Theoretical Studies On NMR Spectroscopic Parameters

The nuclear magnetic shielding constant and the indirect nuclear spin-spin coupling constant are two of the most important parameters for interpreting NMR spectra. The operators that determine the nuclear magnetic shielding and spin-spin coupling constants probe the regions close to the nuclei. For this reason, changes in the electronic structure due to relativistic effects are important for these properties. As an example, we note that the relativistic correction to the hydrogen shielding in hydrogen iodide is about 15 ppm at the Hartree-Fock level, an effect that exceeds the normal shielding range of protons. For the shielding of heavy elements, the neglect of scalar relativistic effects leads to an underestimation of the mercury shielding by almost a factor of 3. Thus, in calculations of the shieldings of molecules containing heavy elements, relativistic effects must be taken into account to obtain even qualitative agreement with experimentally observed trends. However, there have been few theoretical attempts to calculate NMR parameters of these heavy elements, due to the extra difficulty of the relativistic effect, in addition to the effects of the basis set and electron correlation and the gauge origin dependence. Relativistic effective core potential methods are not adequate for calculating heavy-element NMR parameters. In the present thesis, two-component quasi-relativistic ab initio calculations were performed for the 憕 nuclear magnetic shielding constants and chemical shifts (in section 1 of Chapter 2), and for ~25Te nuclear magnetic shielding constants and chemical shifts (in section 1 of Chapter 2). The numerical results show the importance of relativistic effects for reproducing the absolute shielding constants and chemical shifts of heavy elements. The nuclear shielding and spin-spin coupling constants are highly sensitive probes of the electronic structure of a molecule. At the same time, these parameters, in particular the nuclear shieldings, are also sensitive probes of intermolecular interactions and solvent effects. An understanding of how the surroundings affect the NMR parameters will thus provide valuable help for understanding observed NMR spectra and thus a case ab initio study of solvent effect on 29i NMR shieldings of chlorosilanes in the 20% (v/v) solution of acetone were carried out (in section 2 of Chapter 2). III + DOCTORAL DISSERTATION To quantitatively reproduce the experimental nuclear spin-spin coupling constants, it is usually necessary to use a very large basis set and to treat electron correlation at a very high level in the ab initio calculations. For example, Sekino et al. employed the full fourth-order many-body perturbation theory (MBPT) method and equation of motion coupled-cluster (EOM-CC) method to evaluate the Fermi contact term and non-contact terms. They concluded that, using fairly large basis sets, the EOM-CCSD provides results that agree with experimental indirect nuclear spin-spin coupling constants to within an average error of 13%. Unfortunately, it is currently difficult to carry out such high level of ab initio calculations on large molecules. Therefore, it is still interesting to develop reliable and accurate semiempirical methods that could be used for large molecule calculations. A generalized semiempirical relationship for calculating nuclear spin-spin coupling constants (慗, 2J and 3J) we...