The electronic states of oxazole studied by VUV absorption and electron energy-loss (EEL) spectroscopies, and ab initio configuration interaction methods

The oxazole VUV absorption spectrum over the range 5-12 eV shows intense bands centred near 6.3, 7.5, 8.3, 9.6 and 10.8 eV. The electron energy-loss (EEL) spectrum shows additional structure with a strong peak (∼1.4 eV) arising from resonant vibrational excitation of the molecule via a shape resonance, and a spin-forbidden 3ππ∗ state at 4.6 eV. Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. The CI studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for low-lying Rydberg states are relatively close (SD 0.38) to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. Two of the three lowest electronic states arise from ππ∗ excitation of the outer (3a″ and 2a″) π-orbitals, with one state (LPNπ∗) originating from the lone pair on nitrogen (15a′) between them.