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

Abstract

Electronic structures of 1,2,5-thiadiazoles (X 2C 2N 2S, where X = H, F, Cl, CH 3, 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 C 2v 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 2B 1(ground state) < 2B 2 < 2A 2 < 2A 1. 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.