Rotational barriers base effect

The electron correlation repulsion theory can be used to explain the configurations of many molecules. We have also used it to explain base strengths in cyclic compounds and the relationships of group Va, Via, and Vila bases. A. number of experiments come to mind to test its application to the latter problem, such as observing the effect of protonation on rotational barriers. It remains to be seen whether the facts will support this simple notion. 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

It may be pertinent to mention here a recent criticism of the gear effect by methyl groups, since most of the known examples used in the study of the gear effect are in aromatic molecules. Based on dynamic NMR data on the internal rotation of the 9-methyl group in 42, which indicated a far lower barrier for 42a than for 42b, Oki... 

A method for calculating the barriers to internal rotation has recently been proposed (Scott and Scheraga, 1965). It is based on the concept that the barrier arises from two effects, exchange interactions of electrons in bonds adjacent to the bond about which internal rotation occurs, and nonbonded or van der Waals interactions. The exchange interactions are represented by a periodic function, and the nonbonded interactions either by a Buckingham 6-exp or Lennard-Jones 6-12 potential function, the parameters of which are determined semi-empirically. (The parameters of the nonbonded potential energy functions are discussed in Section VB.)... 

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