| A novel method for the interpretation of nuclear magnetic shielding tensors calculated from ab initio molecular orbital theory, called Natural Chemical Shielding (NCS) analysis, has been developed, and is implemented in a computer program. NCS analysis uses the theory and methods of Natural Bond Orbital (NBO) analysis to determine the magnetic contributions of natural bond orbitals to the calculated shielding tensors of atomic nuclei in molecules. The nuclear shielding tensors are independently obtained from the gauge-including atomic orbitals (GIAO) method, which has gained wide acceptance and is employed by many ab initio quantum chemistry program packages.;The chemical bond and lone pair NBO contributions to the shielding tensors of nuclei in methane and carbon monoxide agree closely with those calculated by the individual gauge for localized orbitals (IGLO) method. In the case of formaldehyde, however, the IGLO and NCS contributions are in wide disagreement, due to their different underlying description of the oxygen lone pair orbitals.;The NCS analysis has been performed on calculated shielding tensors of proton, carbon, and nitrogen nuclei in hydrogen bonded clusters of HCN. An important result is that there is only a minor contribution from the delocalized electron density associated with the H-bonds. Surprisingly, the H-bond lone pairs yield negative, rather than positive contributions to the H-bonded proton chemical shielding. A statistical thermodynamic model of liquid HCN, which treats the clusters as persistent species, predicts the right order of magnitude and correct sign of the shielding vs. temperature relationship for each of the three nuclei.;The NCS analysis is used to investigate the effect of electronic delocalization on the alkyne carbons in substituted acetylenes, as part of a larger study to investigate the nature of the unusually small anisotropy of the terminal carbon of phenylacetylene, which is due in part to the Ramsey diamagnetic components of the shielding tensor, as opposed to the paramagnetic part of the tensor. The NCS analysis of the acetylenic shielding tensors finds only very weak contributions from delocalized features of the electronic structure.;Finally, the last chapter of this dissertation describes the hardware and software of a liquid-state NMR spectrometer developed in this laboratory. |
