Theoretical Studies on Photodissociation and Isomerization Dynamics of Diazines Following Ultraviolet Excitation

Photodissociation of diazines following UV excitation provides a prototypical system for examining unimolecular reaction dynamics on vibrationally excited ground-state potential energy surfaces. In this work, we present a comprehensive theoretical study of the isomerization and dissociation dynamics of the isomers of diazines (C 4 H 4 N 2 ), with particular emphasis on pyrazine, after rapid internal conversion to the ground electronic state. A unified ground-state potential energy surface is constructed using CCSD(T)/CBS energies, and microcanonical rate constants and product branching ratios are evaluated using Rice–Ramsperger–Kassel–Marcus theory in combination with microcanonical variational transition-state theory. The results reveal a strong energy dependence of the photodissociation dynamics arising from the interplay between isomerization and competing fragmentation pathways. At lower excitation energies (248 nm), rapid isomerization funnels population into pyrimidine, and dissociation proceeds predominantly through pyrimidine-centered channels. At intermediate energies (193 nm), direct dissociation of pyrazine becomes competitive, with concerted three-body fragmentation producing acetylene and hydrogen cyanide accounting for approximately 36% of the total product yield. At higher energies (157 nm), the dynamics approach a statistical limit dominated by concerted three-body dissociation. These results demonstrate that product branching in diazine photodissociation is governed by a coupled isomerization–dissociation network and cannot be inferred from isolated reaction pathways.