Dipole moment and helium(I) photoelectron spectroscopic studies of the conformation of di-2-pyridyl and diphenyl dichalcogenides R2X2(X = S, Se, or Te)

Abstract

The dipole moments of di-2-pyridyl dichalcogenides in benzene solution at 25 and 45° and the He^I photoelectron spectra of di-2-pyridyl and diphenyl dichalcogenides R₂X₂(X = S, Se, or Te) were investigated to obtain information about the conformation of these molecules in solution and in the gas phase. The calculated C–X–X–C dihedral angles (100–110°) for the dipyridyl dichalcogenides, for which µ_calc=µ_exp, are at least 10° larger than those for the diphenyl derivatives. These results indicate that the π-interactions between the X atoms and the pyridyl groups are more effective than between the X atoms and the phenyl rings, reducing, as a consequence, repulsion between the lone electron pairs of the chalcogen (X) atoms and lowering the torsional barriers about the X–X bonds. The variation of the dihedral angles between the phenyl derivatives and the corresponding dipyridyl dichalcogenides as derived on the basis of photoelectron spectroscopic data, is in disagreement with the results from dipole moment data. This difference between the gas phase and solution results may be caused by solvent and temperature effects or by secondary interactions between the π and σ orbitals. The dipole moment and photo-electron spectral data indicate that the diaryl dichalcogenides are conformationally flexible with a skew equilibrium conformation and very little interaction between the two aryl groups.