High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of phenol

Abstract

The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145 λphot 206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O-H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at λphot 248 nm show a feature centered at TKER ∼6500 cm-1. These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1 S0 (π* π) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2 (π 1*) potential energy surfaces (PESs) at larger RO-H, en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes 16a and 11 can both promote nonadiabatic coupling in the vicinity of the S0 S2 CI. Spectra recorded at λphot ≤248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER ∼12 000 cm-1. These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1 (π 1 π*) and repulsive S2 (π 1*) PESs. Parent mode 16b is identified as the dominant coupling mode at the S1 S2 CI, and the resulting phenoxyl radical cofragments display a long progression in 18b, the C-O in-plane wagging mode. Analysis of all structured TKER spectra yields D0 (H-O C6 H5) =30 015±40 cm-1. The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of π 1 * states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the π 1 * state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in "indirect" predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs. © 2006 American Institute of Physics.