title: Controlling electronic product branching at conical intersections in the UV photolysis of para-substituted thiophenols.
abstract: H (Rydberg) atom photofragment translation spectroscopy and high-level ab initio electronic structure calculations are used to explore the photodissociation dynamics of three para-substituted thiophenols (p-YPhSH; Y = CH(3), F, and MeO). UV excitation in the wavelength range 305 > λ(phot) > 240 nm results in S-H bond fission and formation of p-YPhS radicals in their ground (X̃(2)B(1)) and first excited (Ã(2)B(2)) electronic states; the X̃/Ã state product branching ratio, Γ, varies with para-Y substituent and excitation wavelength. Excitation at λ(phot) < 265 nm results in direct population of the dissociative 1(1)πσ* potential energy surface (PES). Γ falls across the series p-CH(3)PhSH > p-FPhSH > p-MeOPhSH. Branching is ultimately determined at the conical intersection (CI) formed by the 1(1)πσ* and ground (S(0)) PESs at extended R(S-H) bond length but is sensitively dependent on the orientation of the S-H bond (relative to the ring plane) in the S(0) molecules prior to photoexcitation. Excitation at λ(phot) > 265 nm populates quasi-bound levels of the respective 1(1)ππ* states, which predissociate rapidly by tunneling under the lower diabats of the 1(1)ππ*/1(1)πσ* CI at short R(S-H). Less extreme X̃/Ã product branching ratios are measured, implicating intramolecular vibrational redistribution within the photoexcited 1(1)ππ* molecules prior to their sampling the region of the 1(1)πσ*/S(0) CI.
authors.affiliation: School of Chemistry, University of Bristol, Bristol, United Kingdom.
authors.forename: Thomas A A
authors.initials: TA
authors.lastname: Oliver
authors.forename: Graeme A
authors.initials: GA
authors.lastname: King
authors.forename: David P
authors.initials: DP
authors.lastname: Tew
authors.forename: Richard N
authors.initials: RN
authors.lastname: Dixon
authors.forename: Michael N R
authors.initials: MN
authors.lastname: Ashfold
mesh_terms: D004583:Electrons; D007536:Isomerism; D008958:Models, Molecular; D008968:Molecular Conformation; D010636:Phenols / Q000737:chemistry*; D010782:Photolysis; D011789:Quantum Theory; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D010636:Phenols; D013438:Sulfhydryl Compounds; C042983:thiophenol