Most importantly,the energetic cost

Most importantly,
the energetic cost of a 90
twist (38.8 kcal
mol at the
B3LYP level and 39.1 kcal
mol at the MP2 level) is much less than the strength of a CC π bond itself. As illustrated
here and examined elsewhere, decreasing the dihedral angle
from 180
results in a less pronounced distortion of the π bond
if it is accompanied by an out-of-plane bending—
pyramidalization—of the sp2 carbon atoms (6, 7). The degree
of pyramidalization appears in Figure 3.