Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: Intramolecular orbital alignment of the phenylthiyl radical.

The photoinduced hydrogen (or deuterium) detachment reaction of thiophenol (C₆H₅SH) or thiophenol-d₁ (C₆H₅SD) pumped at 243 nm has been investigated using the H (D) ion velocity map imaging technique. Photodissociation products, corresponding to the two distinct and anisotropic rings observed in the H (or D) ion images, are identified as the two lowest electronic states of phenylthiyl radical (C₆H₅S·). Ab initio calculations show that the singly occupied molecular orbital of the phenylthiyl radical is localized on the sulfur atom and it is oriented either perpendicular or parallel to the molecular plane for the ground (B₁) and the first excited state (B₂) species, respectively. The experimental energy separation between these two states is 2600±200 cm^-1 in excellent agreement with the authors’ theoretical prediction of 2674 cm^-1 at the CASPT2 level. The experimental anisotropy parameter (β) of -1.0±0.05 at the large translational energy of D from the C₆H₅SD dissociation indicates that the transition dipole moment associated with this optical transition at 243 nm is perpendicular to the dissociating S–D bond, which in turn suggests an ultrafast D+C₆H₅S·(B₁) dissociation channel on a repulsive potential energy surface. The reduced anisotropy parameter of -0.76±0.04 observed at the smaller translational energy of D suggests that the D+C₆H₅S·(B₂) channel may proceed on adiabatic reaction paths resulting from the coupling of the initially excited state to other low-lying electronic states encountered along the reaction coordinate. Detailed high level ab initio calculations adopting multireference wave functions reveal that the C₆H₅S·(B₁) channel may be directly accessed via a ¹(n_π,σ^*) photoexcitation at 243 nm while the key feature of the photodissociation dynamics of the C₆H₅S·(B₂) channel is the involvement of the ³(n_π,π^*)→³(n_σ,σ^*) profile as well as the spin-orbit induced avoided crossing between the ground and the ³(n_π,σ^*) state. The S–D bond dissociation energy of thiophenol-d₁ is accurately estimated to be D₀=79.6±0.3 kcal/mol. The S–H bond dissociation energy is also estimated to give D₀=76.8±0.3 kcal/mol, which is smaller than previously reported ones by at least 2 kcal/mol. The C–H bond of the benzene moiety is found to give rise to the H fragment. Ring opening reactions induced by the π-π^*/n_π-π^* transitions followed by internal conversion may be responsible for the isotropic broad translational energy distribution of fragments. [ABSTRACT FROM AUTHOR]