A theoretical study of the electronic spectrum of s-tetrazine

Abstract

The vertical electronic spectrum of s-tetrazine has been studied using multiconfigurational wavefunctions (CASSCF) and second-order perturbation theory (CASPT2) to include effects of dynamic electron correlation. The spectrum is complex with a large number of excited states at low energies (10 singlet valence states between 2eV and 6eV). They include n → π* and π → π* excited states, but also two doubly excited states of the n, n → π*, π* type. The lowest doubly excited state appears already at 4.37 eV. In all, 47 excited states have been characterized, comprising both valence (singlet and triplet) and Rydberg states. The calculations are complicated due to strong correlation effects in the four lone-pairs and between the lone-pairs and the π electrons. Thus large active spaces were needed. In some cases it was necessary to use the multi-state CASPT2 method to unravel mixing of valence and Rydberg states at the reference CASSCF level. The computed energies have been used to assign the electronic spectrum and good agreement between computed and experimental vertical transition energies is found, the largest deviation being 0.3 eV.