Concentration and solvent modulated intermolecular Fermi resonance

This study resolves the long-standing controversy over the origin of CO stretching vibration splitting in cyclopentanone by integrating argon matrix isolation, annealing-controlled aggregation, and polarized Raman spectroscopy. For the first time, cyclopentanone monomers were isolated at ppm concentrations, revealing single CO peaks and excluding Fermi resonance in individual molecules. Controlled annealing tracked dimer formation in situ, unveiling a quadruplet splitting pattern (1749/1750 and 1732/1734 cm⁻¹) as a spectroscopic signature of intermolecular dipole coupling and Fermi resonance activation. Solvent-specific modulation was demonstrated: aprotic solvents (CCl₄, CH₃CN) induced concentration-dependent blueshifts, while protic CH₃OH formed hydrogen-bonded clusters that suppressed resonance, with matrix isolation confirming no intermolecular interaction below 70 K. Light-driven aggregation control was achieved, where 365 nm irradiation selectively enhanced resonance through dimerization, whereas 254/405 nm light suppressed it. Supported by DFT calculations, this work redefines the mechanistic framework of Fermi resonance, establishing a universal strategy for probing intermolecular dynamics and designing tunable optoelectronic materials through environmental and optical modulation.