5b, X= CH, 2, 3, and 4, both sets of interfering substituents in 9, 9'-bitriptycyls are directed toward each other. In addition, the interfering groups are small hydrogen atoms. This minimizes ground-state strain. The combination of a high rotational transition state energy with an uncongested ground state is most favorable for a large rotational barrier. 2b As anticipated, 2, 2'-dimethyl-9, 9'-bitriptycyl (5b) was found to have an enormous rotational ...
