Quantum Wave Packet Propagation Study of the Photochemistry of Phenol: Isotope Effects (Ph-OD) and the Direct Excitation to the ¹πσ* State

Abstract

An earlier time-dependent quantum wave packet propagation study of the photochemistry of Ph-OH [J. Chem. Phys. 2005, 122, 224315] is extended to investigate isotope effects (for Ph-OD) and the dynamics initiated by direct (vibronically induced) excitation to the (1)πσ* state. The isotope effect is significant only when the initially excited state is (1)ππ*, that is, there are noticeable changes not only in the time scale but also in the branching ratio (Ã/X̃) for the electronic states of the product Ph-O radical. In contrast, the isotope effect on the dynamics initiated by direct excitation to the (1)πσ* state is very small. Our most important observation for the dynamics initiated by direct excitation to the (1)πσ* state is that the initial excitation of the O-H stretch mode does not result in a noticeable enhancement of the product Ph-O radical in the à state, which corresponds to a dissociating H atom with low kinetic energy. The initial excitation of the CCOH torsion mode is the main reason for the enhancement of the product Ph-O radical in the à state that was observed in a vibrationally mediated two-photon experiment [J. Chem. Phys.2008, 128, 104307].

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

“…Despite the lack of quantitative reliability, qualitative reliability is sometime enough to shed insight on dynamical details. For example, qualitative results from theoretical study [10,18] on the dependence of the branching ratio between a phenoxyl radical (Ph-OÁ) on the initial vibration level of phenol in the ground state and on the identity of the initially excited electronic state, provided a clear explanation for the experimental results obtained by the IR1UV two photon methods. [11] When the size of the target molecule is small, the spectrum of a photoabsorption process contains rich fine structures corresponding to vibrational progressions, as shown in the X-NO systems (X-NO stands for the molecules like Cl-NO, HO-NO, and CH 3 O-NO, etc.…”

“…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).…”

“…[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.…”

“…All possible initially excited electronic states then have to be considered in the following theoretical dynamics, as shown in a recent study of the photochemistry of phenol. [10] In this study, not only the first excited bright state pp* but also the nearby second excited dark state pr* were considered as the initial excited state, and theoretical values were compared with experimental results by using two photon (IR 1 UV) methods. [11] Values of the adiabatic excitation (aT e ) energies are also very helpful if available.…”

“…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.…”