THE ELECTRONIC STATES OF THE AZINES .4. PYRAZINE, STUDIED BY VUV ABSORPTION, NEAR-THRESHOLD ELECTRON ENERGY-LOSS SPECTROSCOPY AND ABINITIO MULTIREFERENCE 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 pi-pi* and n-pi* states. However, the 1B1u/1B2u states derived from the 1E1u 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 compared, with a larger basis set.