Effects of Intermolecular Interactions and Intramolecular Dynamics on Nuclear Resonance

There are several facets to our interest in temperature, solvent, and isotope effects on NMR chemical shifts. Excepting molecular beam studies, the application of NMR to the elucidation of molecular structure and mechanisms of molecular reactions nearly always involves observations of molecules in some environment. Before NMR can be used to obtain structural and mechanistic information unambiguously, the effects of the environment of a molecule on its NMR spectrum must be understood. For example, when a chemical shift is observed in an enzyme-substrate complex for a nucleus in the substrate, the shift can be used to infer structural (e.g., conformational) changes in the substrate induced by the enzyme, but first this requires a separation of the effect due to a change in environment. At the same time, NMR studies which are necessary to understand and thereby separate out the unavoidable complications due to the effects of the environment may provide fundamental information on the environment itself. This is an equally worthwhile objective, the use of NMR as a probe of intermolecular forces and intramolecular force fields. Thirdly, such studies are intimately connected with the nuclear magnetic shielding function itself, its variation with molecular geometry or intermolecular separation. While closely identified with the NMR technique, nuclear magnetic shielding is a molecular property in the same class as such other properties as electric polarizability and magnetic susceptibility which depend upon the molecular electronic wave functions and are interesting in their own right. A nuclear resonance signal is a site-specific sensor of intermolecular forces and intramolecular dynamics. As such, it holds the promise of being useful as a unique probe of the intramolecular potential (anharmonic force field) of a single molecule, of the intermolecular potential between two molecules, and of the structure of fluids and solutions. It offers possibilities that are unique in comparison to other physical techniques. One sees only X-A and X-X (not A-A) interactions when using a nucleus in molecule X as a probe, so that it is possible to observe primarily X-A interactions by making the mixture an infinitely dilute solution of X in A. Experimental NMR measurements are capable of extremely high resolution and precision. Furthermore, a great deal of redundancy in information can be obtained so that strong restrictions can be placed on any theoretical interpretation: one can use several nuclei in a colliding pair of molecules as probes, thereby obtaining