To test the validity of our methods we calculated the energy
of the 90 twisted triplet 2-butene diradical relative to
the energy of trans-2-butene (Table 1). This structure, illustrated
in Figure 1, arguably represents the barrier to rotation
about a double bond (11). These values are in good agreement
with experimental estimates of the π-bond strength in
ethylene of approximately 65 kcalmol (12). However, ab initio
molecular dynamics calculations suggest that trans–cis
isomerizations of ethylene and stilbene involve structures that
are not planar: they are both twisted and pyramidalized (13).
The authors would like to emphasize that the preceding
discussion is not new to the literature. Pyramidalization in
twisted alkenes has been well documented (3–7). Our purpose
in writing this article is to provide instructors with a
simple vehicle for rectifying the common misrepresentation
of CC double bonds as rigid and inflexible. A suitable entry
point for introducing students to the idea of a flexible
CC double bond is in conjunction with discussions of cis
and trans isomers of cycloalkenes (14).