The proton relaxation and Overhauser effect in the nuclear magnetic resonance spectra of some amides

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The relaxation behavior of some amides was investigated in their high resolution spectra by the direct saturation–recovery and Overhauser effect methods. The recovery curves and steady-state Overhauser effects were obtained for solutions of dimethylformamide with double- and triple-resonance irradiations. The results were analyzed for 3- and 4-spin systems and the values of relaxation parameters were calculated. It was shown that for dimethylformamide the one-exponential recovery curve approximation gives the T1 values with a precision better than experimental error (±2%). The observed difference in relaxation times for methyl group protons was explained by different correlation times and more restricted rotation of 1 of the groups. The correlation times for dipole–dipole interactions of methyl groups with the formyl proton were also evaluated with some reasonable assumptions.The change of the relaxation rate of N-methyl protons in C6H6 and C6D6 solutions was explained by formation of the collision complex with amide molecules and specific intermolecular dipole–dipole proton interaction. This effect could be useful for determination of the cis- and trans-conformations in N-methyl substituted amides and peptides as an absolute method because it does not require measurements on both isomers.