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

摘要

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 secondorder 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 CH_3CO 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.