Vibrational Spectroscopy of the Water–Nitrate Complex in the O–H Stretching Region

Abstract

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3(-)·H2O, NO3(-)·HDO, and NO3(-)·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm(-1) (2300-2700 cm(-1)) for NO3(-)·H2O (NO3(-)·D2O) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both NO3(-)·H2O and NO3(-)·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.

Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

“…6), which acts to break the strong H bond of one of the OH groups while strengthening the other. When this occurs, the frequencies of the two OH stretches, which are nearly degenerate at θ = 0°, dramatically split apart with increasing θ, with one of them evolving toward the OH stretch position in isolated water while the other red-shifts by hundreds of wavenumbers (18,24,26 (18), in the context of a VA model involving a separation between the OH stretching degree of freedom and the displacements associated with the rocking mode. This treatment invokes the VA potential energy curves displayed in Fig.…”

Nonadiabatic vibrational dynamics in the HCO2−⋅H2O complex

“…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.…”

“…[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.…”

“…[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.…”