Solvent effects in the proton resonance spectra of dimethyl-formamide and dimethyl-acetamide

Changes in the nuclear magnetic resonance spectra of dimethyl-formamide and dimethyl-acetamide have been studied in various solvents over a range of concentrations. It is shown that addition of aromatic solvents causes the relative chemical shifts of the two methyl resonances to change sign. This shows that no rapid reorientation of the dimethyl amino group occurs in these solutions at room temperature, and the shifts observed are interpreted in terms of the formation of loose complexes between the amide and the aromatic molecule.