We are exploring the mechanism and potential application of the effect of isotopic substitution of NMR chemical shifts of nuclei that are several bonds away from the site of substitution. Studies of a number of differently substituted binuclear aromatic molecules (including cis- and trans-stilbenes, trans-azobenzene, diphenyl-acetylene and diphenylethane) show isotope-induced changes in carbon-13 chemical shifts through as many as 10 chemical bonds and as small as 0.64 parts per billion. Ab initio molecular orbital calculations for trans-stilbene demonstrate that a small change in the average length of the C-H/C-D bond (simulated by a decrease of 0.012 Angstrom unit on deuteration) causes changes in the computed carbon-13 chemical shifts that agree very well with experimental observation. Studies with liquid crystal solvents are being used to study weak molecular interactions. This method permits the measurement of very small distortions in molecular shape. Although extremely small distortions of other highly symmetric molecules (such as methane) are readily observed, fullerene (C-60) has been found to retain its spherical symmetry.
