Abstract

Graph theory based method is proposed for constructing a complete set of homodesmotic reactions (HDR) for studying the molecular energy of organic compounds. The efficiency of the method is shown by the example of ring strain energy Es calculation for a test set of 65 cyclic compounds. The absolute enthalpies of the cycles as well as of reference compounds used in HDRs were calculated using the composite methods G3 and G4. Depending on the structure of the studied cyclic compound, the thermal effect of HDR was shown to be a combination of uncompensated non-valence effects: ring strain, gauche interaction, hydrogen bonding, as well as difference in far environment of atoms, which is observed as the effect of small molecules. Within the framework of a complete set of HDRs, it is possible to separate all of these effects and determine Es free from secondary interactions. The obtained Es set allows tracing its relation with different structural factors such as cycle size, presence of hetero- or sp2-hybridized atoms in the cycle, effect of the substituent etc.

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