Activation energies amino group restricted rotation

Kleinpeter and coworkers79 have recently studied restricted rotation and ring inversion in highly substituted anilines, 23-31 (Figure 10), prepared by the reaction of cyclic ylidene malonitriles with acetylene (di)carboxylic acids. Table 8 presents the experimental and calculated activation energies for the restricted rotation of the amino group in 23-31. 

TABLE 8. Experimental and calculated activation energies for the restricted rotation of the amino group in 23-31... 

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.64,86,88,89 194 Shoup et al. 89,90 measured the activation parameters for the rotation of the dimethylamino group in N8-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 Reese195 for 26c. In this last case, however, the method by which the activation energy was obtained has not been described (usually approximate... 

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