In other words, the strain energy in trans cycloalkenes may
be manifested in the aliphatic portion of the ring (effects 1
and 2 above), the CC double bond (effects 3 and 4 above),
or both.
Olefin strain energy (OSE) has been defined as the difference
in strain energy between an alkene and its hydrogenated
product. OSE can be obtained computationally by
comparing the calculated heat of hydrogenation of an alkene
with the calculated heat of hydrogenation of an
unstrained reference alkene (3, 16). For example, in calculating
the OSE caused by the pyramidalized double bond in
X, XI (Figure 9) would be used as the unstrained reference
alkene. OSE is ΔHhydrogenation(X) − ΔHhydrogenation(XI). We used
the cis isomer as the reference for each trans cycloalkene isomer.
Thus, the calculated energy difference between the cis
and trans isomers of each cycloalkene was used as part of the
OSE value. The cis isomer is not exactly a perfectly matched
unstrained reference because there is a difference in energy
between unstrained cis and trans double bonds that needs to
be taken into account. Mathematically we calculated the OSE
for the trans isomers as ΔE(cis-trans) cycloalkene − ΔE(cis-trans) 2-butene.