Intermolecular Force-Field Parameters for Boron Hydrides

Intermolecular atom–atom force-field parameters of the (exp-6-1) type for B and H atoms in boron hydrides were determined. They were obtained by full-weighted least-squares minimization of 116 forces in 15 observed crystal structures of boranes, the heat of sublimation of B10H14 and data from ab initio wavefunction calculations for diborane. Net atomic charges were obtained by fitting them to molecular electric potentials calculated from ab initio wavefunctions. Charges of terminal hydrogens were usually negative and those of bridging hydrogens usually positive. Repulsion-energy calculations for the B2H6 dimer provided the exponential dependence of H...H repulsion. Using the resulting force field, minimum-energy crystal structures were found with structural parameter values close to those of the observed structures. For diborane, energy minimization beginning with randomly oriented molecules placed initially in an 8 × 8 × 8 body-centered orthogonal cell led to the observed crystal structure and monoclinic space group.