Tunnelling under a conical intersection: Application to the product vibrational state distributions in the UV photodissociation of phenols

• DOI: 10.1063/1.3585609
• Publication Date: 2011-05-20T23:39:36Z
• Authors: Richard N. Dixon; Thomas A. A. Oliver; Michael N. R. Ash...
• Keywords: Conical intersection; Vibronic coupling; Potential energy surface

When phenol is photoexcited to its S(1) (1(1)ππ∗) state at wavelengths in the range 257.403 ≤ λ(phot) 275.133 nm O-H bond dissociates yield an H atom and a phenoxyl co-product, with available energy shared between translation well characterised product vibration. It accepted that dissociation enabled by transfer S(2) (1(1)πσ∗) state, for which potential surface (PES) repulsive stretch coordinate, R(O-H). This PES cut near R(O-H) = 1.2 Å S(0) ground 2.1 Å, give two conical intersections (CIs). These have each been invoked-both theoretical studies interpretation of experimental vibrational activity-but considerable controversy. paper revisits dynamic mechanisms underlie photodissociation substituted phenols light symmetry restrictions arising from torsional tunnelling degeneracy, has neglected hitherto. places tighter constraints on dynamics around CIs. The non-rigid molecular group G(4) necessitates vibronic interactions a(2) modes enable coupling inner, higher (S(1)/S(2)) CI, or b(1) outer, lower (S(2)/S(0)) CI. data following excitation through many levels demonstrate effective role ν(16a) (a(2)) ring mode enabling fission. requires under S(1)/S(2) hindering barrier ∼5000 cm(-1) associated geometric phase effect. Quantum calculations using new ab initio PESs provide quantitative justification this conclusion. fates other excited are also rationalised, revealing both spectator intramolecular redistribution modes. A common feature cases observation extended, odd-number only, progression (i.e., parent enables tunnelling), we explain as Franck-Condon consequence major change active vibration frequency. comprehensive results serve confirm hypothesis fission involves CI-in accord conclusions reached recent correlation lifetimes (and phenols) corresponding vertical gaps their PESs.