Abstract
We report a combination of experimental (velocity map imaging measurements of the methyl (Me) radical products) and ab initio electronic structure studies that explore the influence of substituents (Y) on the dynamics of S-Me bond fission following excitation to the first excited S1 states of thioanisole and three 4-substituted thioanisoles (4-YPhSMe, with Y = H, Me, MeO and CN). In all bar the case that Y = CN, the resulting 4-YPhS products are found to be formed predominantly in their excited (Ã) electronic state. In all cases, the relative yield of X̃ state products increases upon tuning to shorter excitation wavelengths and, in the specific case of bare thioanisole (as found previously by Lim and Kim, Nat. Chem., 2010, 2, 627), jumps when exciting on the parent resonance assigned to the S1(v7a = 1) level. Two conical intersections (CIs) in the RS-Me stretch coordinate are crucial to rationalising all of the observed dynamics. The first, (CI-1, between the diabatic (1)ππ* and dissociative (1)πσ* potential energy surfaces (PESs) at RS-Me∼ 2 Å) lies above the S1(v = 0) level in energy, and the calculated minimum energy path through this barrier involves substantial deviations from planarity in all bar 4-CNPhSMe. Beyond this barrier, the potential is quite steeply repulsive, and Me + 4-YPhS(Ã) products are the inevitable products if the molecular framework is unable to re-planarise within the time it takes for the dissociating molecules to pass through the region of CI-2 (between the diabatic (1)πσ* and ground (S0) states) where the product electronic branching is determined. The gradual increase in the yield of 4-YPhS(X̃) radicals upon tuning to shorter photolysis wavelengths, the much increased branching into PhS(X̃) products when exciting the PhSMe (S1, v7a = 1) level and the dominance of 4-CNPhS(X̃) products in the specific case that Y = CN can all be understood in terms of a (relative) lowering of the effective barrier associated with CI-1, thereby allowing access to the dissociative region of the PES at closer-to-planar geometries.
Highlights
An ability to ‘tailor’ photochemistry could impact on many fields of chemistry
For bare PhSMe only, 1-D potential energy cuts (PECs) along RS–Me for the S0 and first four singlet excited electronic states were calculated using complete active space self-consistent field (CASSCF) and with second order perturbation theory (CASPT2) and the above AVTZ basis set, with the phenyl ring frozen at its optimised ground state geometry
The relative yield of Xstate products in all cases increases upon tuning to shorter excitation wavelengths and, in the specific case of bare PhSMe, jumps dramatically when tuning to the parent resonance assigned to the S1(v7a = 1) level.[14]
Summary
An ability to ‘tailor’ photochemistry could impact on many fields of chemistry. Such ambitions have underpinned much of the recent interest in coherent control, i.e. the use of appropriately designed laser pulses to drive a photochemical process to some particular target outcome.[1,2,3] Strategic substitution remote from the reaction centre offers another route to tuning the outcome of a chemical process, and much of the recent effort exploring and understanding the fragmentation dynamics of heteroaromatic and heteroatom containing aromatic molecules (azoles, phenols, thiophenols, etc.) following ultraviolet (UV) photoexcitation has focussed on such issues.[4,5,6,7,8,9,10,11] Bond fission in these types of molecule often relies on the interaction between a bound 1pp*. Paper reported use of ion imaging methods to study the photodissociation of thioanisole[14] (a derivative of thiophenol, in which the thiyl ( termed sulfenyl) hydrogen is replaced by a methyl group, denoted as PhSMe) and thioanisole-d315 following excitation to the 11pp* state.[16] The PhS fragments from S–Me bond fission are formed predominantly in their electronically excited A2B2(2A0) state but, in both cases, these workers identified one or more parent vibronic resonances that yield strikingly different electronic branching between the Aand ground X2B1(2A00) states of the PhS product These states of the radical are distinguished by having the odd electron in, respectively, the in-plane and out-of-plane singly occupied molecular orbitals which are largely localised on the sulphur atom. Such variation in the electronic branching in the products of a photochemical reaction (S–Me bond fission) with change of substituent, and/or by change of excitation wavelength, is considered in light of the detailed topographies of the relevant PESs and the way these influence the nuclear motions that lead to the eventual dissociation
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