Molecular dynamics with quantum transitions for proton transfer: Quantum treatment of hydrogen and donor–acceptor motions

Semiquantal time-dependent Hartree approach to condensed phase chemical dynamics: application to the system-bath model

A semiquantal analysis of condensed phase chemical dynamics, outlined recently for a double-well linearly coupled to dissipative harmonic bath, is formulated in detail to clarify its general features as well as the specifics of the linear and quadratic coupling cases. The theory may be called a "semiquantal time-dependent Hartree (SQTDH)" approach, as it assumes a factorized product of the squeezed coherent state wave packets for the variational subspace of the many-dimensional time-dependent wave function. Due to this assumption, it straightforwardly satisfies the canonicity condition introduced by Marumori et al. is described by a set of Hamilton equations of motion in an extended phase space that includes auxiliary coordinates representing the wave packet widths. The potential in the extended phase space provides a pictorial understanding of the quantum effects affected due to the bath coupling, e.g., suppression of the wave packet spreading in terms of the potential wall developing along the auxiliary coordinates. The idea is illustrated by prototypical models of quartic double-well and cubic metastable potentials linearly and quadratically coupled to the bath. Further applications and extensions, where the SQTDH method will offer a practical approach for introducing quantum effects into realistic molecular dynamics simulations, are also discussed.

Test of the quantum instanton approximation for thermal rate constants for some collinear reactions

Two variants of the recently developed quantum instanton (QI) model for calculating thermal rate constants of chemical reactions are applied to several collinear atom-diatom reactions with various skew angles. The results show that the original QI version of the model is consistently more accurate than the "simplest" quantum instanton version (both being applied here with one "dividing surface") and thus to be preferred. Also, for these examples (as with other earlier applications) the QI results agree well with the correct quantum rates (to within approximately 20% or better) for all temperatures >200 K, except for situations where dynamical corrections to transition state theory (i.e., "re-crossing" dynamics) are evident. (Since re-crossing effects are substantially reduced in higher dimensionality, this is not a cause for serious concern.) A procedure is also described which facilitates use of the METROPOLIS algorithm for evaluating all quantities that appear in the QI rate expression by Monte Carlo path integral methods.

H atom transfer along an ammonia chain: Tunneling and mode selectivity in 7-hydroxyquinoline⋅(NH[sub 3])[sub 3]

Excitation of the 7-hydroxyquinoline(NH(3))(3) [7HQ(NH(3))(3)] cluster to the S(1) (1)pi pi(*) state results in an O-H-->NH(3) hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S(1)<-->S(0) spectra of the 7HQ.(NH(3))(3) and the d(2)-7DQ.(ND(3))(3) clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S(1)<--S(0) spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200-250 cm(-1) for 7HQ.(NH(3))(3) accelerates the HAT reaction by an order of magnitude or more. The d(2)-7DQ.(ND(3))(3) cluster exhibits a more gradual threshold from 300 to 650 cm(-1). For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H-->NH(3) coordinate. The barrier results from a conical intersection of the optically excited (1)pi pi(*) state with an optically dark (1)pi sigma(*) state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH(3) distance and the energetic separation between the (1)pi pi(*) and (1)pi sigma(*) states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the (1)pi pi(*)<-->(1)pi sigma(*) interaction to a much smaller extent.