Extreme population inversion in the fragments formed by UV photoinduced S-H bond fission in 2-thiophenethiol

H atom loss following near ultraviolet photoexcitation of gas phase 2-thiophenethiol molecules has been studied experimentally, by photofragment translational spectroscopy (PTS) methods, and computationally, by ab initio electronic structure calculations. The long wavelength (277.5 ≥ λ_phot ≥ 240 nm) PTS data are consistent with S–H bond fission after population of the first ¹πσ* state. The partner thiophenethiyl (R) radicals are formed predominantly in their first excited òA′ state, but assignment of a weak signal attributable to H + R([X with combining tilde]²A′′) products allows determination of the S–H bond strength, D₀ = 27 800 ± 100 cm⁻¹ and the Ö[X with combining tilde] state splitting in the thiophenethiyl radical (ΔE = 3580 ± 100 cm⁻¹). The deduced population inversion between the à and [X with combining tilde] states of the radical reflects the non-planar ground state geometry (wherein the S–H bond is directed near orthogonal to the ring plane) which, post-photoexcitation, is unable to planarise sufficiently prior to bond fission. This dictates that the dissociating molecules follow the adiabatic fragmentation pathway to electronically excited radical products. π* ← π absorption dominates at shorter excitation wavelengths. Coupling to the same ¹πσ* potential energy surface (PES) remains the dominant dissociation route, but a minor yield of H atoms attributable to a rival fragmentation pathway is identified. These products are deduced to arise via unimolecular decay following internal conversion to the ground (S₀) state PES via a conical intersection accessed by intra-ring C–S bond extension. The measured translational energy disposal shows a more striking change once λ_phot ≤ 220 nm. Once again, however, the dominant decay pathway is deduced to be S–H bond fission following coupling to the ¹πσ* PES but, in this case, many of the evolving molecules are deduced to have sufficiently near-planar geometries to allow passage through the conical intersection at extended S–H bond lengths and dissociation to ground ([X with combining tilde]) state radical products. The present data provide no definitive evidence that complete ring opening can compete with fast S–H bond fission following near UV photoexcitation of 2-thiophenethiol.