[4]Paracyclophane: MNDO and STO‐3G molecular structure and strain energy

The molecular structure of [4]paracyclophane was optimized at the semiempirical MNDO and the ab initio STO‐3G level. A comparison of the results showed that the benzene ring is much less bent at the STO‐3G level. Although some bond alternation is predicted at both levels of theory, the aromatic carbon–carbon bond lengths are still in the range typical of highly delocalized compounds. The calculated strain energy [ SE (tot.)] of [4]paracyclophane is larger for the STO‐3G structure. Nevertheless, the distributions of SE (tot.) over the bent benzene ring [ SE (bb.)] and the oligomethylene bridge [ SE (br.)] are the same for the MNDO and STO‐3G structures.