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Photodissociation Dynamics of Ortho-Substituted Thiophenols at 243 nm

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journal contribution
posted on 2019-03-11, 00:00 authored by Jean Sun Lim, Hyun Sik You, Songhee Han, Sang Kyu Kim
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 1πσ* state or to the 21ππ* state followed by efficient coupling to the 1πσ* state. The calculated potential energy curves for the 1πσ* or 21ππ* 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 1πσ* 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.