Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: A Trajectory Surface-Hopping Study

The photodissociation of methyl hydroperoxide (CH₃OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH₃O(²E) + OH(²Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S₀) and first excited singlet (S₁) states become degenerate. The O-H bond dissociation of CH₃OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH₃OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S₁/S₀ leading to ground-state products CH₃OO(X ²A″) + H(²S) in addition to adiabatic dissociation in the first excited singlet state, S₁, correlating to products CH₃OO(1 ²A') + H(²S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH₃OOH at 193 nm.