Exchange calculations ring-flips

Quantitatively, line-shape analysis was used to determine rate data for these stereoisomerizations in terms of the two-ring flip mechanism. 32> The associated free energies of activation for the various exchange processes at 20 °C are shown schematically in Fig. 10. For the equilibrium BB AA, AG°2o is 0.3 kcal/mol. For the conversion of BB to AA, the calculation yielded AG 0 16.2 kcal/mol. and for the reverse reaction (AA- -BB), AGt0 15.9 kcal/mol. The barrier to enantiomerization of B and B is AG%B 14.6 kcal/mol. Thus, at 20 °C the enantiomerization of B and B is energetically more favorable by 1.6 kcal/mol than that of A and A. 

Quadrupole echo lineshapes for the phenylene ring-flip motion (or any jump model) are simulated with an n-site exchange calculation one must also include the exchange-caused relaxation between the pulses (echo distor-... 

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