The vibrational Fermi resonance of two liquids, methanol (CH,OH) and dichlo
romethane (CH2Cl2), is investigated by measuring changes in the position an
d intensity of Fermi-coupled Raman bands as a function of pressure, in a di
amond anvil cell. The Fermi resonance of interest occurs in the 2900 cm(-1)
spectral region, where coupling between the CH symmetric stretch fundament
al and a CH bend overtone gives rise to two prominent bands. The methanol r
esults reveal a pressure induced transition through exact resonance at 1.25
GPa, where the two coupled states decompose into a pair of fully mixed hyb
rid bands. In dichloromethane, on the other hand, the two coupled states ar
e driven farther apart and become less mixed with increasing pressure. The
Fermi resonance coupling coefficient, W, is found to be constant in each li
quid 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 o
f the fundamental and Fermi resonance corrected overtone. The results are c
ompared with those of previous Fermi resonance studies using solvent, phase
, isotope, temperature, and pressure variation. In addition to yielding a r
obust method for quantifying Fermi resonance, pressure variation is shown t
o offer a powerful aid to the resolution of spectral assignment ambiguities
.