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

The electronic states of pyridine have been investigated experimentally, using VUV absorption and near-threshold electron energy loss spectroscopy, and theoretically, with ab initio multi-reference configuration interaction calculations. The theoretical energies relate well to the experimental observations, allowing firm identification of all low-lying pp* and np* excited states, including an optically forbidden np* ¹A₂ state, detected at 5.43 eV in electron energy loss. Fine structure in the VUV spectrum, associated with Rydberg-state excitation, matches that observed in high resolution photoelectron spectra, facilitating Rydberg-state assignments. Good energies are calculated for n = 3, 4 members of Rydberg series. The Cl and MC SCF results confirm that the first ionisation potential in pyridine is ²A₁ corresponding to removal of a non-bonding electron, in agreement with experiment. A triple zeta valence plus polarisation SCF + all-valence electron Cl has led to good agreement for various one-electron operators of the wavefunctions with experiment. © 1990.