Switching intermolecular Fermi resonance on and off via dipolar coupling or hydrogen bonding

Fermi resonance (FR) represents a fundamental anharmonic coupling mechanism, yet its activation and suppression mechanisms in condensed phases remain poorly defined. Here, we show that the characteristic carbonyl doublet of cyclopentanone in Raman spectra originates from an anharmonic FR between the C = O stretching fundamental and a ring-breathing overtone, and this coupling can be reversibly switched on and off by the local intermolecular environment. Concentration-dependent Raman measurements, together with anharmonic vibrational perturbation theory calculations and molecular dynamics simulations, reveal a systematic evolution of the FR doublet peaks. The results indicate that the apparent FR emerges under conditions where intermolecular interactions bring the C = O stretching vibration (∼1745cm^-1) into near degeneracy with a ring-breathing overtone (∼1726cm^-1). In contrast, specific solvent associations in protic environments perturb this near-degeneracy and substantially suppress the effective FR splitting.