Strong Fermi resonance associated with proton motions revealed by vibrational spectra of asymmetric proton bound dimers.

Experimental infrared spectra between 2600 to 3800 cm -1 for a series of asymmetric proton bound dimers with protonated trimethylamine (TMA-H + ) as the proton donor were recorded and analyzed. Based on conventional wisdom, the frequency of the N-H + stretching mode is expected to red shift as the proton affinity of proton acceptors (Ar, N 2 , CO, C 2 H 2 , H 2 O, CH 3 OH, and C 2 H 5 OH) increases. The observed band, however, shows a peculiar splitting of ~300 cm -1 with the intensity shifting pattern resembling a two-level system. Theoretical investigation based on ab initio anharmonic algorithms reveals that the observed band splitting and its extraordinarily large gap of ~300 cm -1 is a result of strong coupling between 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 simple and general theoretical model to link the strong FR coupling to the quasi-two-level system behavior in the observed band intensity. Since the model does not depend on the molecular specification of TMA-H + , the strong coupling we observed here is an intrinsic property associated with proton motions in a wide range of molecular systems.