Abstract
The NMR shieldings in molecular environments of various magnetic nuclei from the first, second and third rows have been calculated using approximate molecular orbital techniques. These techniques comprise the use of CNDO or INDO molecular wavefunctions, gauge invariant atomic orbitals and Rayleigh-Schrodinger perturbation theory. Considerable use is made of a version of the CNDO/s molecular orbital program, specially extended to cope with molecules containing second and third row atoms and iodine. Algorithm to calculate the dia- and paramagnetic contributions to NMR shielding have been appended to this program. Because of doubts surrounding the parameters employed in the CNDO/s program, a non-empirical CNDO program has also been constructed, with the same repertoire of atoms. This program uses atomic parameters obtained from atomic orbital calculations, while resonance integrals are calculated in situ by an exact method. As well as the dia- and paramagnetic contributions, an algorithm to calculate the spin-orbit coupling contribution to NMR shielding has also been appended to the non-empirical CNDO program. This algorithm treats spin-orbit coupling interactions as an additional magnetic perturbation, the effect on shielding being thus obtained from third order perturbation expressions. The performance of the contributions calculated from the two programs in explaining the observed chemical shift trends in [13]C, [27]Al, [71]Ga, [73]Ge and [75]As containing series has been compared. This comparison affirms the critical and often dominant role played by spin-orbit coupling interactions whenever heavy atom substituents are present, as well as indicating the overall superiority of the non-empirical CNDO program. The remaining shortcomings of the shielding contributions as calculated by the non-empirical CNDO program are assumed in the light of their ability to explain the chemical shift trends in [13]C, [27]Al, [33]S, [73]Ge, [75]As and Se containing series. In addition, shielding contributions have been obtained using the INDO/s molecular orbital program for series of [15]N and [19]F containing molecules. For some model compounds in the [15]N series, the solvaton method has been used to assess the effect of a polar solvent on NMR shielding.