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 delocalized 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 sigma-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 Si(3)H(8) (6.3 kcal mol(-1)) and Si(4)H(10) (4.2 kcal mol(-1)). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported.
Wu
Wei
W
Department of Chemistry, College of Chemistry and Chemical Engineering, The State Key Laboratory for Physical Chemistry of Solid States, Center for Theoretical Chemistry, Xiamen University, Xiamen, Fujian 361005, China. weiwu@xmu.edu.cn
Ma
Ben
B
I-Chia Wu
Judy
J
Schleyer
Paul von Ragué
Pv
Mo
Yirong
Y