Dynamics and mechanisms of nonradiative photodissociation of glycine: A theoretical study

Abstract

Photochemistry of small biomolecules is fundamental to understand the photochemical reactions of larger molecules, such as proteins. In this work, the dynamics and mechanisms of photodissociations of glycine conformers were studied using the DFT, TD-DFT and CASPT2 methods with the aug-cc-pVDZ basis set and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH). This study emphasized unimolecular dissociations and isomerization, nonradiative S1 → S0 relaxations that generate molecular products and the interplay between photoenergies and thermal energies. Four nonplanar conformers were suggested by the TD-DFT/B3LYP/aug-cc-pVDZ and CASPT2(8,14)/aug-cc-pVDZ methods to be stable in the S1 state. The potential energy curves obtained from the TD-DFT/B3LYP/aug-cc-pVDZ and CASPT2(8,14)/aug-cc-pVDZ methods and SA-CPMCSCF/6-31G(d) conical intersection optimizations suggested S0/S1 intersections for distorted glycine structures, Cα–N and NH dissociations and NH → OC isomerization. The NVE-MDSH results showed that although no dissociated molecular product was observed in the S1 state, isomerization-mediated dissociation and exothermic S1 → S0 relaxation temperature could lead to eight molecular product channels, in which the vertically excited Wigner-sampled structures generated eleven reactive and stable molecular products in different temperature ranges. These thermally selective reactions favored the lowest energy conformer (structure Ip) as the precursor, from which reactive and stable molecular products were formed above the threshold exothermic relaxation temperature. These theoretical results showed in detail for the first time how unimolecular photodissociations and nonradiative S1 → S0 relaxations create molecules in cold interstellar media without heat transfer from the surroundings. The results in this work could be used in future theoretical and experimental studies on the same or similar systems.