A Nuclear Magnetic Resonance Investigation of Molecular Complexes of Polar Molecules in Aromatic Solvents

In order to confirm the existence of specific molecular complexes in solutions of polar solutes in aromatic solvents previously proposed, the temperature variation of the proton resonance shifts of the solutes acetonitrile, p-benzoquinone, and N,N-dimethylformamide in 5 mole% concentration in toluene were measured. The large temperature coefficients of the solute shifts observed strongly support molecular complex formation. For the same solutes dissolved in methylcyclohexane the temperature coefficients were negligibly small. The temperature at which onset of free rotation of the N-dimethyl group of N,N-dimethylformamide occurs is solvent and concentration dependent. These observations are consistent with the proposed geometry of the molecular complex in the aromatic solvents.