Exploring nuclear motion through conical intersections in the UV photodissociation of phenols and thiophenol

• Published in: Proceedings of the National Academy of Sciences - PNAS 2008-09, Vol.105 (35), p.12701-12706
• Main Authors: Ashfold, Michael N.R, Devine, Adam L, Dixon, Richard N, King, Graeme A, Nix, Michael G. .D, Oliver, Thomas A.A
• Format: Article
• Language: English
• Subjects: Atoms & subatomic particles; Chemical compounds; Chemical reactions; Chemicals; Physical Sciences; Reaction kinetics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0800463105

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths (λphot), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by XFormula H (X = O, S) bond fission after excitation at their respective S₁(¹ππ*)-S₀ origins and at all shorter wavelengths. The achieved kinetic energy resolution reveals population of selected vibrational levels of the various phenoxyl and thiophenoxyl coproducts, providing uniquely detailed insights into the fragmentation dynamics. Dissociation in all cases is deduced to involve nuclear motion on the ¹πσ* potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to λphot and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long λphot is rationalized in terms of radiationless transfer from S₁ to S₀ levels carrying sufficient OFormula H stretch vibrational energy to allow coupling via the conical intersection between the S₀ and ¹πσ* PESs at longer OFormula H bond lengths. In contrast, H + C₆H₅O(X²B₁) products formed after excitation at short λphot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared ¹ππ* excited state and the ¹πσ* PES. The fragmentation dynamics of thiophenol at all λphot matches the latter behavior more closely, reflecting the different relative dispositions of the ¹ππ* and ¹πσ* PESs. Additional insights are provided by the observed branching into the ground (X²B₁) and first excited (²B₂) states of the resulting C₆H₅S radicals.