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 propanes with substituents of various nature. The SE values

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... er 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.