Friday, July 28, 2006

Phys. Chem. Chem. Phys., 2006, 8, 2733 - 2743

An NMR and relativistic DFT investigation of one-bond nuclear spin–spin coupling in solid triphenyl group-14 chlorides

Mathew J. Willans, Bryan A. Demko and Roderick E. Wasylishen

A solid-state nuclear magnetic resonance and zeroth-order regular approximation density functional theory, ZORA-DFT, study of one-bond nuclear spin–spin coupling between group-14 nuclei and quadrupolar 35/37Cl nuclei in triphenyl group-14 chlorides, Ph3XCl (X = C, Si, Ge, Sn and Pb), is presented. This represents the first combined experimental and theoretical systematic study of spin–spin coupling involving spin-pairs containing quadrupolar nuclei. Solid-state NMR spectra have been acquired for all compounds in which X has a spin-1/2 isotope—13C, 29Si, [117/119]Sn and 207Pb—at applied magnetic fields of 4.70, 7.05 and 11.75 T. From simulations of these spectra, values describing the indirect spin–spin coupling tensor—the isotropic indirect spin–spin coupling constant, 1J(X,35/37Cl)iso and the anisotropy of the J tensor, 1J(X,35/37Cl)—have been determined for all but the lead–chlorine spin-pair. To better compare the indirect spin–spin coupling parameters between spin-pairs, 1Jiso and 1J values were converted to their reduced coupling constants, 1Kiso and 1K. From experiment, the sign of 1Kiso was found to be negative while the sign of 1K is positive for all spin-pairs investigated. The magnitude of both 1Kiso and 1K was found to increase as one moves down group-14. Theoretical values of the magnitude and sign of 1Kiso and 1K were obtained from ZORA-DFT calculations and are in agreement with the available experimental data. From the calculations, the Fermi-contact mechanism was determined to provide the largest contribution to 1Kiso for all spin-pairs while spin-dipolar and paramagnetic spin–orbit mechanisms make significant contributions to the anisotropy of K. The inclusion of relativistic effects was found to influence K(Sn,Cl) and K(Pb,Cl).

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