Bond stretching force constants and compressibilities of nitride, oxide, and sulfide coordination polyhedra in molecules and crystals

Abstract

Molecular quadratic stretching force constants are calculated for a variety of MX bonds (X = N, O, S) in coordinated polyhedra containing row 1 and 2 metal atoms, M, using SCF molecular orbital methods and 6-31G* basis sets. The resulting data scatter along three distinct trends, depending on whether the bonds involve row 1 atoms, row 1 and row 2 atoms, or row 2 atoms. When compared with spectroscopically determined force constants, the calculated force constants are found to be ∼ 20% larger. A single trend seems to obtain when the calculated force constants are plotted as a function of the effective nuclear charges of the bonded atoms and their interatomic separations. Scaled force constants calculated for the SiO bond are in rough agreement with values provided by spectroscopic measurements for silicic acid molecules and silicate crystals. Polyhedral compressibilities for nitride-, oxide-, and sulfide-coordinated polyhedra are inversely related with force constants calculated for their MX bonds. The close similarity of these compressibilities and those recorded for chemically similar crystals suggests that force constant-compressibility relationships in chemically similar molecular and crystalline systems are not significantly different.