a modified IOis absorbance of the background obtained with the empty Perkin-Elmer Model 12C Spectrometer, using a double-pass nionochromator with a CsBr prism. Il’ater vapor was recell. moved by blowing dry air through the spectrometer. A cell with CsEr windows was used for all studies. It was decided to remeasure the intensity of the one-mm. This cell was the one described in ref. 5 , with no metal spacer. Br-Br stretching frequency in Brz-benzene com- For the chlorine complex a KBr prism was used, with the plex and to study more bromine complexes. Since same cell. Experimental Difficulties. Bromine Solutions.-The the bromine molecule is homonuclear, the comfrequencies of the bromine complexes come a t the plexes it forms should provide a further test of the absorption lower limit of transmission by the CsBr prism and very little generality of the correlation between added effective light is available. In very concentrated bromine solutions, charge and relative change in force constant. layers of the yellow CsBrl were formed on the cell walls, Furthermore, data on the halogen-halogen stretch- causing scattering and a large shift in backgrounds. Furthermore, the intensities of absorption by the complexes ing vibrations in such complexes were extremely were quite low. Figure 1 sliows a typical spectrum of broscarce, with only the work of Collin and D’Or,* mine in benzene, obtained directly from the recorder. This our own preliminary study,6 and a study of ben- spectrum illustrates these difficulties. The mechanical slit zene-12 and pyridine-12 by Plyler and Mulliken13 of the monochromator was opened as far as possible ( 2 mm.). The rapid decrease in transmittance by the monochromator available. Also, since Collin and D’Orsa had not is readily apparent in the 20curve. reported their experimental conditions, we could This spectrum also illustrates the “zero point shift” in not determine whether the data for Clz-benzene t h a t the Io and 2 curve do not coincide a t the extremes. fit our correlation, and we decided to repeat that Figure 2 shows two replots from recorder traces such as Fig. 1. The curve in Fig. 2 for the 0.03 M solution is a replot of study. In addition several new iodine mono- Fig. 1, and the dashed line shows how the base line was adchloride complexes were studied. justed to correct for the “zero point shift.” The intensity was obtained by integrating the area between the dashed Experimental line and the curve with a planimeter, and niultiplying by Chemicals.-Chemicals were from the same sources listed 2.303. in references 2 and 3, except for the following. Bromine was Figure 3 shows the absorption spectrum of a n extreitiely Mallinckrodt reagent grade, used without further purificaconcentrated solution of tetrdbutylanlmoniuni tribromide in tion. Benzene was dried over CaCL and distilled, to give a benzene. n’hen a n attempt was made to prepare a 1 A6 large middle fraction, b.p. 80” (745 rnm.). Chlorobenzene, solution of this tribrornide in this solvent, separation into o-dichlorobenzene, p-bromoanisole, o-bromoanisole, benzo- two liquid layers occurred. This figure is the spectrum of the phenone and biphenyl were reagent grade chemicals (Eastmore dense, highly colored layer. This band was also noted nian or Mallinckrodt) and were used without further puri- in very conceiitrated cesium bromide pellets of tetrabutylfication. Methylene chloride was washed with aqueous ninmonium tribromide. It will be noted t h a t this band is sodium thiosulfate, aqueous sodium carbonate, dried over fairly broad with a band center a t 312 cm.-l. The spectruili phosphorus pentoxide. and then distilled from fresh phos- of tetrabutylammonium bromide showed no absorption iii phorus pentoxide, b.p. 39.5” (730 mm.). Tetrakis-(pbrothis region, indicating that the new band is due to the Br3mopheny1)-ethylene, 1,2-bis-(p-methoxyphenyl)-1,2-di- I o n . Anderson and Person have explained this band as R phenylethylene, l,l-bis-(p-bromophenyl)-2,2-dibromoethyl- combination of the symmetric stretching frequency and the m e , and tetraphenylethy1enec:were available from previous bending frequency of the ion.’4 The weakness of this band wnrk.‘ accounts for the lack of interference in the spectra of the Preparation of Solutions .-Iodine monochloride solutions bromine complexes. The major effect there is the scattcriiig, were prepared by pipetting known volumes of a standard which is evident in Fig. 1 and also in Fig. 3. iodine monochloride solution in carbon tetrachloride into a _ _ _ ~ ( 1 3 ) E. K. I’lyier arid I
