Evaluation of bonding energies in organic compounds by quantum mechanical methods

Full text: In this paper we proposed a quantum mechanical evaluation model for bonding energies in organic compounds. Values of bonding energies were determined as homolytic dissociation energies. The molecular structures were built and geometrically optimized, in the first stage, by the M M2, MM+, AMBER molecular mechanics methods. Energetically minimizations were refined using semiempirical methods. In agreement with the chosen molecular structure we used, methods like CNDO, INDO, PM3, AM1. Binding energies were evaluated by simulation of homolytic dissociation processes. Using quantum-mechanical methods we characterized molecular structures and fragments resulted from dissociation of chemical bonds. Total binding energies (TBE) of un-optimized fragments, the fragments geometrically minimized post-fragmentation, transitional state and initial molecular structures were determined and analyzed. The enthalpy of reaction (ΔH) was determined from the difference between TBE of fragments and TBE of system. The value of activation energy (Ea) is indicated by the difference between transitional state TBE and TBE of system. ΔH and Ea are correlated with binding energy. Method's accuracy and application fields of different evaluation modalities (optimized, non-optimized, transition states) were achieved by intercomparison of obtained values with the experimental data in the literature. More than 74 organic and inorganic compounds (alkanes, alkenes, alkynes, aromatic structures, halogenated compounds, alcohols, peroxides, aldehydes and ketones, carboxylic acids, ammines, nitro derivatives, nitrates, nitrites, thiols, thioethers, water, hydrogen peroxide, ammonium, hydrazine etc.) were analyzed. (author)