HALOGEN NUCLEAR-QUADRUPOLE COUPLING-CONSTANTS - COMPARISON OF AB-INITIO CALCULATIONS WHICH INCLUDE CORRELATION, WITH EXPERIMENT

Ab initio determination of the electric field gradient (EFG) tensors a t halogen and other centres enabled determination of the nuclear quadr upole coupling constants (NQCC) for a diverse set of axially symmetric (C-3 nu, C-infinity nu, D-infinity h and other symmetries) inorganic and organic molecules, where the heavy elements are Cl, Br, and I with C, Si, Ge, and Sn hydrides. The latter elements are in an approximate ly tetrahedral environment. The study presents results at a standardis ed level of calculation, triple-zeta in the valence space (TZV) plus p olarisation functions (TZVP) for the equilibrium geometry stage; all-e lectron MP2 correlation is included in all these studies. f-Orbital ex ponents were optimised for both Br and I centres in the methanes; the atomic populations of the f-orbital components are very small for the Brand I-atoms, confirming their role as polarisation functions rather than having any bonding character. The EFG are determined at equilibri um with the TZVP basis set, except Sn and I centres where the basis se t is TZV + MP2. For the bromo and iodo compounds, especially the latte r, it is essential to allow for core polarisation, by decontraction of the p,d-functions. This is conveniently done by initial optimization of the structure with a partly contracted basis, followed by reestabli shment of the equilibrium structure with the decontracted basis. A clo se correlation of the observed (microwave spectral) data with the calc ulations was observed, using the 'best' values for the atomic quadrupo le moments for Cl, Br, and I; thus there seems no need to postulate th at the value of Q(Br) for Br-79 and Br-81 are in error. The SCF and MP 2 wave-functions were converted into localised molecular orbitals by t he Boys Method. This allowed a study of the differing s/p/d-hybridisat ion ratios, and the centroid positions, to be compared with the quadru pole coupling constants. The charge distributions for the atoms were c onverted into local bond dipoles, which in turn are correlated with th e electronegativity differences of the bonded atoms.