Rotational barriers ground state energy, effect

Having obtained stable rotamers of compound 6, Staab and Lauer (45) extended the work to see whether rotamers of amides that normally have lower barriers as a result of a disfavored canonical structure 2 due to electronic effects are also isolable. They found that the rotamers of 2,4,6-trwm-butylbenzoben-zimidazolide (7) were isolable, but those of the corresponding imidazolide (8) were not. The barrier to rotation of the former in hexachlorobutadiene solution was 28.7 kcal/mol for the E Z process at 80°C. The barrier to rotation of the latter was estimated at less than 23 kcal/mol. It is possible to attribute this result to electronic effects that raise the ground state energy, because the aromatic... 

The study of the rotational barriers in captodative-substituted radicals leads to the following conclusions the barriers are noticeably lower or higher than in cases of dicaptor or didonor substitution. This can be interpreted as the consequence of a captodative effect in these systems. However, the amount of special influence on the barrier height in energetic terms is small and may sometimes not exceed the numerical uncertainties. A derivation of absolute values for stabilization energies of captodative-substituted radicals by this procedure is not possible, since both ground and transition states are affected by substitution. The lowering of the barriers... 

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