ASSOCIATION STUDIES OF N-ACETYL-AMINO ACID N,N-DIMETHYLAMIDES IN CARBON-TETRACHLORIDE

The self‐association of N‐acetylglycine N,N‐dimethylamide, N‐acetyl‐L‐valine N,N‐dimethylamide, and N‐acetyl‐L‐phenylalanine N,N‐dimethylamide in carbon tetrachloride was investigated by using ir and 1H‐nmr methods. It was concluded from ir measurements that the associated species is the dimer formed as a result of the simultaneous formation of two intermolecular hydrogen bonds. This is supported by the results of 1H‐nmr measurements. Thermodynamic quantities for the association were determined from the temperature and concentration dependence of the NH proton chemical shifts of the sample solutions. Compared with the Gly derivative, L‐Val and L‐Phe derivatives have larger values of −ΔH for association, which shows good correlation with ΔvNH values, the difference between the maxima of the monomer and dimer bands, obtained from ir spectra. This is due to the less stable monomer conformation and to the stronger intermolecular hydrogen bonding of the dimers in L‐Val and L‐Phe derivatives. The line shapes of both methyl proton resonances of L‐Val residue and methylene proton resonances of L‐Phe residue were found to vary with concentration and temperature of the sample solutions. These data indicate that the rotation about the Cα—Cβ bond is restricted by the steric hindrance present in the associated dimers. All these experimental results can be related to the fact that L‐Val and L‐Phe derivatives have a warped framework because of the bulky side chains, whereas the Gly derivative has a planar framework. Copyright © 1979 John Wiley & Sons, Inc.