IR Study of C12 in CsHBand CCI,
The Journal of Physical Chemistry, Vol. 83, No. 9, 7979
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Spectroscopic Studies of Chlorine in Benzene and Carbon Tetrachloride Solutions. 1, Infrared Spectrumt T . 4 . Jao$ Department of Chemistry and Center for Energy and Environment Research, University of Puerto Rico, Mayaguez, Puerto Rico 00708
and W . B. Person*$ Department of Chemistry, University of Florida, Gainesville, Fiorida 3261 1 (Received July 25, 1977; Revised Manuscript Received September 26, 1978) Publication costs assisted by the National Science Foundation
New measurements of the infrared spectrum of C12 dissolved in benzene-CC1, solutions are reported. The absorption by the Cl-C1 stretching vibration from C12in pure CCl, is broad (Aul12= 88 cm-I), weak (A = 0.78 km mol-l), and close to the gas-phase wavenumber (545 cm-’ in solution vs. 555 cm-’ in the gas phase). When benzene is added to the CC14,the C1-C1 vibration changes drastically in wavenumber (to 527 cm-l in pure benzene), half-width (Au,,, = 41 cm-l in pure benzene), and in apparent intensity ( A = 3.33 km mol-l for Clzin pure benzene). These values for C12in benzene are different from those reported earlier (Person, Erickson, and Buckles) and the new study provides a much more detailed picture of the changes in the spectrum of C12that occur as the solvent is changed from CC14 to benzene.
Introduction Spectroscopic studies of halogens in benzene solutions diluted with different relatively inert solvents have been carried out quite extensively in the last quarter century. The experimental methods include ultraviolet, visible, Raman, and infrared spectroscopictechniques. In the early stages Mulliken’s charge transfer theory1 was a ~ p l i e d l - ~ quite successfully to interpret the experimentally observed spectral changes. However, Hanna4i5 suggested that electrostatic effects alone were sufficient to account for observed effects in the infrared spectra of different halogens with benzene. More recently, ab initio quantummechanical calculations by Lathan and Morokuma6suggest that electrostatic effects may dominate those observed for weak complexes. In examining the experimental infrared data2 for C12-benzene complexes, however, one realizes the need for more careful and more controlled experiments on this system. We haw, therefore, repeated the study of the infrared spectra of chlorine in benzene. It is important to be able to compare the observed infrared spectrum of C12 in benzene with the infrared spectrum of C12 in an “inert” solvent, so we have also carried out a study of the spectrum of C12 in carbon tetrachloride. We have also made a more refined calculation of electrostatic effects on the infrared spectrum, using more reasonable values for the polarizability derivatives determined from our own recent experimental measurements of the Raman spectrum, with ab initio values for quadrupole moment derivatives’ for the chlorine molecule. These calculations are presented in the following paper and compared there with1 the new experimental results reported here.
Experimental Section The chlorine gas used for this work was either research or ultra-high-purity grade from Matheson. Benzene and carbon tetrachloride were spectrophotometric grade from Mallinckrodt. The infrared liquid cell was composed of two undrilled KBr windows fastened to a 3-mm teflon Experimental work done at the University of Florida.
* Senior Visiting Fellow and Visiting Professor, Department of
Chemistry, Royal Holloway College, University of London, 1978. 0022-3654/79/2083-1181$01 .OO/O
spacer which was connected to a glass joint which in turn could be connected to a vacuum line. The components were held together inside brass tubes and against two brass plates tightened with four screws of appropriate lengths8 Two matched liquid cells were used. The KBr windows were polished after each spectrum was taken. A Perkin-Elmer Model 621 infrared spectrophotometer was used for this study. The chlorine solutions studied varied in concentration from 0.3 to 0.9 M. At these high concentrations photochemical reaction occurs very easily. Extreme care was taken to avoid this reaction by working in the dark and by adding oxygen gas to the solution to act as a radical quencher. The concentration of each chlorine solution was determined just before the spectrum was taken by withdrawing a 5-mL portion of the solution and titrating with a 0.1 M standard solution of sodium thiosulfate. In order to minimize the change in chlorine concentration during the course of recording the spectrum and also the growth of the photochemical product sufficient to distort the absorption band from the C1-Cl stretching vibration, we chose to sacrifice the signal-to-noise ratio (S/N), reducing it to around 125, with a spectral resolution of about 2 cm-‘. The total scan time for each measurement from 650 to 350 cm-l could thus be reduced to about 15-20 min. The concentrations of chlorine of some solutions were also determined after measurements were made to establish the actual concentrations of the solutions during the measurement. We also remeasured the spectrum of the solution after the chlorine gas was removed from the solution in the cell by pumping through the vacuum line. In this way we could determine how much photochemical product was formed during the measurement.
Experimental Results The spectrum of 0.33 M chlorine in benzene in the 3-mm cell is shown in Figure 1B recorded vs. pure benzene in the reference beam in a matched cell. That spectrum is compared with the spectrum (A) of benzene in the same cell, recorded vs. air in the reference beam, and also with a baseline (C) of benzene vs. benzene in the 3-mm cells. Benzene absorbs almost totally above 575 cm-’ and below 410 cm-l as seen in Figure lA, so that we cannot measure
0 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
T.-C. Jao and W. B. Person
WAVENUMBER (cm')
WAVENUMBER
Figure 1. Infrared spectrum of 0.33 M chlorine dissolved in pure benzene, taken at room temperature with a 3-mm liquid cell: (A)
spectrum of pure benzene vs. air; (B) spectrum of CI,-benzene solution vs. pure benzene in matched cells; (C) spectrum of benzene vs. benzene in those matched cells.
(m?)
Figure 4. Infrared spectrum of 0.31 M chlorine dissolved in pure CCI4 (3-mm cell): (A) spectrum of pure CCI, vs. air; (6)spectrum of chlorine solution vs. CC14 in matched cells; (C) spectrum of CCI, vs. CCI, in those same matched cells.
i
1001 400
450
550
500
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600
WAVENUMBER (cm-9
(ern")
Figure 2. Infrared spectrum of 0.445 M chlorine dissolved in a solution of 60% (v/v) benzene and 40% ( v h ) CCI, (3-mm liquid cell): (A) spectrum of solvent vs. air; (B) spectrum of chlorine solution vs. solvent in matched cells; (C) spectrum of solvent vs. solvent in those matched cells.
Figure 5. Infrared spectra of 0.66 (C) and 1.2 M (B) chlorine dissolved in pure CCI4 (3-mm cells); see caption to Figure 4.
,
loot
I
500
450
I
550
650
600
WAVENUMBER (cm'l) 450
500
550
600
Figure 6. Infrared spectra (B) of 0.31 M chlorine dissolved in pure CCI, (1.6-mm cells); see caption to Figure 4.
WAVENUMBER (cm-9
1
Figure 3. Infrared spectra of 0.296 M chlorine and of 0.99 M chlorine
dissolved in a solution of 20% (v/v) benzene in 80% (v/v) CCI, (3-mm liquid cell): (A) spectrum of solvent vs. air; (B) 0.99 M chlorine solution vs. solvent in matched cells; (D) solvent vs. solvent in those same matched cells.
the absorption spectrum for the chlorine-benzene solution in this cell except in the region from about 410 to 575 cm-'. Spectra of chlorine solutions in benzene diluted by carbon tetrachloride are shown in Figures 2 and 3. As benzene was gradually diluted by the addition of carbon tetrachloride, less solvent absorption was found above 575 cm-l, so that more spectral information could be obtained concerning the high-frequency wing of the chlorine absorption band. However, the strong absorption by carbon tetrachloride in the 3-mm cell below 500 cm-l made it difficult to observe the low-frequency wing of the chlorine absorption band in these solutions. The infrared absorption band of chlorine dissolved in pure carbon tetrachloride was also observed, and it is shown in Figure 4. The infrared absorption band (B) of chlorine in this solution is very broad and weak and is quite different from that of chlorine in the benzene solutions. If one did not use a long pathlength liquid cell, this broad and weak absorption could easily be confused with some
0.5
,
4
60% ("/vIC& 40% ( V v ) C C 1 4
pure C6Hs
4
0.0
600
xxl WAVENUMBER
400 1crn-l)
600
500
400
WAVENUMBER (crn-1)
Figure 7. Replotted normalized infrared spectra of chlorine in benzene-CCI, solutions; the concentration of chlorine in each solution is 0.5 M and pathlength is 3 mm. The dashed line indicates the wavenumber reading of 530 cm-' in order to illustrate the frequency shift.
baseline shift of unknown origin. To make sure that the absorption is in fact due to chlorine, we measured the absorption band from solutions of chlorine in CC14 at several different concentrations, and also from the same solution at different pathlengths. These results are shown
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
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...','.F.
.w /'
y'
-... F' \,-*.., I ... ......,.;:.=>?0.0
560
480
520
WAVENUMBER
.......... ... .. .. .. .. . . . . .' .. E. . ... . ',". '... ) , . , . . . . , ............... ~
,""".*e.*.
0.0 ..'..
, .......... .. -
',
440
(cm")
WAVENUMBER
(cm')
--
Figure 8. Lorentzian curve (F, -) fitted to the observed normalized absorption spectrum of chlorine in benzene (I, 000). Curve E is the difference (absolute value) between I and F. (-e)
Figure 10. Lorentzian curve (F, ---) fitted to the observed normalized absorption band (I, 000)of chlorine in carbon tetrachloride. Curve E is the difference (absolute value) between I and F. (-a)
TABLE I: Integrated Infrared Molar Absorption Coefficients ( A ) and the Parameters of the Lorentzian Function for the Cl-C1 Vibration of Chlorine in Benzene-CC1, Solutions Lorentzian curve parametersb
s,c
-
benzene concn, M
A,a km mol-'
mol-' cm-'
u,,,
ASlil:
cm-'
cm-'
11.3 (pure
3.33
49.95
527.1
40.9
3.26 2.74 1.85 1.22 0.78
48.84 41.10 27.74 18.29 11.72
528.4 530.0 531.0 532.3 545.0
37.1 35.3 30.9 33.2 87.6
benzene)
WAVENUMBER (crri')
--
Figure 9. Lorentzian curve (F, -) fitted to the observed normalized absorption band (I, 000)of chlorine in 60% (v/v) benzene and 40% (v/v) carbon tetrachloride!. Curve E (.-) is the difference (absolute value) between I and F.
in Figures 5 and 6. The experimental spectra (linear in transmittance) were converted to plots of absorbance vs. wavenumber to produce spectra such as those shown in Figure 7 . In that figure the dashed line used as a reference marks 530 cm-l in order to illustrate the frequency shift of the chlorine absorption band in various solutions. In order to characterize these spectral results, we have fitted the observed spectrum for C1, in each solution by a Lorentz curveg (In (Io/I) = SAvlj2/2a[(v - vo)2 + ( A V ~ / ~ / with ~ ) ~ parameters ]) chosen to give the best fit possible. Here Avllz is the fwhm (full-width at halfmaximum), vo the wavenumber of the band center, and ~ S / T A Vis~ the / ~ maximum absorbance. Typical fits are shown in Figures 8,9, and 10 for absorption curves for C12 in 1:0, 3:2, and 0:l volume-to-volume solutions of benzene in CC1& The Lorentz curves were then integrated to obtain the apparent integrated molar absorption coefficients ( A = (l/nl)Jln ( l o / I ) dv where n is the total molar concentration of CIZin eolution, 1 is pathlength) given in Table I, together with the frequency maxima and the half-intensity bandwidths for the Lorentz curves. As can be seen from Figures 8-10, the integrated area under the Lorentz curve, S, is always an underestimate for the total band area, perhaps by about 8-10%. Since the uncertainty in the concentration is estimated to be about f8%, and that in 1 about *2%, we estimate the overall uncertainty in the value of A to be about +13%, -8%. It should also be noted here that the experimental absorption curves that were fit by the Lorentz curves were obtained from the experimental data by normalizing the experi-
9.04 6.78 4.52 2.26 0.0 (pure
cc14 1 a Apparent integrated molar absorption coefficients of chlorine. The values are believed to be reliable to t 13% or -8% (see text). All parameters were obtained from curves normalized to 0.5 M (see text). The values (except for chlorine in carbon tetrachloride) are believed to The reliability of the be underestimated by !-lo%. frequency maximum ( u o ) is believed to be k0.5 cm-'. e The half-band widths ( A T , , , ) are believed to be reliable to t 2 cm-'.
mental absorbance to 0.5 M C12 concentration by multiplying it by 0.5/na,, where nclzis the concentration in the actual experiment. Calculation of the Equilibrium Constant. The apparent intensities, A , in Table I can, in principle, be used to obtain a value for an equilibrium constant for the very weak benzene-C1, complex. Andrews and KeeferlO have analyzed the ultraviolet absorption by this complex to obtain Kf N 0.034 L mol-l, with a maximum molar extinction coefficient emax at 280 nm of 8000 L cm-l mol-1 or with a value of Kc,,, = 270 L2 cm-' m o P . We have repeated these experiments8 with considerable difficulty, obtaining values of Kernax= 275 in a 1-cm cell and Ktma = 321 in a 0.1-cm cell, suggesting the order of the uncertainty in our analysis of our own ultraviolet data. Values of K and t from the Scott plots range for K from 0.01 to 0.03 L mol-l and for E from 8000 to 25000 L cm-l mol-l. These results are typical of those expectedll for such weak complexes. It is no surprise, therefore, that our data from the infrared studies summarized in Table I do not provide any additional information about the equilibrium constant of this "complex". Figure 11 shows a plot of the apparent molar absorption coefficient A as a function of concentration of benzene. If there were appreciable complex formation, and if the infrared intensity for the Cl-C1
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b
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I ~ 4 40 1 1
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0
2
4
6
8
IO
12
Concentration of Benzene
Figure 12. Plot of vo and for the CI-CI stretching vibration from 0.5 M CI, (3-mm cell) in benzene-CCI, solutions, as a function of benzene concentration.
0
2
4
6
8
10
12
M
Concentration of Benzene Figure 11. Plot of apparent integrated molar absorption coefficient A of the CI-CI stretching vibration from 0.5 M C12 in benzene-CCI, solutions (3-mm cell), as a function of benzene concentration. The error bars show the error of +13, - 8 % on only one point; the same uncertainty applies to each point.
stretching vibration derived only from the C12 in the complex, we should expect this plot to be a curve through the origin (see Figure 1 of ref 11). If the Cl, absorption intensity were due to electrostatic interactions of Cl, with the solvent which were the same in CCl, and in benzene, then we might expect Figure 11to be a constant horizontal line. Instead, Figure 11 suggests that the apparent intensity of Clz in benzene is indeed greater than in CCl,, but the lack of curvature (within experimental error) allows no information about whether the intensity is due to complex formation or just to differences in electrostatic interactions.
Discussion It seems quite clear from our data presented here for the infrared spectrum of Cl, in benzene (and also that cited8 for the study of the ultraviolet spectrum) that the benzene-C12 interaction is a “contact”.12 Nevertheless, there are still some items of interest. From the values of the apparent integrated molar absorption coefficients A in Table I and shown in Figure 11, there is no doubt, whatsoever, that a dramatic increase occurs in the intensity of the C1-C1 stretching vibration as beneene is added to a solution of C12 in CClb We believe there are two possibilities: (1)either this vibration has a much greater intensity due to electrostatic interactions, for C12 surrounded by (but not complexed by) benzene molecules than it does for Clz surrounded by CC14 molecules, or (2) benzene and Cl, form a complex (assumed to be one-to-one, mostly for the sake of simplicity) and the C1-C1 stretching vibration is enhanced vibronically as a result of this specific interaction, as suggested previously by Friedrich and Person3 (see also Mulliken and Person12 and Person14). This question cannot be resolved exper-
imentally since the complex is so weak, but we can investigate the first possibility in more detail in a model calculation presented in the following paper.15 The extent of the changes in the spectral properties (wavenumber, half-intensity width, and intensity) of the C1-C1 stretching vibration for C1, as the solvent is changed from pure CC14 to pure benzene is shown in Figure 12 (Figure 11 for the intensity). We see in Table I and in Figure 11 that the apparent integrated molar absorption coefficient A (based on total Cl, concentration) increases by a factor of 4 from 0.78 to 3.3 km mol-I as the solvent changes from C C 4 to benzene. There is a change in wavenumber of maximum absorption from 545 to 527 cm-l for the corresponding solvent change, but much of this change (15 cm-l) occurs on the first addition of benzene. Perhaps the most significant change is the change in half-intensity width (fwhm) of the C1-C1 vibration band from 88 to about 30-40 cm-l from CC14to benzene (again occurring mostly as the first bit of benzene is added). This half-intensity width is somehow related to the lifetime of the interaction; the Heisenberg uncertainty principle (cAvljzT N 1/2x) suggests a lifetime of about 0.13 ps for Clz in CC4, and about 0.26 ps for C12 in benzene. If the interaction responsible for the absorption by C12were just electrostatic interaction with the solvent occurring during the lifetime of a collision between C12 and a solvent molecule, one would expect the lifetime of the collision with the heavier solvent molecule (CCl,, in this case) to be slightly longer than that with the lighter solvent molecule. It is all very puzzling, and is discussed further in the following paper.15 We may close here by drawing attention once more to the drastically different intensity for the infrared absorption due to the C1-C1 vibration of C12 in benzene, compared to C12 in CCl,. The new measurement for this absorption band from Clz in pure benzene is different ( A = 3.3 km mol-l than the earlier result ( A 1.5 km mol-’) and we believe our present measurement is much more reliable. We believe the comparison between Clz in benzene and Clz in CC14 and the variation as the solvent changes gradually have been especially important and potentially instructive.
Model for IR Study of (>I, in C6H6and cci,
Acknowledgment. Partial financial support from the National Science Foundation (Research Grant No. GP 17818) is gratefully acknowledged. We are grateful to Dr. Derek Steele (Royal Holloway College, University of London) for many helpful discussions, to the Science Research Council of the United Kingdom for the support (as Senior Visiting Fellows) that made them possible, and to the Chemistry Dlepartment of Royal Holloway College for the support and hospitality provided to us while we were there (1978). References and Notes (1) R. S. Mulliken, J . Am. Chem. Soc., 74, 811 (1952). (2) W. 8. Person, R. E. Erickson, and R. E. Buckles, J. Am. Chem. Soc., 82, 29 (1960).
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(3) H. B. Friedrich and W. B. Person, J. Chem. Phys., 44, 2161 (1966). (4) M. W. Hanna, J . Am. Chem. Soc., 90, 285 (1968). (5) M. W. Hanna and D. E. Williams, J. Am. Chsm. Soc., 90,5358 (1968). (6)W. A. Lathan and K.Morokuma, J. Am. Chem. Soc.. 97.3615 (1975). (7) T. C. Jao, N. Beebe, W. 8 . Person, and R. J. Sabin, Chem.'Phys. Lett.. 26. 474 119741. -,- , (8) T. C. Jao, Ph.D. Dissertation, University of Florida, 1974. (9) A. S.Wexler, Appl. Spectrosc. Rev., 1, 29 (1967); see also, for example, D.A. Ramsay, J. Am. Chem. Soc., 74, 72 (1952). (10) L. J. Aridrews and R. M. Keefer, J. Am. Chem. Soc., 73,462 (1951). (11) W. B. Person, J . Am. Chem. Soc., 87, 167 (1965). (12) L. E. Orgel and R. S.Muliiken, J. Am. Chem. Soc., 79, 4839 (1957). (13) R. S. Mulliken and W. B. Person, "Molecular Complexes", Wiley, New York, 1969. (14) W. B. Person in "The Spectroscopy and Structure of Molecular Complexes", J. Yarwood, Ed., Plenum Press, London, 1973, Chapter 1. (15) T. C.Jao and W. B. Person, J. Phys. Chem., following article in this issue. I
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Spectroscopic Studies of Chlorine in Benzene and Carbon Tetrachloride Solutions. 2. A Model Calculation of Electrostatic Effects on the CI-CI Stretching Vibration T.-C:. Jaot Department of Chemistry and Center for Energy and Environment Research, University of Puerto Rico, Mayaguez, Puerto Rico 00708
and Willis B. Person*+ Department of Chemistry, University of Florida, Gainesville, Norida 326 I 1 (Received September 26, 1978) Publication costs asslsted by the University of Puerto Rico, Mayaguez Campus, and the NIH
A niodel for electrostatic collision-induced infrared absorption by the C1-C1 stretching vibration of Clz in benzene and in CC14 solutions is presented. This model is based upon the treatment by Fahrenfort and by van Krankendonk of collision-induced absorption in the gas phase, modified for solutions, with an approximate averaging over all orientations for the colliding molecules. Parameters for evaluating the collision-induced intensities are all evaluated from other kinds of experimental data, except for the derivative of the quadrupole moment of C12with respect to the C1-C1 stretching normal coordinate, which is taken from a quantum mechanical calculation. The predicted integrated molar absorption coefficient A for C12in CCll is 0.55 km mol-', compared to an experimental value of 0.78 km mol-'; and for Clz in benzene the calculated value is A = 1.4 km mol-', compared to 3.3 km mol-' from experiment. These results are discussed and some variations on the model calculation are presented. We believe that most of the intensity increase for the C1-C1 stretching vibration of Cl2 that occurs from CC4to benzene solution is due to the different electrostatic collision-inducedintensity, but that some of the increase may also be due to one-to-one complex formation between benzene and Clz.
Introduction In the preceding paper' we have presented some results from a new study of the infrared absorption spectrum of the C1-C1 stretching vibration for Clz in benzene, in CC14, and in solutions of benzene in CC4. The results are summarized there in Figures 11 and 12. The significant result from that study is that the C1-C1 stretching vibration, which is forbidden in the infrared spectrum for the isolated molecule, appears with measurable intensity for Clz dissolved both in pure benzene and in pure CC14 (the apparent integrated molar absorption coefficient, based on the total C12 concentration in each solution, is 3.3 km mol-l for Clz in benzene and 0.78 km mol-1 for C12 in CC14, varying more or less linearly with benzene concentration in the rnixed solvents). While it is true that these intensities are rather low (the intensity for the infrared-active C-C1 stretching fundamental in CC14, for Senior Visiting Fellow and Visting Professor, Department of Chemistry, Royal Holloway College, University of London, 1978. 0022-3654/79/2083-1185$01 .OO/O
example, is 390 km they differ significantly from zero, and the intensity must be due to the interaction between the Clz molecule and the solvent. This infrared activity for C12 in benzene has been explained previously in various ways. The activity for this vibration for C12dissolved in CC14has not previously been reported, and it requires some modification of some of the previous explanations for the infrared activity. One of the previous explanations has been that the benzene molecules form a weak one-to-one complex with Clz molecule^.^ The lower symmetry of the C12molecule in the complex allows the C1-C1 stretch to become active and quite strong; the weakness of the apparent molar absorption coefficient is because only a small fraction of the total C12 is complexed. Carbon tetrachloride is not expected to form a complex with C12, so the appearance of the C1-C1 stretching vibration in CC14 solution must be attributed to some other cause. Hanna and c0-workers~9~ have suggested that the infrared activity of the C1-C1 stretching vibration for C12in
0 1979 American Chemical
Society
