Simplified Treatment of Electronic Structures of the Lowest Singlet and Triplet States of Didehydropyrazines

Experiments on the lowest lying singlet and triplet states of didehydropyrazine isomers (that focus on energy gaps, geometries, and vibrational frequencies) have been carried out computationally by implementing the improved virtual orbital-based multireference (MR) ab initio methods. Pyrazines possess a reasonable MR nature making their description challenging with the conventional quantum chemical approaches. Although wave functions of the diradicals usually have two dominant configurations, a larger reference space is warranted to consistently and accurately describe pyrazine diradicals indicating the complex nature of the systems. Present calculations predict a singlet ground state for ortho- and para-pyrazines, while a triplet ground state is suggested for the meta isomer. The adiabatic singlet-triplet energy gap for the para form is found to be notably higher (by 28.4 kcal/mol) compared to ortho (2.0 kcal/mol) and meta isomers (-5.1 kcal/mol). Accurate and reliable computations are imperative for forecasting the state-ordering in such diradicals. The structural properties obtained from the present calculations lend strong support toward future experimental explorations on these systems.