Theoretical study of the thermochemistry of sulfur molecular crystals II. Lowest energy allotropes of polymeric ω-sulfurs

Abstract

Extended Huckel theory (for the repulsive part of the energy) and a method derived from classical perturbation expansions (for the long-range attractive part of the energy) were combined and the adequate parametrization has been set up in Part I. In this first part of the series, the method has been tested with success on ω-sulfur and on an example of two interacting helices. Here, we apply this method to determine the lowest energy conformations for all 1D, 2D and real 3D structures of polymeric ω-sulfurs considered from our structural hypotheses. We examine how considerations on the intermolecular repulsive interactions can lead to reliable structural information. Of course, the dispersive (attractive) part of the energy (including dipole-dipole, dipole-quadrupole and three-body terms) is necessary to determine quantitatively the stability of these sulfur structures. We find that ω2-sulfur is more stable than ω1-sulfur, where each of these helical chains is built up from a three-atom unit cell. For both 3D allotropes, the calculated enthalpy of sublimation is in good agreement with the experimental values.