Calculation of the Formation Enthalpy and the Ionic Equilibrium Constant of Organic Substances by Quantum Chemistry Methods.

Using composite methods of quantum chemical calculations and the atomization method, the values of the enthalpy of formation for a test set of 79 compounds in gas phase have been calculated with accuracy of 3.8, 4.5, and 4.7 kJ/mol for the G4, G4MP2, and CBS-QB3 methods, respectively. According to the results of statistical analysis of deviations of theoretical values from experimental ones, the presence of an error occurring at the geometry optimization step due to neglected correlation effects has been revealed. For aliphatic amines and alcohols from the considered set of compounds, the values of the pKa for aqueous solutions have been calculated using the G4 method including solvent effects according to the CPCM model. The existence of a correlation between the error in the pKa calculation and the differences in energy values between the highest occupied and lowest unoccupied molecular orbitals for conjugated acid and base forms in the solution phase has been revealed. A method has been suggested to refine the calculation results, using which the pKa values for 20 amines and 20 alcohols have been determined with accuracy of 0.67 and 0.87 pKa units, respectively. [ABSTRACT FROM AUTHOR]