Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton‐Bound Dimers

Infrared spectra for a series of asymmetric proton‐bound dimers with protonated trimethylamine (TMA‐H+) as the proton donor were recorded and analyzed. The frequency of the N–H+ stretching mode is expected to red shift as the proton affinity of proton acceptors increases. The observed band, however, shows a peculiar splitting of approximately 300 cm−1 with the intensity shifting pattern resembling a two‐level system. Theoretical investigation reveals that the observed band splitting and its extraordinarily large gap of around 300 cm−1 is a result of strong coupling between the fundamental of the proton stretching mode and overtone states of the two proton bending modes, that is commonly known as Fermi resonance (FR). We also provide a general theoretical model to link the strong FR coupling to the quasi‐two‐level system. Since the model does not depend on the molecular specification of TMA‐H+, the strong coupling we observed is an intrinsic property associated with proton motions. Huge Fermi resonance coupling was found in the experimental IR predissociation spectra of proton‐bound dimers based on protonated trimethylamine (TMA‐H+). An ab initio anharmonic vibrational analysis could explain the quasi‐two‐level behavior of the multiple states in the Fermi resonance as the proton affinity of proton acceptors varies.