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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.

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Composite method G4 was used to calculate strain energies (SE) of mono-and disubstituted fluoro-, chloro-, methyl-, methoxycyclopropanes using the authors' methodology of a complete set of homodesmotic reactions. A QTAIM topological analysis of the electron density distribution (16 compounds with the previously studied ones) was carried out in order to reveal the quantitative "structure-activity" relationship of SE in the test set of cyclopropanes with substituents of various nature. The SE values (per one C 3 -cycle) vary within wide limits, from 103.9 kJ/mol for acetylcyclopropane to 164.9 kJ/mol for 1,1-difluorocyclopropane. The QSAR for the SE values was examined using three structural indices of cyclopropane compounds, namely: the deviation of the bond critical point from the bond line (DBCP), the difference in the bond path length and the distance between atomic attractors (BPL -GBL), and the Laplacian of the electron density at the critical point of the cycle ( 2 ). With the exception of bicyclobutane (SE per cycle = 141.5 kJ/mol), an excellent linear correlation of SE versus all indices in the entire range of strain energies was established: SE = (8 ± 5) + (1900 ± 70) × (BPL − GBL) kJ/mol, R = 0.991, maximal deviation MAX = 11, mean absolute deviation MAD = 5 kJ/mol. The same quality was observed in the case of the 2 index. The brief discussion on the nature of the observed dependencies was given. The main factor of the SE increasing was suggested to be enhanced polarization of the cyclopropane C-C bonds due to effect of both electron donor and acceptor substituents. The found fair correlations make it possible to propose the established correlations for the express estimation of strain energies in the compounds containing cyclopropane fragments.

Organic free oxyl radicals are intermediates of many important biological and technological processes. Determination of their thermochemical characteristics makes it possible to analyze and predict these processes, and is also a non-trivial task of modern physical organic chemistry. In this work the determination of the standard enthalpies of formation (∆fH°) of organic oxyl radicals was carried out using the homodesmotic methodology. The radicals of the normal and branched structure containing from two to nine carbon atoms in the chain and having a primary or secondary radical center were selected as a test set of compounds. The sets of group exchange homodesmotic reactions were constructed for each structure. Thermal effects of the reactions were calculated using absolute enthalpies of all participants of the formal process obtained in the M062X/cc-pVTZ approximation, and then were applied to obtain ∆fH° of radical under study using known values of ∆fH° for reference compounds. The calculated enthalpies of formation for oxyl radicals were found to be in good agreement with available literature data. Based on the found values of the enthalpies of formation of oxyl radicals, the O-H bond dissociation energies in alcohols were calculated, being 435.6 ± 0.2 kJ/mol for the set of primary alcohols and 441.8 ± 0.9 kJ/molfor secondary ones. The obtained results indicate a ~6 kJ/mol stronger O-H bond in secondary alcohols and an insignificant effect of the carbon chain length on the O-H bond strength in the homologous series for both primary and secondary alcohols.