The electronic states of the azines. IV. Pyrazine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

The present paper extends the number of known low-lying electronically excited states of pyrazine through comparison of VUV absorption and near-threshold electron energy-loss spectra. Assignments of these, and assessment of previously established states, have been carried out by all-valence electron CI studies using multi-reference multi-root approaches in a double zeta+Rydberg function basis. In general, the first members of Rydberg series are well-determined by calculation, as are the lowest pp* and np* states. However, the ¹B_1u/¹B_2u states derived from the ¹E_1u state in benzene are not well-calculated. It seems that the paucity of MOs of symmetry a_u lies behind this and that an iterative natural orbital approach may be necessary to obtain a better fit of theory with experiment. In the assignment of cationic states, there are some differences between present large-scale CI and earlier Green function (GF) and Tamm-Dancoff (TDA) results; in particular, the CI calculations produce many more shake-up states and with a lower onset energy than the TDA method with the same basis set. One-electron properties of the pyrazine ground electronic state have also been computed, with a larger basis set. © 1991.