HIGH-PRESSURE RAMAN-SPECTRA OF THE ACETONE CARBONYL STRETCH IN ACETONE METHANOL MIXTURES

The isotropic and anisotropic Raman bands of the carbonyl mode in acetone do not coincide, a phenomenon caused by resonant transfer of vibrational energy coupled with an angular dependent intermolecular potential. We report on the frequency shifts, line width changes, and noncoincidence of these bands in mixtures with methanol as the density, temperature, and acetone mole fraction (X(A)) are independently varied. Increased density red shifts both signals; it also narrows the anisotropic band while not greatly altering the isotropic band. Increased temperature blue shifts both bands and broadens them (considerably, for the anisotropic mode). Dilution with methanol causes red shifts and broadening of both bands. The noncoincidence shift increases as the density Of X(A) increases. In pure acetone, temperature has little effect; in dilute solutions, increased temperature causes a pronounced decrease in noncoincidence. These observations can be consistently explained through the formation of strong acetone-methanol hydrogen bonds, balanced against weaker acetone dipole-acetone dipole interactions.