STRUCTURAL-CHANGES AS A FUNCTION OF TORSIONAL MOTION STUDIED BY AB-INITIO CALCULATIONS .5. B(2)X(4)(X=F, CL, BR)

Structural changes as a function of the torsional angle about the B-B bond angle been studied for the diborane tetrahalides X(2)B-BX(2)(X = F, Cl, Br) by ab initio calculations. The calculations predict the per pendicular form with D-2d symmetry to be the most stable conformation when the double-zeta basis 6-31G set is used. A triple-zeta basis set augmented with diffuse functions and polarization functions of the ty pe 6-311+G gives the planar D-2h conformation to be the most stable c onformation for B2F4 in agreement with experimental results. Inclusion of correlation through the MP2 level of theory affects mostly the B-F bond length, and only to a small degree the barrier height. For inter mediate states with one X-B-B-X torsional angle constrained to lie bet ween 0 and 90 degrees a pyramidal arrangement around the boron atoms i s found with an overall symmetry of C-2, and with two unequal B-X bond distances and two unequal B-B-X bond angles. Vibrational frequencies have been calculated and compared with experimental assignments. Calcu lated structure parameters are compared with experimental values.