The electronic states of thiazole studied by VUV absorption spectroscopy and ab initio configuration interaction methods

The thiazole VUV absorption spectrum over the range 5-12 eV shows intense bands near 5.4, 6.0, 7-7.8, 8.2-8.8, 9.2-10.1 and 10.2-11.0 eV; there is marked vibrational structure in the 7.5-8.5 and 10.2-11 eV regions. A number of Rydberg states have been identified, largely from IE1, but also tentative values for ones from IE2 and IE4. Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods, and these have been compared with the VUV envelope. Calculated energies for low-lying Rydberg states are close to those expected, and there is generally a good correlation between the theoretical intensities and calculated density of states, with the experimental envelope. The CI studies used a triple zeta + polarization basis set, augmented by diffuse (Rydberg) orbitals. The lowest absorption bands are dominated by intense ππ* (A') valence states, together with LPNπ* and πσ* states, which are relatively weak. The lowest Rydberg states arise from excitation of the occupied MOs in the sequential order π4 < σ18 (LPN) < π3 < π2 < σ17 (LPS) this same order is determined by CI for ionisation in the UV-PES spectrum. Adiabatic structures of the σ- and π-triplets, -cations and the π-anion have been compared with the ground state structure. The known phosphorescence of thiazole is from the 3A triplet state (C1 symmetry) where significant twisting of C5H and S to opposite sides of the mean plane occurs. Some theoretical molecular properties of thiazole are described, which give good agreement with microwave spectral data