The molecular and electronic states of 1,2,4,5-tetrazine studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction studies

The VUV electronic absorption spectrum of 1,2,4,5-tetrazine has been re-investigated, and together with electron energy-loss spectra has led to identification of a number of new excited states. The valence and Rydberg excited states have been studied by multi-reference multi-root configuration interaction studies using MRDCI techniques. Initial studies with the RPA and TDA methods gave almost identical results for the excitation energies, but there is a substantial energy-lowering in the MRDCI calculations, which improves agreement with experiment substantially; these differences are a result of the double, triple and quadruple excited reference configurations included in the reference set of the latter method. The ¹pp* excitations are calculated rather higher than experiment, except for the lowest-lying (weak) ¹B_2u state at 5.0 eV. The calculated order for the next three pp* states is ¹B_1u (weak) followed by ¹B_2u (strong) and ¹B_1u (strong), the last two bands being responsible for the dominant absorption near 7.5 and 8.5 eV. All of this group of four bands involve excitations from the pair of MOs 1b_2g and 1b_1g into the 1a*_u and 4b*_3u VMOs. The sequence of np* states are in a similar order to the pp* excitations, with respect to the upper state, and the two lowest singlet states, ¹B_3u and ¹A_u are reasonably well determined. The triplet states follow a similar order to the singlets, and again the dominance of the effect of the two lowest VMOs is demonstrated, but considerable differences between the weighting of leading configurations occurs between singlet and triplet manifolds. The non-diagonal TDA method has been used to reconsider the UV-photoelectron spectrum. The ionisation potentials for tetrazine are reinterpreted with the first three bands being regrouped into 1, 2, 2 ionisations respectively. The ground state properties of tetrazine suggest that the NQR spectrum will show a principal axis ¹⁴N quadrupole coupling constant larger than the other known azines. The decline in diamagnetic susceptibility anisotropy in tetrazine, relative to benzene and pyridine, is indicative of lower resonance energy.