| Ab initio computational methods and solid-state NMR spectroscopy were used to examine the 13C chemical shielding in a variety of small organic compounds. In the spirit of Ramsey's perturbation formalism, the IGAIM method was used to partition the total shielding into the paramagnetic and diamagnetic contributions using a nucleus-centered gauge origin. In addition, IGAIM was used to calculate the shielding on a molecular orbital (MO) basis. It was found that the shielding variation within a large number of compounds is due almost exclusively to the paramagnetic part, and that the paramagnetic contribution in turn can be simply explained in terms of the interactions of occupied MOs with virtual MOs via the angular momentum operator.;The correlation between shielding and electron density is re-examined within a group of monocyclic aromatic ions (including benzene). In this series, the shielding depends upon the electron population of the p-orbital perpendicular to the molecular plane rather than the total electron density.;Although the sign of the diamagnetic part of the total shielding is always positive (shielding), the sign of the paramagnetic part can be either positive or negative. For example, in compounds such as FCl, CH3F, FC ≡ CH, FNH3+ and CH3NH3 +, the signs of the paramagnetic terms at two atoms with different electronegativies depend on the relative phases of the atomic orbitals centered at these two nuclei.;The shielding trends in allene (along with the higher cumulenes) were studied, and the difference between the shielding along the carbon bond axis in allene and acetylene was examined in more detail. Also, the substituent effect on shielding for a large number of mono-substituted acetylenes was investigated. In accord with expectation, it was found that substituents which maintain strict cylindrical symmetry (e.g., F, Cl, CN) do not affect the shielding along the acetylene bond (z-axis). They do, however, significantly alter the isotropic shielding at the acetylenic carbons by changing the shielding components perpendicular to the molecule. Planar substituents which interact with the pi-system of the acetylene group, on the other hand, give marked changes in the z-axis component at Cbeta but, interestingly, not at Calpha. This is easily understood in terms of the symmetry and nodal distribution of the MOs in these compounds. In addition, the principal shielding tensor components for the acetylene carbons in propiolaldehyde, trimethylsilylacetylene, methoxyacetylene (Calpha only), and acetonitrile, were obtained from their static powder patterns.;The final chapter describes an investigation into the conformational properties of several mono-substituted cyclohexanes. The experimental conformational enthalpies, determined at 157 K, were found to be (kcal/mol) 1.76 +/- 0.10 (methyl), 1.54 +/- 0.12 (ethyl), and 1.40 +/- 0.15 (isopropyl). |
