Solvent Effects on Nuclear Magnetic Resonance Chemical Shifts

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Abstract
The basic nuclear magnetic resonance (NMR) experiment involves the subjection of a macroscopic assembly of atoms or molecules to the influence of a strong static magnetic field and of a much weaker one derived from a radiofrequency signal. If the magnitude of either the former field or the frequency of the latter is maintained constant and the other varied, conditions may be achieved where the combined interactions of the two fields with magnetic nuclei in the sample lead to discrete changes in the energy of the nuclei. These energy changes can be detected by various means [1,2] and a spectrum of nuclear resonance conditions may be recorded. At an early stage in the development of the technique it was discovered [3] that the resonance condition for a particular nuclide was influenced by its chemical environment. Subsequently, it was established that a compound containing several nuclei of the same type could produce a spectrum exhibiting separate absorption bands which were chemically shifted, depending on the environments of the nuclei. Moreover, it was found that the chemically shifted bands could be composed of multiplet lines [4] due to spin interactions between the various nuclei. A substantial proportion of the use of NMR has been devoted to the determination of chemical shifts (6) and spin-spin coupling constants (J) and their interpretation in terms of the functional group type of the nuclei involved, their dispositions, and their relative quantities. However, the appearance of NMR spectra is not influenced by intramolecular factors alone, but by intermolecular effects also which can modify chemical shifts significantly and, to a lesser extent coupling constants. Consequently, before NMR can be used to elucidate molecular structure unambiguously, the effects of the environment of a molecule on its spectrum must be understood. While this may be a satisfactory objective per se, it should not be forgotten that a full understanding of medium effects additionally may provide new tools for the determination of molecular structure and also new understanding of the behavior of the medium itself, which may have greater fundamental importance.