Theoretical Study of the Thermochemistry of Sulfur Molecular Crystals. I. Method and Application to α- and 1D-Polymerized Sulfurs

Abstract

The combination of two semiempirical methods, the "extended Hu ¨ckel theory" for repulsive energy and a method derived from classical perturbation expansions for the dispersion energy, is used to evaluate the weak interaction energy between covalent rings or chains of sulfur compounds. In this paper, the parametrization method of both parts of the energy is set up. The volume of the elementary cell and the heat of sublimation of R-sulfur are in correct agreement with experimental values. As a test for the study of polymerized ω-sulfur, a simple 1D-example shows correct behavior for interchain distance and cohesive energy. In each case, a detailed analysis of the contribution of each term involved in the total intermolecular potential function is given. In particular, we show the importance of three-body terms in the energetics of such structures.

Spherical and Aspherical Intermolecular Force Fields for Sulfur Allotropes

“…Using the above-described procedures, we optimized A and B of a spherical sulfur nonbonded potential to 11 S n crystal structures, using the computer program nbp . The heat of sublimation of α-S 8 has been measured experimentally; − we adopt the value of 104.4 kJ mol -1 reported by Chastel and Ezzine . Since α-S 8 was the only structure for which heat of sublimation data was available, the energy side condition was fitted exactly.…”

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

“…The choice of the number of k points,24 for the sampling of the reduced Brillouin zone, has been discussed in Part I. 16 Concerning the second part of the energy, application of time-dependent perturbation theory leads to a dispersion multipole expansion, which in the case of two-body terms, is expressed as :…”

“…The retained parameters for the repulsive part (EHT calculations) are equal to : eV and 29 In the case of a-sulfur, the S dipole polarizability was estimated at 5.39 as shown in Part I of this study. 16 The S polarizability Ó3 in polymerized sulfur is assumed to be identical to that found for a-sulfur since the local environment of S atoms is the same in both structures.…”

“…He proposed that A and C are conrotatory helices (rotating both in the same direction ; let us choose the righthand one, for example : rr) and that B and D are also conrotatory with respect to each other but are rotating in the ¤ Part I is ref. 16 opposite direction (the left-hand one : ll). The structure of S q2 is based on a centred quadratic unit cell with successive parallel planes of helical chains where the axes of helices in one plane are perpendicular to those of helices in the two neighbouring ones.…”