Ground and ionic states of 1,2,5-thiadiazoles: An UV-photoelectron spectroscopic and theoretical study

Electronic structures of 1,2,5-thiadiazoles (X2C2N2S, where X = H, F, Cl, CH3, CN) have been investigated in the gas phase by He I and He II UV-photoelectron spectroscopy, as well as by calculations at the B3LYP and SAC-CI levels using aug-cc-pV(T+d)Z basis sets. The ground state geometry of the neutral molecules has been obtained from quantum-chemical calculations at the B3LYP level. Thiadiazoles are predicted to be planar, not considering methyl-group hydrogen atoms, with C2v symmetry. Equilibrium structures of the four lowest energy cationic states have been determined at the UB3LYP level, and calculations predicted that the sequence of ionic states is 2B1(ground state) < 2B2 < 2A2 < 2A1. Photoelectron spectroscopy and theory applied to thiadiazoles has provided information on the valence occupied levels, on the ionisation energies, and on the fundamental vibrations of the cations.