SynopsisSemi-empirical energy calculations for an internal Pro-Pro dimer are presented that take into account the nature of the flexibility of the proline ring due to its puckering. Calculations show that three stable conformations are available for the dimer: the cis (w = O", + = 160'); the trans (w = 180", $ = 160", also referred to as trans'); and the cis' (w = 180°, tb = -40') conformations. The best conformational pathways between these stable conformations are determined. Calculations also show that the barrier for cis'-trans' conversion is of the same order of magnitude as that for cis-trans conversion.

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The energy barriers to chair-chair interconversion in tri-N-methyl-, -ethyl-, -isopropyl-, and -t-butyl-hexahydrotriazine have been measured by the study of the temperature-dependence of their nuclear magnetic resonance spectra. The free energies of activation vary with the substituent upon the nitrogen atoms, and the rate of ring inversion is increased by substituents that facilitate nitrogen inversion. Solvent effects confirm that the observed rate process is ring inversion and not hindered nitrogen inversion.

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