Near-UV photolysis of substituted phenols Part II. 4-, 3- and 2-methylphenol

Abstract

The photodissociation of jet-cooled 4-, 3- and 2-methylphenol molecules has been investigated using the experimental techniques of resonance enhanced multiphoton ionisation and H (Rydberg) atom photofragment translational spectroscopy. O–H bond fission is found to occur, via a repulsive ¹πσ* state, in a manner analogous to that occurring in phenol and 4-fluorophenol. Excitation to the ¹ππ* manifold results in H-atom loss either directly (via a ¹ππ*/¹πσ* conical intersection) or indirectly, following internal conversion to the ground state and subsequent coupling to the ¹πσ* state via a second conical intersection at extended O–H bond lengths. The resulting methylphenoxyl radicals are created with specific vibrational excitation, reflecting the nuclear distortions required to access the ¹πσ* potential energy surface and the geometry changes induced by subsequent H atom loss. The position of the methyl group on the benzene ring is observed to influence the product vibrational energy disposal—not least through its influence on the mode(s) that are activated as a result of coupling to the repulsive ¹πσ* state. O–H bond strengths are reported for 4-, 3- and 2-methylphenol. These are in good agreement with values derived from recent combustion calorimetry studies and serve to highlight the relative destabilisation of the radical caused by methyl substitution at the 3-position.