Pressure dependent vibrational Fermi resonance in liquid CH3OH and CH2Cl2

The vibrational Fermi resonance of two liquids, methanol (CH,OH) and dichloromethane (CH2Cl2), is investigated by measuring changes in the position and intensity of Fermi-coupled Raman bands as a function of pressure, in a diamond anvil cell. The Fermi resonance of interest occurs in the 2900 cm(-1) spectral region, where coupling between the CH symmetric stretch fundamental and a CH bend overtone gives rise to two prominent bands. The methanol results reveal a pressure induced transition through exact resonance at 1.25 GPa, where the two coupled states decompose into a pair of fully mixed hybrid bands. In dichloromethane, on the other hand, the two coupled states are driven farther apart and become less mixed with increasing pressure. The Fermi resonance coupling coefficient, W, is found to be constant in each liquid up to pressures exceeding 1 GPa (W approximate to 52.6 and 22.3 cm(-1) in CH3OH and CH2Cl2, respectively). The anharmonic shift of the CH bend is about 10 cm(-1) in both liquids, determined by comparing the frequencies of the fundamental and Fermi resonance corrected overtone. The results are compared with those of previous Fermi resonance studies using solvent, phase, isotope, temperature, and pressure variation. In addition to yielding a robust method for quantifying Fermi resonance, pressure variation is shown to offer a powerful aid to the resolution of spectral assignment ambiguities.