A dipole moment and helium(I) photoelectron spectroscopic study of the conformations of chalcanthrenes

Abstract

The dipole moments (µ) of thianthrene, selenanthrene, and telluranthrene in benzene solution at 25° and the He^I photoelectron spectra of all four chalcanthrenes were investigated to obtain information about the conformation of these molecules in solution and in the gas phase. The calculated angles of fold along the chalcogen–chalcogen axes, for which µ_calc.=µ_exp., are 163.8°(O), 142.4°(S), 139.0°(Se), and 124.6°(Te). These angles are appreciably larger than the solid-state values reported for these molecules and some of their perfluoro-derivatives. These larger values are probably caused by butterfly motions of the tricyclic molecules, for which the barrier is expected to be small in solution. The photoelectron spectra show the expected six π-bands and one σ-band in the low ionization region. The empirical assignments of these bands are supported by results of π-only semi-empirical calculations (SCF-PPP). The trends in the average values of the first two ionization energies and their energy differences in this series of chalcanthrenes indicate that these molecules adopt a folded conformation in the gas phase also.