Ab initio characterization of through-space indirect nuclear spin-spin coupling tensors for fluorine-X (X = F, C, H) spin pairs

Recent experimental and theoretical studies have focused on elucidating the nature of indirect nuclear spin-spin couplings (J) observed for nuclei in the absence of a formal covalent bond between them. In this contribution, we present systematic high-level ab initio (multiconfigurational self-consistent field, MCSCF) calculations of the 'through-space' fluorine-fluorine, fluorine-carbon, and fluorine-hydrogen indirect nuclear spin-spin coupling tensors for three related heterodimeric systems comprised of methane or fluoromethane and hydrogen fluoride. Both the isotropic and anisotropic portions of these tensors are analyzed as a function of the geometry of the model systems. In general, the isotropic coupling constants are dominated by the Fermi-contact mechanism and the anisotropies are dominated by the FC X SD mechanism. Substantial anisotropy is calculated for the J(F-19, F-19) tensors. A discussion of the through-space J tensors in the hydrogen fluoride-methane system in the context of the relevant overlap integrals is presented. While covalent bonding is not required for J coupling, non-zero overlap integrals are required for sizable couplings to be observed. (C) 2002 Elsevier Science B.V. All rights reserved.