Relativistically parameterized extended Hückel calculations

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Abstract
Spin-spin coupling tensors for one-bond coupling between main group elements are calculated using a sum-over-states of relativistically parameterized extended Hückel (REX) wavefunctions and the relativistic analogue of Ramsey's theory. All parameters, including the tabulated magnetic hyperfine integrals, come from relativistic (or non-relativistic) atomic Hartree-Fock calculations, affording a systematic comparison of these two cases. The results reconfirm the strong relativistic increase of K(XH) in XH4, X = C-Pb, and the rôle of the new, isotropic term K ps . The relativistic increase of the relative anisotropy R = (K ‖-K ⊥)/K is found to be a rather general phenomenon. Its origin is traced to the s-tp 1/2 contribution and, ultimately, to a phase factor for p 1/2 orbitals. Thus, for example, the R(PbC) in Pb(CH3)4 is predicted to be larger than the corresponding R(SnC) in Sn(CH3)4. For molecules having lone pairs, both the increase of Z along a column and the relativistic effects diminish the 1 K, making it large and negative for the heaviest members of the group, as found experimentally for (CH3)2Se and (CH3)2Te. The K ss term, corresponding to the non-relativistic Fermi-contact term, is found to dominate almost invariably.