Nuclear Magnetic Resonance Studies of Solvent Effects on the Hindered Internal Rotation in N,N-Disubstituted Amides. II. Higher Alkyl Amides

The temperature dependence of the chemical shifts due to hindered internal rotation was measured for diethylformamide, diethylacetamide, diisopropylformamide, diisopropylacetamide, and for solutions of these molecules in various solvents. In all cases, the chemical shifts showed rather unusual behavior. Conventional methods, coupled with certain approximations, were used to calculate ``activation energies'' as estimates for the ``rotational barrier'' to the internal rotation about the N–CO bond. These values were found to lie in the range of 5 to 26 kcal/mole. However, it is difficult to determine the real significance of these numbers; indeed, the behavior of the chemical shifts were such that no underlying concepts could be developed that would correlate the observed results except for the conclusion that the solvent plays a dominant role in the relationship between chemical shift and temperature.