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Is Cyclopropane Really the σ‐Aromatic Paradigm?
Author(s) -
Wu Wei,
Ma Ben,
IChia Wu Judy,
Schleyer Paul von Ragué,
Mo Yirong
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200900586
Subject(s) - cyclopropane , aromaticity , delocalized electron , sigma , chemistry , computational chemistry , cyclobutane , ab initio , sigma bond , molecule , physics , ring (chemistry) , organic chemistry , quantum mechanics
Dewar proposed the σ‐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 ‘σ‐aromaticity’,”—other analyses, also indirect, resulted in wide‐ranging estimates of the σ‐aromatic stabilization energy. Moreover, the aromatic character of “in‐plane”, “double”, and cyclically delocalized σ‐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 σ‐ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the σ‐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 σ‐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 σ‐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.