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

The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145 >=lambda(phot)>= 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 lambda(phot)>= 248 nm show a feature centered at TKER similar to 6500 cm(-1). These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S-1 <- S-0 (pi(*)<-pi) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S-0) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S-0 and S-2((1)pi sigma(*)) potential energy surfaces (PESs) at larger RO-H, en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes nu(16a) and nu(11) can both promote nonadiabatic coupling in the vicinity of the S-0/S-2 CI. Spectra recorded at lambda(phot)<= 248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER similar to 12 000 cm(-1). These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S-1((1)pi pi(*)) and repulsive S-2((1)pi sigma(*)) PESs. Parent mode nu(16b) is identified as the dominant coupling mode at the S-1/S-2 CI, and the resulting phenoxyl radical cofragments display a long progression in nu(18b), the C-O in-plane wagging mode. Analysis of all structured TKER spectra yields D-0(H-OC6H5)=30 015 +/- 40 cm(-1). The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of (1)pi sigma(*) states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the (1)pi sigma(*) 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. (c) 2006 American Institute of Physics.