Electronic excitation spectrum of s-tetrazine:: An extended-STEOM-CCSD study

The theoretical vertical electronic excitation spectrum of 1,2,4,5-tetrazine is obtained using the extended similarity transformed equation-of-motion coupled cluster method and is compared to previous CASPT2 and MRCI results. In extended-STEOM-CCSD, all types of excitations that occur in s-tetrazine, notably n --> pi*, pi --> pi*, Rydberg transitions and doubly excited states are obtained from a single calculation, and in a balanced way. All features in the experimental vacuum UV spectrum up to about 11 eV are assigned to calculated dipole-allowed transitions. Additional features found in the electron energy loss spectrum are assigned to dipole-forbidden singlet and triplet transitions of ungerade symmetry. An interesting convoluted feature in the experimental vacuum-UV spectrum that extends between about 8.15 and 8.5 eV is assigned to a pair of states of B-2u symmetry, calculated at 8.28 and 8.54 eV, that are essentially the plus and minus combination of a singly excited pi --> pi* and a doubly excited n,n --> pi*,pi* configuration that can be expected to exhibit extensive vibronic interaction.