Nonadiabatic vibrational dynamics in the HCO2−⋅H2O complex

Abstract

Based on extensive ab initio calculations and the time-propagation of the nuclear Schrödinger equation, we study the vibrational relaxation dynamics and resulting spectral signatures of the OH stretch vibration of a hydrogen-bonded complex, HCO − 2 H 2 O. Despite their smallness, it has been shown experimentally by Johnson and coworkers that the gas-phase infrared spectra of these types of complexes exhibit much of the complexity commonly observed for hydrogen-bonded systems. That is, the OH stretch band exhibits a significant red shift together with an extreme broadening and a pronounced substructure, which reflects its very strong anharmonicity. Employing an adiabatic separation of time scales between the three intramolecular high-frequency modes of the water molecule and the three most important intermolecular low-frequency modes of the complex, we calculate potential energy surfaces (PESs) of the ground and the first excited states of the high-frequency modes and identify a vibrational conical intersection between the PESs of the OH stretch fundamental and the HOH bend overtone. By performing a time-dependent propagation of the resulting system, we show that the conical intersection affects a coherent population transfer between the two states, the first step of which being ultrafast (60 fs) and irreversible. The subsequent relaxation of vibrational energy into the HOH bend and ground state occurs incoherently but also quite fast (1 ps), although the corresponding PESs are well separated in energy. Owing to the smaller effective mass difference between light and heavy degrees of freedom, the adiabatic ansatz is consequently less significant for vibrations than in the electronic case. Based on the model, we consider several approximations to calculate the measured Ar-tag action spectrum of HCO − 2 H 2 O and achieve semiquantitative agreement with the experiment. Based on extensive ab initio calculations and the time-propagation of the nuclear Schrödinger equation, we study the vibrational relaxation dynamics and resulting spectral signatures of the OH stretch vibration of a hydrogen-bonded complex, HCO − 2 · H 2 O. Despite their smallness, it has been shown experimentally by Johnson and coworkers that the gas-phase infrared spectra of these types of complexes exhibit much of the complexity commonly observed for hydrogen-bonded systems. That is, the OH stretch band exhibits a significant red shift together with an extreme broadening and a pronounced substructure, which reflects its very strong anharmonicity. Employing an adiabatic separation of time scales between the three intramolecular high-frequency modes of the water molecule and the three most important intermolecular low-frequency modes of the complex, we calculate potential energy surfaces (PESs) of the ground and the first excited states of the high-frequency modes and identify a vibrational conical intersection between the PESs of the OH stretch fundamental and the HOH bend overtone. By performing a time-dependent propagation of the resulti...

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

“…The dramatic simplification of the spectra in the OD stretching region of the deuterated isotopologues indicates that strong nuclear quantum effects are at play in this system, which are not evident when CH 3 NH 3 + is placed in the crown pocket. We consider these effects in the context of similar behavior reported previously on the formate monohydrate binary complex along with new results for the partially and fully deuterated complex (17,18). In that case, very strong anharmonic coupling between the OH stretch and a soft intermolecular mode is treated in the context of vibrationally adiabatic (VA) potential energy surfaces.…”

“…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.…”

“…Because the formate monohydrate spectra display such a clear manifestation of isotope effects in anharmonic behavior, it is valuable to review the key features of the theoretical models that accurately predicted its behavior, and then apply these methods to the more complex H 3 O + (18C6) system. There is consensus (18,25) that this progression is due to combination bands arising from excitation of the OH stretch fundamental along with several quanta of the low-frequency (∼70 cm ) water rocking mode (angle θ as defined in Fig. 6), which acts to break the strong H bond of one of the OH groups while strengthening the other.…”

Nonadiabatic effects in electronic and nuclear dynamics

“…3, where we studied vibrational conical intersections and the quality of the adiabatic picture for the hydrogen-bonded ${\text{HCO}}_2^{-}·{\mathrm{normalH}}_2\mathrm{O}$ complex. That molecular system has been studied extensively in the gas-phase by Johnson and coworkers, revealing very peculiar IR spectra of the OH-stretch vibration of water 178,183,197 .…”

“…It has recently been shown 3 that a similar approach to the one that is used in the treatment of electronic-nuclear nonadiabatic processes can also be employed to identify vibrational conical intersections between potential energy surfaces along specific vibrational modes.…”