Time-dependent quantum wave-packet description of the π1σ* photochemistry of phenol

Abstract

The photoinduced hydrogen elimination reaction in phenol via the conical intersections of the dissociative 1pi sigma* state with the 1pi pi* state and the electronic ground state has been investigated by time-dependent quantum wave-packet calculations. A model including three intersecting electronic potential-energy surfaces (S0, 1pi sigma*, and 1pi pi*) and two nuclear degrees of freedom (OH stretching and OH torsion) has been constructed on the basis of accurate ab initio multireference electronic-structure data. The electronic population transfer processes at the conical intersections, the branching ratio between the two dissociation channels, and their dependence on the initial vibrational levels have been investigated by photoexciting phenol from different vibrational levels of its ground electronic state. The nonadiabatic transitions between the excited states and the ground state occur on a time scale of a few tens of femtoseconds if the 1pi pi*-1pi sigma* conical intersection is directly accessible, which requires the excitation of at least one quantum of the OH stretching mode in the 1pi pi* state. It is shown that the node structure, which is imposed on the nuclear wave packet by the initial preparation as well as by the transition through the first conical intersection (1pi pi*-1pi sigma*), has a profound effect on the nonadiabatic dynamics at the second conical intersection (1pi sigma*-S0). These findings suggest that laser control of the photodissociation of phenol via IR mode-specific excitation of vibrational levels in the electronic ground state should be possible.

Basics of the time‐dependent wave‐packet propagation for photodissociations of polyatomic systems

“…[52] Dependence of the dissociation time constant on isotope substitution or the initial vibrational level is often anticipated and estimated on the basis of kinetic theory, but such expectations may be incorrect if several features with significant nonadiabatic effects occur in the dynamic process. A typical example is the effect on hydrogen generating dynamics of phenol [10,18] and thiophenol [19,44] systems. The wave-packet propagation method is very useful here because isotope substitution in experiments can be hindered due to technical difficulties.…”

“…Detail discussion for the molecules are in Ref. 1), [10,18,19,[42][43][44]52,53] is an essential ingredient for easier and systematic iteration of the steps from (2) to (5). The changes here may include adjustments of critical points, or addition of other critical points, change or addition of coordinates and their corresponding kinetic operator, adjustment of the relationships among PESs including the NACT and consequent diabatic Hamiltonians, change in the initial state of the nuclear wave-packet just after photoabsorption (the identity of the initially excited electronic state and the relative position with respect to the FC region).…”

“…See Refs. [10,18,48,62] for similar figures of the PESs, the snapshot, the population dynamics (and branching ratio), and the autocorrelation function (and spectrum), respectively. necessary to search for the transition state (TS) structure connecting the ground electronic state of the parent molecule and its isomer structure.…”

“…1) is the reaction coordinate. [18] However, the reaction coordinate becomes more complicated as the distance between the photon absorbing moiety and the dissociating bond increases, and usually becomes curvilinear as the combination of several distances and angles.…”

“…1). [18,19] After the selection of appropriate coordinates, the KEO for the coordinate system has to be determined. When the selected coordinates correspond to a kind of Jacobian coordinates, then there are several KEOs that can be applied, [20][21][22][23][24] and the reduced masses of the KEOs can be derived by applying systematic procedures.…”