Exploring the influence of π–hydrogen bonding on the Fermi resonance of benzene in methanol/dichloromethane solutions via Raman spectroscopy and DFT simulations

Using Fermi resonance as a probe of the external field environment to reflect changes in hydrogen bonds. • It discusses the effects of π-hydrogen bonds in different directions on molecular vibrations and vibrational coupling. • A combination of experimental and computational methods was used to verify the differences in π-hydrogen bonds. In this study, the influence of π–hydrogen bonding (π-HB) on the Fermi Resonance (FR) of benzene in mixtures with methanol or dichloromethane was investigated using Raman spectroscopy combined with density functional theory (DFT) calculations, including reduced density gradient (RDG) analysis. The results reveal that the π-HBs formed in the two systems exhibit fundamentally different characteristics: axis-asymmetric π-HBs in the benzene–methanol system and axis-symmetric π-HBs in the benzene–dichloromethane system, leading to distinct FR behaviors. In the benzene–methanol system, the Fermi doublet shows a red shift with increasing benzene concentration, accompanied by an increase in the coupling constant W from 9.70 to 10.19 cm -1 , which is attributed to π-electron redistribution and a broadened vibrational potential induced by asymmetric π-HB formation. In contrast, the benzene–dichloromethane system exhibits a blue shift of the Fermi doublet with no significant change in W, resulting from axis-symmetric π-HB interactions that concentrate electron density without altering the coupling strength. These results demonstrate that FR can serve as a sensitive probe for distinguishing different types of intermolecular interactions in weakly interacting molecular systems.