Abstract:
SCF-MS-Xα calculations have been carried out on the trihalide ions I–3, IBr–2, ICl–2, Br–3, BrCl–2 and Cl–3 and on XeF2. The nuclear quadrupole coupling constants calculated from the Xα wavefunctions agree well with experimentally determined values. In the case of the X–3 ions (X = Cl, Br or I) calculations have been carried out for unsymmetrical structures as well as for symmetrical D∞h structures, and it is found that the experimentally observed dependence of the quadrupole coupling constants on geometry is well reproduced by the Xα calculations. For D∞h I–3 the valence σ wavefunctions are very similar to those of the three-centre four-electron bonding scheme originally proposed by Pimentel. The bonding involves the valence p orbitals almost exclusively, s and d orbitals being involved to only a limited extent. The unbalanced p-orbital populations calculated for these species from the Xα wavefunctions agree reasonably well with those obtained from nuclear quadrupole coupling data using the Townes and Dailey equations, but systematic differences are observed, particularly for the central halogen atoms. Analysis of the various contributions to the Xα calculated electric field gradients shows that most of the approximations involved in the derivation of the Townes and Dailey equations are justified. However, it is shown that a slight contraction of the π atomic orbitals on the central halogen atoms results in a larger field gradient and quadrupole coupling constant at this atom. This has the consequence that the unbalanced p-orbital populations calculated for the central halogen atoms from the Townes and Dailey equations are systematically too high. Similar calculations have been carried out for the IF–2 ion, which has recently been identified by matrix-isolation infrared spectroscopy. In this case the I 5d orbitals become involved to a greater extent in the bonding, so that the I orbital populations are midway between those expected for the delocalized three-centre four-electron bonding scheme involving p orbitals only and a localized bonding scheme involving pd hybridization on iodine.
