Role of Triplet States in the Photolysis of Proteogenic Amino Acids.

This investigation delves into the UV photodissociation of pivotal amino acids (Alanine, Glycine, Leucine, Proline, and Serine) at 213 nm, providing insights into triplet-state deactivation pathways. Utilizing a comprehensive approach involving time-dependent density functional calculations (TD-DFT), multi-configurational methods, and ab-initio molecular dynamics (AIMD) simulations, we scrutinize the excited electronic states (T1 , T2 , and S1 ) subsequent to 213 nm excitation. Our findings demonstrate that α-carbonyl C-C bond-breaking in triplet states exhibits markedly lower barriers than in singlet states (below 5.0 kcal mol-1 ). AIMD simulations corroborate the potential involvement of triplet states in amino acid fragmentation, underscoring the significance of accounting for these states in photochemistry. Chemical bonding analyses unveil distinctive patterns for S1 and T1 states, with the asymmetric redistribution of electron density characterizing the C-C breaking in triplet states, in contrast to the symmetric breaking observed in singlet states. This research complements recent experimental discoveries, enhancing our comprehension of amino acid reactions in the interstellar medium.

Elucidating the N-N and C-N Bond-breaking Mechanism in the Photoinduced Formation of Nitrile Imine.

In this study, we revealed the significance of chemical bonding for the photochemically induced mechanism of 2-phenyl tetrazole derivatives generating nitrile imines. The correlated electron localization function shows that the formation of imine nitrile involves two key bond events: (i) the heterolytic C-N breakage taking place in the T1 state and (ii) the homolytic N-N rupture occurring in the T2 excited state. In particular, a cation-radical specie results from the C-N cleavage, whereas the N-N rupture creates a biradical resonant form of imine nitrile. Additionally, we noticed that the substantial pair delocalization of the C-C-N bonded structure could play a significant role in the conversion of the biradical imine nitrile into both the propargylic and allenic forms via the T1 →S0 deactivation.