Photoelectron and X-ray emission (PAX) spectroscopy and electronic structure. Thiourea

Abstract

X-Ray emission (C K_α, N K_α, S K_β and S L_2,3M) and X-ray photoelectron spectra have been measured for thiourea. The X-ray spectra, which are generated by valence-to-core transitions, have been aligned on a common (molecular orbital) energy scale and, together with the ultraviolet photoelectron spectrum, have been used, within the assumptions of Koopmans' theorem, to indicate the electronic structure of thiourea. The spectroscopic results have been compared with a GAUSSIAN 86 STO-3G ab initio calculation. Evidence has been found for extensive π delocalisation over the carbon, sulphur and nitrogen atoms. Sulphur 3s and 3d orbitals are shown to play no part in covalent bonding in the molecule. Discrepancies between these conclusions and theoretical predictions were ascribed to the use of a limited basis set and to the neglect of relaxation effects.