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

Abstract 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, CH 3 and COCH 3 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 CH 3 and COCH 3 fragment elimination reactions. Calculations of the excited-state PES reveal that the most of H, CH 3 and COCH 3 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 CH 3 detachment, which makes relaxation through the S 1  − S 0 conical intersection unfavorable.