The O-H bond fission is the main radiationless deactivation pathway of phenol under UV light [Sobolewski et al., Phys. Chem. Chem. Phys. 2002, 4, 1093-1100]. The photodissociation dynamics involves two conical intersections or avoided crossings of the potential energy surfaces — one involving the bright bound 1ππ* state (which is S1 in the Franck-Condon region) and the dark repulsive 1πσ* state (S2) and the other involving S2 and the ground 1ππ state (S0). We performed multistate trajectory simulations of the dynamics, including coherence and decoherence using the coherent switching with decay-of-mixing algorithm, and including multidimensional, multistate tunneling by the army ant tunneling algorithm. We used full-dimensional anchor-points reactive potential surfaces and state couplings fit [Yang et al. Chem. Sci. 2014, in press, DOI: 10.1039/C4SC01967A] to electronic structure calculations including dynamical correlation by MC-QDPT/jul-cc-pVDZ. The calculations successfully reproduce the experimentally observed bimodal character of the total kinetic energy release spectra [Dixon et al. J. Chem. Phys. 2011, 134, No. 194303] and confirm the interpretation of the most recent experiments that the photodissociation process is dominated by tunneling. Analysis of the trajectories uncovered an unexpected dissociation pathway for one quantum excitation of the O-H stretching mode of the S1 state, namely tunneling in a coherent mixture of states starting in a smaller ROH (~ 0.9-1.0 Å) region than has previously been invoked. The simulations also show that most trajectories do not pass close to the S1–S2 conical intersection (they have an average minimum-energy gap greater than 0.6 eV), they provide statistics on the out-of-plane angles at the locations of the minimum energy adiabatic gap, and they reveal information about which vibrational modes are most highly activated in the products. This work was supported in part by the U. S. Department of Energy, Office of Basic Energy Sciences, under grant DE-SC0008666.

Photodissociation dynamics of phenol

249th National Meeting (2015)

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