Open AccessExploring nuclear motion through conical intersections in the UV photodissociation of phenols and thiophenol
Author(s) -
Michael N. R. Ashfold,
Adam L. Devine,
Richard N. Dixon,
Graeme A. King,
Michael G. D. Nix,
Thomas A. A. Oliver
Publication year - 2008
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.0800463105
Subject(s) - photodissociation , conical intersection , thiophenol , chemistry , excited state , dissociation (chemistry) , photochemistry , fragmentation (computing) , population , heteroatom , vibronic coupling , atomic physics , potential energy , excitation , photoexcitation , chemical physics , molecular physics , physics , ring (chemistry) , demography , organic chemistry , sociology , medicinal chemistry , computer science , operating system , quantum mechanics
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 XH (X = O, S) bond fission after excitation at their respectiveS 1 (1 ππ*)–S 0 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 the1 πσ* 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 fromS 1 toS 0 levels carrying sufficient OH stretch vibrational energy to allow coupling via the conical intersection between theS 0 and1 πσ* PESs at longer OH bond lengths. In contrast, H + C6 H5 O(X2 B 1 ) products formed after excitation at short λphot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared1 ππ* excited state and the1 πσ* PES. The fragmentation dynamics of thiophenol at all λphot matches the latter behavior more closely, reflecting the different relative dispositions of the1 ππ* and1 πσ* PESs. Additional insights are provided by the observed branching into the ground (X2 B 1 ) and first excited (2 B 2 ) states of the resulting C6 H5 S radicals.