Photoelectron spectroscopy of sulphur-containing heteroaromatics and molecular orbital calculations

Abstract

The photoelectron spectra of the aromatic systems (I)–(VI) have been compared with those of the sulphur-containing systems (VII)–(XIII) in which, nominally, a sulphur atom replaces two adjacent carbon atoms of the systems (I)–(VI). The correlations of π-orbital levels between corresponding members of the two series is so striking as to lead to the conclusion that a sulphur atom behaves like a carbon–carbon double bond in so far as the orbital energies are concerned. This experimental finding is not conceptually new since simple molecular orbital calculations based on such an idea have been carried out previously on thiophen but these photoelectron results provide good experimental evidence for the idea that sulphur can donate two electrons to a conjugated system. Molecular orbital calculations using the CNDO/2 method, described here, indicate a closely related interplay of 3p-orbitals and 3d-orbitals on sulphur. The experimental result is also of interest in ultra-violet spectroscopy where the spectra of sulphur-containing conjugated systems can be interpreted in terms of the spectra of the corresponding olefinic compounds although this also requires repulsion integrals to be similar or cancel. Orbital energy levels were not predicted satisfactorily by the CNDO/2 method but the PPP method gave a reasonable account of them. The CNDO/2 method did give good predictions of dipole moments.