Pressure Dependent Vibrational Fermi Resonance in Liquid CH₃OH and CH₂Cl₂

Abstract

The vibrational Fermi resonance of two liquids, methanol (CH3OH) 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 ≈ 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.

Facet-specific interaction between methanol and TiO 2 probed by sum-frequency vibrational spectroscopy

“…2, which shows that A r+ and A FR vary differently with pressure. The discordance arises because A r+ and A FR depend on the FR coupling strength (44,50), which originates from the anharmonicity of the intramolecular potential and is sensitive to the environment of the molecule (51)(52)(53)(54). Specifically, the FR coupling strength varies with the methanol vapor pressure and thus contributes to the dependence of A r+ and A FR on pressure.…”

“…Note, however, the original amplitude of A 0 2δ should be very weak without FR coupling. To disengage the FR coupling effect and find A 0 r + and A 0 2δ , we use the well-established two-level model with anharmonic coupling, which was applied to methanol in the liquid phase (50,51). Fermi resonance arises from intramolecular coupling, but is expected to be perturbed by molecules' interactions with neighboring molecules or the environment (52,53).…”

Effects of mechanical compression on the vibrational spectrum of a self-assembled monolayer

“…In the case of liquid methanol, Raman intensity ratios of the methyl resonances change measurably with less than 1 GPa of pressure applied. 24 Since SFG intensity scales as the product of the dipolar and Raman cross sections, 16 the intensity of r FR ϩ relative to r ϩ is expected to be particularly pressure sensitive.…”

Fermi resonance as a means to determine the hydrogen-bonding status of two infrared probes

“…In contrast to vibrational coupling between two spectrally active fundamental modes, a defining characteristic of Fermi resonance is the sharing of vibrational excitation energy between a (bright) fundamental transition and an overtone or combination mode, which is otherwise dark or has a negligible absorption cross section. According to the treatment of Bertran et al 20 as well as Devendorf et al 21 the frequency gap (Δ) between an observed Fermi resonance doublet is related to the corresponding unperturbed frequency spacing (Δ 0 ) and the anharmonic coupling strength ( W ) by the equation: $$\mathrm{\Delta }=\sqrt{{{\mathrm{\Delta }}_0}^2+4W^2}.$$…”