Reese, Martin

Rotational restriction of the amino group has been observed by the NMR spectroscopy technique in a number of cytosines in solution as well as in the crystalline state. Shoup et al. ° measured the activation parameters for the rotation of the dimethylamino group in N -dimethylcytosine derivatives. With one exception, the range of activation energies is 15-18 kcal/mole. The activation energies found for 26a and 26b are about twice those reported by Martin and Reese for 26c. In this last case, however, the method by which the activation energy was obtained has not been described (usually approximate... 

If carbonyl protonation fails to occur in aromatic amides where extra resonance stabilization of the carbonium ion is possible, one would scarcely think that it would be important for aliphatic amides. However, the data of Martin and Reese (248) show that increased branching of the acyl portion of the molecule (from acetamide to pivalamide) decreases the basicity to perchloric acid titration in acetic acid as would be expected for hyperconjugative stabilization of a carbonium-like species formed by oxygen protonation. Again, solvation may be the deciding factor. In contrast, the stabilities of iodine complexes of a few acyl substituted A, iV-dimethyl acetamides follow inductive rather than hyperconjugative order (97). 

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