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Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton‐Bound Dimers
Author(s) -
Huang QianRui,
Shishido Ryunosuke,
Lin ChihKai,
Tsai ChenWei,
Tan Jake A.,
Fujii Asuka,
Kuo JerLai
Publication year - 2021
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.202012665
Subject(s) - fermi resonance , overtone , proton , chemistry , protonation , resonance (particle physics) , spectral line , coupling (piping) , molecular physics , molecular vibration , nuclear magnetic resonance , atomic physics , infrared spectroscopy , molecule , physics , ion , materials science , astronomy , metallurgy , organic chemistry , quantum mechanics
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.