Interplay of the Ring and Steric Strains in the Highly Substituted Cyclopropanes

Using the concept of a complete set of homodesmotic reactions for the analysis of molecular energetics of polysubstituted methyl- and fluorocyclopropanes allows assessing the strain energy SE of cyclopropanes, free from interfering effects, in full accordance with the IUPAC definition ("relative to a reference ... hypothetical 'strainless' structure"). The correct SE calculation requires quantifying nonvalence interactions in the products of formal homodesmotic reactions (HDRs) using a routine multiregression analysis. The complete HDR set provides the information necessary for the analysis, namely, the heat effects of HDRs calculated by the G4 composite method and the wide set of reference compounds with various combinations of nonvalence effects. We have found that the SE value for methylcyclopropanes lies in the range from 117.0 (1.1-dimethylcyclopropane) to 146.1 kJ/mol (hexamethylcyclopropane). It is the sum of the ring strain energy RSE = 117.9 ± 0.3 kJ mol, which does not depend on the number of methyl substituents, and the Pitzer strain energy of 4.4±0.1 kJ/mol per one contact (the standard deviation is shown as an error of determination). In the series of fluorocyclopropanes, SE varies from 137.9 (monosubstituted cyclopropane) to 260.0 kJ/mol (hexafluorocyclopropane) and well correlates with the ∑DBCP parameter deduced from the QTAIM analysis of the electron density of the compound, representing the total deviation of bond critical points from geometrical C-C bond lines of CC bonds. The ∑DBCP parameter characterizes the curvature of banana-like bonds in cyclopropanes.