Elemental sulfur exists in many crystalline forms, with many of these forms being composed of discrete S n cyclic molecules. Eleven experimentally determined S n crystal structures are used to define a new spherical S···S intermolecular force field of the (exp-6) type. Evidence is presented that the bonded sulfur atom in these structures is not spherical. The asphericity was modeled by placing repulsion bumps at optimized locations on the sulfur atoms to represent lone pair electron repulsion. The bump locations deviate from perfect tetrahedral geometry, being located more perpendicular to the S−S−S plane. A new aspherical sulfur intermolecular force field was derived from eleven S n crystal structures. The aspherical force field gives an improved prediction of the S n crystal structures.

The molecular structures of cycloalkanes (from cyclopropane to cyclodecane) and tetrahedrane were optimized at the Hartree–Fock/6–31G** level and their molecular electric potentials (MEPs) were calculated using a geodesic grid. The MEPs were fitted using net atomic charges and several site charge models. The net atomic charge model gave very poor fits to the MEPs in every case. A model with additional methylene bisector charge sites, similar to one successfully used previously for linear alkanes, greatly improved the fits to the MEPs of these cycloalkanes. The MEPs of the highly strained molecules cyclopropane and tetrahedrane were further studied using ring center and displaced bond charge sites. The fitting of the MEP of cyclopropane was consistent with a banana bond model with asymmetrically displaced electron density in the CC bonds. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 579–585, 1999

Spherical and Aspherical Intermolecular Force Fields for SulfurAllotropes.-Eleven experimentally determined Sn (6 ≤ n ≤ 20) crystal structures are used to define a new spherical S···S intermolecular force field of the (exp-6) type. Evidence is presented that the bonded sulfur atom in these structures is not spherical. A new aspherical intermolecular force field for S···S interaction places bumps on the sulfur atoms to model aspherical lone pair repulsion. The aspherical force field gives an improved prediction of the Sn crystal structures. -(ABRAHA, ARON; WILLIAMS, DONALD E.; Inorg.

ABSTRACT:The molecular structures of cycloalkanes from cyclopropane to . cyclodecane and tetrahedrane were optimized at the Hartree᎐Fockr6᎐31G** Ž . level and their molecular electric potentials MEPs were calculated using a geodesic grid. The MEPs were fitted using net atomic charges and several site charge models. The net atomic charge model gave very poor fits to the MEPs in every case. A model with additional methylene bisector charge sites, similar to one successfully used previously for linear alkanes, greatly improved the fits to the MEPs of these cycloalkanes. The MEPs of the highly strained molecules cyclopropane and tetrahedrane were further studied using ring center and displaced bond charge sites. The fitting of the MEP of cyclopropane was consistent with a banana bond model with asymmetrically displaced electron density in the C-C bonds. ᮊ