The electronic states of 1,2,5-thiadiazole studied by VUV absorption spectroscopy and ab initio configuration interaction methods

The 1,2,5-thiadiazole VUV absorption spectrum over the range 5-12 eV has been obtained for the first time. It shows broad bands centred near 5.0, 7.2, 7.7, 8.7, 9.6 and 10.6 eV; several of these show well resolved vibrational structure. A number of valence states and Rydberg states relating to the first and second ionisation energies have been identified. The doubling of some bands in the 4.7 eV region (previously reported) is confirmed; a 3s Rydberg state shows a similar phenomenon, which we also attribute to a non-planar upper state; a study of the (planar) equilibrium geometry of the corresponding triplet Rydberg state shows it is a saddle point. 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. There is generally a good correlation between the theoretical intensities and the experimental envelope. Reconsideration of the order of cationic states in the UV-photoelectron spectrum, crucial to the Rydberg state interpretation, has led to the sequence: 2B1 < 2B2 < 2A2 < 2A1. Study of the excitation energies to specific upper states, also supports this order. The value of IE1 is refined to 10.111 eV. Equilibrium structures show the π- and σ-cations are planar, in contrast to the lowest triplet state.