The electronic spectrum of s-tetrazine: Structures and vibrational frequencies of the ground and excited electronic states

Abstract

The ground and excited electronic states of the s-tetrazine molecule have been studied using the methods of ab initio electronic structure theory. In particular, complete self-consistent field (SCF) optimizations of the equilibrium structures on the X̃ 1Ag, ã 3B3u, and à 1Au(C2h)/1B3u (D2h) surfaces using both double-ζ (DZ) and DZ+polarization (DZ+P) basis sets have been carried out. Harmonic vibrational frequencies have been analytically evaluated at these stationary points. DZ SCF results for higher excited electronic states are also reported with the optimizations on these surfaces having been restricted to D2h symmetry. Single point configuration interaction energies including single and double excitations relative to the SCF references (CISD) have been used to predict both vertical and adiabatic electronic excitation energies for all states investigated herein. In addition the Davidson correction [CISD(+Q)] and the closed shell coupled cluster singles and doubles (CCSD) method have been used to approximate the effect of higher excitations. Our results for the equilibrium geometries, harmonic vibrational frequencies, electronic excitation energies, and the energies of the photodissociation of s-tetrazine are analyzed and compared to spectroscopic results where available.