Nuclear Magnetic Resonance Studies of Solvent Effects on the Hindered Internal Rotation of N,N-Disubstituted Amides. I. Dimethylformamide

A number of chemical-shift measurements were made on dimethylformamide (DMF) as a pure liquid and in the solvents carbon tetrachloride, acetone-d6, fluorotrichloromethane, and hexamethyldisiloxane. These measurements included (1) the temperature dependence of the methyl-doublet separation, (2) the effect of solvent and concentration on the methyl-doublet separation, and (3) the effect of temperature, solvent, and concentration on the chemical shift of the formyl proton resonance and on the center of the methyl doublet. The results show large solvent effects that are interpreted in terms of dipolar association between DMF and solvent. No evidence for hydrogen-bonded species was found. Activation energies for the barrier to hindered rotation were calculated and were found to depend strongly on the nature of the solvent.