Large anharmonic effects in the infrared spectra of the symmetrical CH3NO2−⋅(H2O) and CH3CO2−⋅(H2O) complexes

Abstract

The observed IR spectra of the CH3NO2−⋅(H2O) and CH3CO2−⋅(H2O) complexes display sequences of up to seven transitions in the region of the OH stretch fundamentals. This is indicative of strong anharmonic coupling between the OH stretch modes and one or more low-frequency modes. Cubic force fields have been calculated for these two complexes using the MP2 method, and these have been used to calculate the vibrational spectra and to identify the key couplings responsible for the “extra” lines in the observed spectra. In addition, a simple adiabatic model involving the OH stretch and intermolecular rock vibrations is introduced and shown to account in an near quantitative manner for the structure in the OH stretch region of the spectra.

Nonadiabatic vibrational dynamics in the HCO2−⋅H2O complex

“…18,[23][24][25][26] In addition in related work, Neumark and coworkers have studied the complex with NO − 3 . 27 Despite the fact that these complexes show all the nontrivial features of the IR spectroscopy of hydrogen bonds, their spectral signatures are particularly simple.…”

“…That is, the OH stretch vibration of the water molecule splits into a single pronounced "Franck-Condon"-like progression due to the strong anharmonic coupling to the water rocking mode. 18,22,23 Furthermore, this Franck-Condon progression sits on a broad background that might indicate an ultrafast relaxation channel. While the Franck-Condon progression has been reproduced using cubic ab initio derived force constants, 18,22 the broad background and its underlying dynamics still awaits an explanation.…”

“…18,22,23 Furthermore, this Franck-Condon progression sits on a broad background that might indicate an ultrafast relaxation channel. While the Franck-Condon progression has been reproduced using cubic ab initio derived force constants, 18,22 the broad background and its underlying dynamics still awaits an explanation. Owing to the smallness of these complexes, one can treat both their quantum chemistry as well as their quantum dynamics on a reasonably high level that should lead to a qualitatively correct picture of the underlying mechanism.…”

“…[30][31][32] The very concept of a conical intersection requires an adiabatic separation of time scales of two sets of degrees of freedom, which is naturally given in the electronic case via the Born-Oppenheimer approximation, but which is less common for vibrational transitions. 18,20,22,[33][34][35][36] In the context of vibrational conical intersections, an adiabatic separation has been introduced between high-frequency OH stretch and bend vibrations on the one hand, and lower-frequency backbone vibrations 28 or torsional modes 37,38 on the other hand within one molecule. Alternatively, in the case of the hydrogenbonded water dimer, the high-frequency intramolecular modes have been separated from the lower-frequency intermolecular modes.…”

“…[6][7][8][9][10][11][12] It is the very anharmonic nature of the hydrogen bond potentials that gives rise to the peculiar vibrational spectroscopy, [16][17][18][19][20][21][22] i.e., in essence the fact that the typical dissociation energy of a hydrogen bond is in the same range as the frequency of its vibrational modes. It has also been shown that the problem is inherently high-dimensional, [17][18][19][20][21][22] which renders the modeling of the quantum dynamics demanding and the interpretation of the computational results complicated.…”