Carbon-13 Nuclear Magnetic Resonance Spectroscopy. Solvent Effects on Chemical Shifts

Abstract

The ^(13)C chemical shift of chloroform has been measured in a variety of solvents. Relative to dilute solution in cyclohexane, all of the solvents studied resulted in downfield shifts which, with the exceptions of benzene and acetonitrile, correlate linearly (r = 0.982) with the changes in proton shifts in the same solvents. The results, taken with the variation of the ^(13)C-H coupling constant with solvent, suggest that the solvent effects arise from changes in the average distance of the bonding electrons in the chloroform C-H bond as the result of intermolecular association. Contributions from the anisotropy of benzene and acetonitrile indicate that measurements of the chemical shift of more than one nucleus in the same molecule may allow detection and estimation of neighbor anisotropy contributions to chemical shifts.

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