Is Cyclopropane Really the sigma-Aromatic Paradigm?

Dewar proposed the sigma-aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts-There is no need to involve 'sigma-aromaticity',-other analyses, also indirect, resulted in wide-ranging estimates of the sigma-aromatic stabilization energy. Moreover, the aromatic character of in-plane, double. and cyclically de-localized sigma-electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J Phys. Chem. A 2007, 111, 8163-8169) challenged the existence of an induced a-ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the sigma-aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc-PVTZ level, the sigma-aromatic stabilization energy of cyclopropane is, at most. 3.5 kcal mol(-1) relative to propane, and is close to zero when n-butane is used as reference. Trisilacyclopropane also has very little sigma-aromatic stabilization, compared to Si3H8 (6.3 kcal mol(-1)) and Si4H10 (4.2 kcal mol(-1)). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported.