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

Abstract

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 (1)πσ* state. The partner thiophenethiyl (R) radicals are formed predominantly in their first excited Ã(2)A' state, but assignment of a weak signal attributable to H + R(X˜(2)A'') products allows determination of the S-H bond strength, D0 = 27,800 ± 100 cm(-1) and the Ã-X˜ state splitting in the thiophenethiyl radical (ΔE = 3580 ± 100 cm(-1)). The deduced population inversion between the à and X˜ 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 (1)πσ* 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 (1)πσ* 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˜) 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.

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

“…135 In similar vein, long wavelength (278    225 nm) photolysis of 2-thiophenethiol results in cleavage of the pendant S-H bond and formation of H + thiophenethiyl products. 137 The deduced dissociation dynamics show many parallels with those for thiophenol and thioanisole (section 2). Most of the radical products are formed in their excited ( Ã ) state, consistent with a non-planar parent geometry on approach to CI-2.…”

“…The TKER distribution changes upon tuning to yet shorter wavelengths, however, in a way that can be explained by again invoking the onset of a rival C-S bond extension pathway that enables non-adiabatic coupling via a conical intersection with the S 0 PES and subsequent unimolecular decay to, in this case, mainly H + thiophenethiyl( X ) products. 137 One key message emerging from the small but growing literature pertaining to photoinduced dynamics in heterocyclic systems is the importance of ring-based * excited states in facilitating non-adiabatic coupling to the ground state: it is clear that incipient ring-expanding distortions can act as a driver for radiationless transfer, even in situations that do not result in the formation of fully ring-opened products.…”

The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects

“…The H Rydberg atom (HRA) PTS setup has been detailed previously, 26,51 so only a brief outline of the apparatus and methodology specific to the present study is given here. The samples (3-MePhSH, 3-FPhSH, 2-MePhSH, and 2-FPhSH, all supplied by Sigma Aldrich) are liquids at room temperature and were used without further purification.…”

Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol

“…Such signals have been observed in many previous experiments involving a range of polyatomic hydrides and are generally attributed to processes where the electronically excited parent internally converts to vibrationally hot S0 levels and then loses an H atom, or to processes involving multiple photons. 7,11,21,34,40,47,48,[62][63][64] Of greater current interest, the spectra also show a number of well-resolved peaks at higher TKER values, the positions and relative intensities of which vary with the photolysis wavelength. For completeness, we note that the TKER spectra recorded at the longest excitation wavelengths (i.e.…”