Gas-phase photoelectron spectroscopic and theoretical studies of 1,2- dichalcogenins: Ionization energies, orbital assignments, and an explanation of their color

Gas-phase photoelectron spectroscopy and theoretical calculations are used to study the electronic structure of 1,2-dichalcogenins. Photoelectron spectra are reported for 1,2-dithiin, 3,6-dimethyl-1,2-dithiin, 3,6- diisopropyl-1,2-dithiin, 3,6-di-tert-butyl-1,2-dithiin, 2-selenathiin, 1,2- diselenin, 3,6-dimethyl-1,2-diselenin, and 3,6-di-tert-butyl-1,2-diselenin and are assigned on the basis of (a) trends in ionization cross sections as the ionization photon energy is varied and (b) shifts of the ionizations as chemical substitutions are made. The calculated properties of 1,2-dithiin and 3,6-dimethyl-1,2-dithiin are compared to experimental results. The first four filled frontier valence orbitals are associated with orbitals that can be described as being primarily carbon π and chalcogen lone pair in character. Comparison of spectra collected with He I, He II, and Ne I ionization sources for each compound indicate that there is a large degree of mixing of chalcogen and carbon character through most of the valence orbitals. The highest occupied molecular orbital of the selenium-containing compounds has more chalcogen character than the highest occupied molecular orbital of the 1,2-dithiins. The photoelectron spectra of 1,2-dithiin and 1,2-diselenin contain a sharp ionization that corresponds to removal of an electron from an orbital that is predominantly chalcogen-chalcogen σ bonding in character. The narrow ionization profile indicates fairly weak chalcogen-chalcogen σ bonding in this orbital, which would result in a corresponding weakly antibonding chalcogen-chalcogen σ* orbital. Computational results show that an orbital that is primarily S-S σ* in character is the lowest unoccupied molecular orbital of 1,2-dithiin, and electronic transition calculations show a low-energy HOMO-to-LUMO transition that can be described as a π/lone pair- to-σ* transition that explains the unusual color of 1,2-dichalcogenins.