Molecular-Orbital Studies of Charge-Carrier Transport in Orthorhombic Sulfur. I. Molecular Orbitals ofS8

Abstract

Stimulated by the interesting transport experiments on orthorhombic sulfur by Spear and co-workers, we have carried out a semiempirical molecular-orbital calculation of the ${\mathrm{S}}_8$ molecule, the building block of the molecular crystal. The molecular orbitals are constructed from linear combinations of atomic $3s$, $3p$ orbitals according to the irreducible representations of the molecular symmetry group. It is found that because of the nonplanar (puckered ring) geometry of the molecule, the mixing of $\sigma$ and lone-pair hybrids in the molecular orbitals is not negligible. The molecular energy-level scheme predicts a set of electronic transitions which can be compared, with certain reservations, to the absorption spectrum of sulfur in hexane. It also predicts two forbidden transitions at 2-3 eV which could be responsible for the yellow color of the crystal. The sensitivities of the molecular orbitals and energies with respect to the variation in the parameter of the calculation are discussed. It is concluded that the results are satisfactory for use as the starting point in the studies of the crystal states, which is carried out in the following paper.