J . Phys. Chem. 1992, 96, 75-79
75
High-pressure Raman Spectra of the Acetone C-C Stretch in Binary Liquid Mixtures with Methanol Michael S. Bradley* and John H. Krech Department of Chemistry, University of Connecticut, U-60, 215 Glenbrook Road, Storm, Connecticut 06269-3060 (Received: June 10, 1991)
Raman spectra of the acetone C-C stretch (780 cm-I) in binary mixtures with methanol (at acetone mole fractions xA = 1.O,0.8,0.6,0.4, and 0.2) under high pressure are analyzed. The vibration shifts to higher frequency with increasing pressure, and with decreasing mole fraction, due to increased collisions. The line broadens continuously with dilution. At a given temperature, the width varies linearly with mole fraction, though the infinite dilution width does not vary with temperature. This is best explained through the decrease in acetoneacetone interactions and the presence of an additional relaxation pathway via the methanol. The percent of Lorentzian character decreases as the concentration and/or the temperature decreases, indicative of increasing intermolecular interactions, which slow motion in the environment.
Introduction Vibrational (IR and Raman) spectra are sensitive to changes in the molecular environment.' Intermolecular interactions in the liquid state are of fundamental chemical importance and have been the subject of many theoretical and experimental reports.2-8 Careful experimental work has been performed on single-component liquids, where the only extensive variables were pressure and temperat~re.~-" In the more complex environment of a binary liquid mixture, where the interactions between unlike molecules can be studied, the composition also becomes an important variable. Spectroscopic investigations of molecular interactions in binary liquid mixtures have been carried out by Bondarev and Mardaeva,12D6ge,13 Fujiyama,14 and Jonas.Is These reports did not extensively investigate each variable (density, temperature, concentration) independently. We were able to obtain extremely high signal to noise ratios for a complete mole fraction-temperaturedensity investigation of a binary liquid mixture within a reasonable time frame by combining a charge-coupled d e ~ i c e ' ~with ~ ' ' automated data collection and analysis. The high-pressure Raman experiment placed several criteria on the selection of liquids. The substances had to be miscible, nonreactive, and liquid over the temperature and pressure range to be studied. Obviously, their Raman bands could not overlap. Strong solutesolvent interactions were also of interest. Acetone (C-C stretch at 780 cm-', C=O stretch at 1704 cm-I, and (1) Rothschild, W. G. Dynamics ofMolecular Liquids; Wiley: New York, 1984. (2) Logan, D. E. Chem. Phys. 1989, 131, 199. (3) Oxtoby, D. W. Adu. Chem. Phys. 1979, XL, 1. (4) Schweizer, K. S.;Chandler, D. J . Chem. Phys. 1982, 76, 2296. (5) Knapp, E. W.; Fischer, S.F. J . Chem. Phys. 1982, 76, 4730. (6) Luck, W. A. P.; Kiimmel, W. K.; Mentel, T. J. Mol. Srrucr. 1990,237, 233. (7) Zerda, T. W.; Thomas, H. D.; Bradley, M.; Jonas, J. J . Chem. Phys. 1987,86, 3219. (8) Schindler, W.; Sharko, P.T.; Jonas, J. J . Chem. Phys. 1982, 76,3493. (9) Jonas, J.; Akai, J. A. J . Chem. Phys. 1977, 66, 4946. (IO) Bradley, M.; Zerda, T. W.; Jonas, J. Specrrochim. Acra 1984,40A, 1117. ( 1 1) Schindler, W.; Jonas, J. J . Chem. Phys. 1980, 72, 5019. (12) Bondarev, A. F.;Mardaeva, A. I. Opt. Spektrosck. 1973, 35, 286. (13) Doge, G.; Amdt, R.; Buhl, H.; Bettermann, G. Z . Nururforsch., A 1980, 35, 468. (14) Fujiyama, T.; Kakimoto, M.; Suzuki, T. Bull. Chem. SOC.Jpn. 1976, 49, 606. (15) Schindler, W.; Jonas, J. J. Chem. Phys. 1980, 73, 3547. (16) Bilkhorn, R. B.; Sweedler, J. V.; Epperson, P. M.; Denton, M. B. Appl. Specrrosc. 1987, 41, 1114. (17) Bilkhorn, R. B.; Sweedler, J. V.;Epperson, P. M.; Denton, M. B. Appl. Spectrosc. 1987, 41, 1125.
0022-3654/92/2096-75$03.00/0
C-(C4)-C breathing motion at 1067 cm-') and methanol (C-0 stretch at 1030 cm-l) met these criteria. The C-H stretches of the two liquids near 3000 cm-' overlapped and were not studied. We present here data on the 780-cm-' Raman band of acetone. First, the theoretical basis of the work is outlined. We then discuss the experimental details and present our results. To anticipate our findings, the peak was strongly influenced by temperature and mole fraction, and less so by density. An increasing amount of hydrogen bonding as the acetone becomes diluted with methanol, and the response of the hydrogen bonds to pressure and temperature are invoked to explain our observations. Successive papers will address the other vibrational modes. Theory Groups of molecules vibrate coherently for some time after e x c i t a t i ~ n . ' ~ ~The - ' ~coherence may eventually be lost through randomization; the rate a t which this occurs is characterized by the correlation time rC. The excitation itself decays through energy relaxation, which establishes an amplitude coherence time (or lifetime) 7,. Two limiting cases (rC >> T , or rC
