Photodissociation Dynamics of Ortho-Substituted Thiophenols at 243 nm.

The photoinduced S-H (D) bond fission dynamics of four ortho-substituted thiophenols, 2-fluoro, 2-chloro, 2-bromo, and 2-methoxythiophenol at a pump wavelength of 243 nm, have been investigated by velocity-map imaging and high-level electronic structure calculations. The D atom images of the deuterated ortho-substituted thiophenols show much reduced X̃/Ã branching ratios of the cofragment radicals over that of bare thiophenol. The angular distributions of the D fragment display negative anisotropies, indicating that transition dipole moments are perpendicular to the fast dissociating S-D bond axis. Initial excitation at 243 nm occurs directly to the πσ* state or to the 2ππ* state followed by efficient coupling to the πσ* state. The calculated potential energy curves for the πσ* or 2ππ* excited states of the ortho-substituted thiophenols along the CCS-D torsion angle (ϕ) display minima at the nonplanar structures, whereas all of the states for bare thiophenol present minima at the planar geometries. This different topology of the ortho-substituted thiophenols in the excited states induces the wide spread of the reactive flux along the ϕ coordinate on the repulsive surface as it should experience significant torque with respect to ϕ during the fragmentation. This encourages the dissociating molecules to follow the adiabatic path at the conical intersection between the ground and the πσ* states at extended S-D bond lengths, giving rise to decreased X̃/Ã branching ratios, demonstrating that the excited-state molecular structure dictates the nonadiabatic transition probability.