AM1 Theoretical Analysis on the Effect of Some Substituents (X) at Carbonyl Carbon Position of the Formamide-Formamidic Acid Tautomerization System. (X = F, Cl, Br, I, CN, NO2, CH3 and CF3)

Heats of formation, entropy changes, free energy changes and equilibrium constant values at 25 °C were calculated by the use of AM1 semiempirical quantum chemical method. The calculations were performed on unsubstituted formamide- formamidic acid tautomeric system, as well as on substituted systems. The equilibrium constant value for the unsubstituted system was found to be 2.21 × 10–9. It is found that the tautomerization equilibria for the substituted systems, for all of the substituents employed in this work, have lower equilibrium constant values compared to that obtained for the unsubstituted system. These values were found to be several orders of 10 less, depending upon the substituent. Geometrical parameters (bond lengths and bond angles) were, also, reported for all of the compounds studied in this work. The results were compared to the available experimental values, and to those obtained from ab initio and DFT obtained at different levels of theory.