An SCF-MS-Xα study of the bonding and nuclear quadrupole coupling in boron trihalides

Abstract

SCF-MS-Xα molecular-orbital calculations have been carried out on the boron trihalides BX₃(X = F, Cl, Br, I) in order to investigate the bonding and nuclear quadrupole coupling in these molecules. Reasonable agreement (to within 30%) is obtained between the calculated and observed halogen coupling constants for X = Cl, Br, I. The agreement is poorer for the boron coupling constants, but these are more dependent on the sphere radii used in the calculations. Similar degrees of in-plane and out-of-plane ‘π bonding’ are present in these molecules and result in transfer of ca. 0.1e from the halogen p_π orbitals to the appropriate boron p orbitals. Previous conclusions, based on the halogen n.q.r. results, about the extent of out-of-plane π bonding are shown to be only fortuitously correct, since the halogen field-gradient asymmetry parameters are not determined primarily by the halogen p_π-orbital-population differences. Previously reported difficulties in reconciling the halogen and boron quadrupole coupling parameters are shown to be caused partly by neglect of in-plane π bonding and partly by contraction of the boron p orbitals resulting in ‘atomic’ field gradients for these orbitals which are larger than the value for the free atom.