Anharmonic coupling behind vibrational spectra of solvated ammonium: lighting up overtone states by Fermi resonance through tuning solvation environments

Abstract

Studies on the vibrational spectra of various ammonium-centered clusters under different solvation environments have raised interests over the last thirty years. The gas-phase infrared photodissociation spectroscopy (IRPD) experiments showed that...

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect

“…A similar scenario occurs for all solvent species, where the bonded OH frequencies of doubly solvated clusters are always higher than those of singly solvated ones. This anticooperative effect, which can be attributed to the decrease of averaged proton affinity upon the increase of solvation numbers, has been observed in many recently studied clusters. , Another intriguing phenomenon seen from the experimental spectra is that, except for CH 3 OH 2 + ···Kr, there is no prominent band splitting in S B or S S /S A . In the previous studies of solvated hydronium ions and ammonium ions, it was shown that FR is an intrinsic characteristic of self-coupled proton motion in OH and NH groups, and it causes a different extent of band splitting depending on the magnitude of coupling as well as the energy matching (resonance) condition between relevant vibrational quantum states. , In the cases of solvated hydronium ions and proton-bound rare-gas dimers, in addition, CB from the couplings between the covalent bond stretching mode and the intermolecular stretching mode is always significant. , Being an analogue to the hydronium ion, the protonated methanol, however, behaves dissimilarly in this aspect.…”

“…The calculations well reproduce this assignment, and the “merged” band is thus denoted as S S /S A (purple dotted line; hereinafter, we denote the absorption bands in capital letters). The trend of band shift is originated from the change of proton affinity of the solvent species: a solvent molecule with a higher proton affinity causes a stronger H bond, weakens the OH covalent bond, and hence lowers the OH stretching frequency. ,, The band intensity increases in the meantime due to a larger dipole moment derivative.…”

“…The trend of band shift is originated from the change of proton affinity of the solvent species: a solvent molecule with a higher proton affinity causes a stronger H bond, weakens the OH covalent bond, and hence lowers the OH stretching frequency. 38,41,42 The band intensity increases in the meantime due to a larger dipole moment derivative.…”

“…The first type is known as Fermi resonance (FR), which describes the anharmonic coupling between the fundamental of stretching mode and the overtone of bending mode (proton motion perpendicular to the bond direction) of the OH/NH covalent bond. This phenomenon is found to be prevailing in many species containing protons, such as water and hydronium ion (H 3 O + ) , and ammonia/amine and ammonium ion (NH 4 + ). − The frequencies of these OH/NH modes are prone to change by the solvation environment, which can be used to finely tune the strengths of the H bond and the covalent bond, thus leading to an effective way to modulate the energy-matching condition for the resonance and yielding different extents of absorption band splitting. The second type is the coupling between the fundamental of the covalent bond stretching and the intermolecular stretching modes, which results in the combination band (CB). , Both FR and CB may borrow intensity from the stretching mode of the covalent bond, which is the major oscillator-strength carrier in the IR spectrum.…”

Spectroscopic Manifestations of Indirect Vibrational State Mixing: Novel Anharmonic Effects on a Prereactive H Atom Transfer Surface

“…There are many theories for describing this so-called spectral broadening. One important source of broadening is due to multiple Fermi resonance interactions − with modes that have approximately half the frequency of the high-frequency mode. Another source are the interactions that modulate the strength of the hydrogen bond, the broad line shape of the OH stretch in liquid water being the most famous example.…”