Excited state photodissociation dynamics of 2-, 3-, 4-hydroxyacetophenone: Theoretical study

Photodissociation of 2-, 3-, and 4-hydroxyacetophenone (2-, 3-, and 4-HAP) has been studied by analyzing excited-state potential energy surfaces (PES). For that, ab initio CASSCF(12,12)/6-31 + + G(d,p) calculations of low-lying excited states have been performed. Molecular beam study of photodissociation of 2-, 3-, and 4-HAP under laser excitation at 193 nm revealed that there are three possible dissociation channels of HAP: separation of H atom, CH3 and COCH3 fragments. In 2-HAP isomer the H separation channel is quenched. In this study we explain the quenching mechanism of the H separation channel and suggest a new model of CH3 and COCH3 fragment elimination reactions. Calculations of the excited-state PES reveal that the most of H, CH3 and COCH3 fragments arise after relaxation into the second, optically "dark" (2)A '' state, while some of the reaction products result from the (1)A '' state dissociation. Dissociation of 2-HAP through H separation in the (2)A '' state is deemed impossible, which explains the absence of deprotonated fragments in the 2-HAP mass spectrum. Breaking of intramolecular hydrogen bond in 2-HAP needs much more energy than the CH3 detachment, which makes relaxation through the S-1- S-0 conical intersection unfavorable.