Are Polarized Cyclobutadienes Non-Aromatic?

Abstract

A series of non-alternant cyclobutadienes, both hydrocarbon and heteroatom-substituted, are examined by means of PM3 semiempirical molecular orbital calculations. The results show that even very substantial polarization has no simply predictable effect on antiaromatic character as measured by the singlet–triplet energy gap. MP2 calculations on selected hydrocarbons lead to the same conclusion.