AB-INITIO CALCULATION OF THE MOLECULAR-STRUCTURE AND CONFORMATIONAL COMPOSITION OF DIHALOTETRASILANES

The ab initio Hartree-Fock pseudopotential DZ+POL method was employed to optimize the structures of 2,3- and 1,4-dihalotetrasilanes. The dihedral angles of the Si-4 skeleton were fixed at selected values. The most stable conformers of 2,3-dichloro- and dibromotetrasilane exhibit C-i symmetry, with maximum separation of the halogen atoms and the terminal SiH3 groups. The relative stability of different conformers of the 1,4-dihalotetrasilanes can be described by dipole-dipole interaction between the polar terminal silicon-halogen bonds. Self-consistent-reaction-field (SCRF) calculations predict a reversed sequence of the two lowest energy conformers in solution relative to that found in the gas phase. Compared with their carbon analogues the barriers to internal rotations in tetrasilanes are considerably lower.