Mechanistic photodecarboxylation of pyruvic acid: Excited-state proton transfer and three-state intersection

Photodissociation dynamics of pyruvic acid experimentally differs from that of commonly known ketones. We have employed the complete active space self-consistent field and its multi-state second-order perturbation methods to study its photodissociation mechanism in the S-0, T-1, and S-1 states. We have uncovered four nonadiabatic photodecarboxylation paths. (i) The S-1 system relaxes via an excited-state intramolecular proton transfer (ESIPT) to a hydrogen-transferred tautomer, near which an S-1/S-0 conical intersection funnels the S-1 to S-0 state. Then, some trajectories continue completing the decarboxylation reaction in the S-0 state; the remaining trajectories via a reverse hydrogen transfer return to the S-0 minimum, from which a thermal decarboxylation reaction occurs. (ii) Due to a small S-1 -T-1 energy gap and a large S-1/T-1 spin-orbit coupling, an efficient S-1 -> T-1 intersystem crossing process happens again near this S-1/S-0 conical intersection. When decaying to T-1 state, a direct photodecarboxylation proceeds. (iii) Prior to ESIPT, the S-1 system first decays to the T-1 state via an S-1 -> T-1 intersystem crossing; then, the T-1 system evolves to a hydrogen-transferred tautomer. Therefrom, an adiabatic T-1 decarboxylation takes place due to a small barrier of 7.7 kcal/mol. (iv) Besides the aforementioned T-1 ESIPT process, there also exists a comparable Norrish type I reaction in the T-1 state, which forms the ground-state products of CH3CO and COOH. Finally, we have found that ESIPT plays an important role. It closes the S-1-T-1 and S-1-S-0 energy gaps, effecting an S-1/T-1/S-0 three-state intersection region, and mediating nonadiabatic photodecarboxylation reactions of pyruvic acid. (C) 2014 AIP Publishing LLC.