THE MOLECULAR AND ELECTRONIC STATES OF 1,2,4,5-TETRAZINE STUDIED BY VUV ABSORPTION, NEAR-THRESHOLD ELECTRON-ENERGY-LOSS SPECTROSCOPY AND AB-INITIO MULTIREFERENCE CONFIGURATION-INTERACTION STUDIES

The VUV electronic absorption spectrum of 1,2,4,5-tetrazine has been r e-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 stu dies with the RPA and TDA methods gave almost identical results for th e excitation energies, but there is a substantial energy-lowering in t he MRDCI calculations, which improves agreement with experiment substa ntially; these differences are a result of the double, triple and quad ruple excited reference configurations included in the reference set o f the latter method. The (1) pi pi excitations are calculated rather higher than experiment, except for the lowest-lying (weak) B-1(2u) sta te at 5.0 eV. The calculated order for the next three pi pi states is B-1(1u) (weak) followed by B-1(2u) (strong) and B-1(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. Th e sequence of n pi states are in a similar order to the pi pi* excita tions, with respect to the upper state, and the two lowest singlet sta tes, B-1(3u) and (1)A(u) are reasonably well determined. The triplet s tates 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 be tween singlet and triplet manifolds. The non-diagonal TDA method has b een used to reconsider the UV-photoelectron spectrum. The ionisation p otentials for tetrazine are reinterpreted with the first three bands b eing regrouped into 1, 2, 2 ionisations respectively. The ground state properties of tetrazine suggest that the NQR spectrum will show a pri ncipal axis N-14 quadrupole coupling constant larger than the other kn own azines, The decline in diamagnetic susceptibility anisotropy in te trazine, relative to benzene and pyridine, is indicative of lower reso nance energy.