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

Abstract

Infrared spectra for as eries of asymmetric protonbound dimers with protonated trimethylamine (TMA-H +)a s the proton donor were recorded and analyzed.T he frequency of the N-H + stretching mode is expected to red shift as the proton affinity of proton acceptors increases.T he observed band, however,s hows ap eculiar splitting of approximately 300 cm À1 with the intensity shifting pattern resembling at wolevel system. Theoretical investigation reveals that the observed band splitting and its extraordinarily large gap of around 300 cm À1 is aresult of strong coupling between the fundamental of the proton stretching mode and overtone states of the two proton bending modes,t hat is commonly knowna sF ermi resonance (FR). We also provide ageneral 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 + ,t he strong coupling we observed is an intrinsic property associated with proton motions.

Vibrational Signature of Dynamic Coupling of a Strong Hydrogen Bond

“…It is also gratifying to note that the quantum treatments also reproduce the ν CH region reasonably well for both TMA–H 2 O and TMA–CH 3 OH. While there is little coupling between the ν CH of TMA and H 2 O or CH 3 OH (Figure S13), there are significant couplings between CH stretches and CH 3 bends within a CH 3 group on TMA, and Fermi resonance is responsible for the extra peaks as reported recently. − , …”

“…While there is little coupling between the ν CH of TMA and H 2 O or CH 3 OH (Figure S13), there are significant couplings between CH stretches and CH 3 bends within a CH 3 group on TMA, and Fermi resonance is responsible for the extra peaks as reported recently. [35][36][37]49 Note that the calculated relative intensities, especially for the ν OH related features, are less satisfactory in comparison to experiment. Whether this is due to the complexity of experiment (IR absorption combined with dissociation, saturation effects, etc.)…”

“…For TMA–CH 3 OH, which is too large to be described by ab initio PES, the anharmonic vibrational spectrum of TMA–CH 3 OH is calculated using the discrete variable representation (DVR) method at the CCSD/AVDZ level of theory (see the SI for details). Such a protocol has been recently applied to study the Fermi resonance in ammonia, monomethylamine, dimethylamine, and protonated trimethylamine clusters. ,,− The calculated anharmonic vibrational spectra of TMA–H 2 O and TMA–CH 3 OH are compared with the experimental ones in Figure .…”

Vibrational Coupling in Solvated H3O+: Interplay between Fermi Resonance and Combination Band

“…A general Hamiltonian has been proposed to explain intensity borrowing between a bright state (such as O–H stretching fundamental) and a set of N dark states (such as bending overtone and combination bands). For N = 2, we can recast the model Hamiltonian, in which all the coupling can be recast into H coup : Here, ε d is the averaged frequency of the two dark states, ε sd = ε s – ε d describes the resonance condition (frequency matching between stretching and centroid of dark states) and Δ = ε d – ε d 1 = ε d 2 – ε d .…”

Disentangling the Complex Vibrational Spectra of Hydrogen-Bonded Clusters of 2-Pyridone with Ab Initio Structural Search and Anharmonic Analysis

“…Although the frequencies of the CH stretch of 2PY are close in frequency with the hydrogen-bonded NH stretch in 2PY-(Me 2 O) and 2PY-(NH 3 ), it does not imply that CH can borrow substantial intensity from NH stretching. In a previous study on 1:1 hydrogen-bonded clusters of protonated trimethylamine (TMA) and water, methanol, and ammonia, the authors found that the strong coupling is very localized between the proton stretching fundamental and bending overtone states. Both theoretical anharmonic analysis and close comparison on the experimental spectra of TMA-H + ···H 2 O and TMA­(d 9 )-H + ···H 2 O are in favor to rule out the role of CH stretching in causing the band splitting.…”