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

The isothiazole VUV absorption spectrum over the range 5-12 eV shows (broad) intense bands centred near 5.17, 6.11, 7.37, 7.75, 9.18 and 10.43 eV. The lowest Rydberg states relating to the first ionisation energy are difficult to identify, but higher members are particularly numerous on the region from 8.4 to 9.6 eV. Electronic excitation energies for valence (singlet and triplet) and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + double polarisation basis set, augmented by diffuse (Rydberg) orbitals. The theoretical study shows the nature of the more intense Rydberg state types, and positions of the main valence and Rydberg bands. By study of the excitation energies to specific upper states, the vertical ionisation energies (IE) are confirmed as π4-1<π3-1<σ18-1(LPN)<σ17-1(LPS). Structures for the π- and σ-cations, and the (neutral) ππ∗-triplet states have been obtained. Calculated energies for low-lying Rydberg states are close to those observed, and there is generally a good correlation between the theoretical intensities and the experimental envelope. The ground state atomic and molecular properties are in good agreement with experiment.