DFT Comparative Study of the Formamide and Fluoro-FormamideTautomers and Their Corresponding Transition States

Seventeen tautomers of formamide [NH3CO] and thirty one tautomers of fluoro-formamide [NH2FCO] were examined by density functional theory (DFT) calculations. Five methods of DFT (B3LYP/ 6-311++G, B3LYP/ 6-311++G(2df,2p), B3LYP/cc-pVTZ, wB97X-D/6-311++G(2df,2p) and wB97X-D/ cc-pVTZ) were used to calculate the energies and optimized geometries of formamide tautomers. B3LYP/cc-pVTZ method was determined as the best method and used to calculate relative energies, optimized geometries, mulliken charge distributions, Gibbs free energies and enthalpy of formations of all proposed fluoro-formamide/fluoro-formamidic acid tautomers. Ten transition states of formamide tautomers and nineteen transition states of fluoro-formamide tautomers were determined by vibrational frequency calculations (they have an imaginary value of frequency). Using B3LYP/cc-PVTZ, the relatively small energies of formamide (A1) and formamidic acid (A6) (-446246 and -446193kJ.mol -1 , respectively) participate to it being more stable than other formamide/formamidic acid tautomers, as well as for fluoro-formamide (C1) (-706947 kJ.mol -1 ) and fluoro-formamidic acid (C3) (-706855 kJ.mol -1 ). The stability of fluoro-formamide at carbonyl and at nitrogen was discussed using two thermodynamic calculations: Enthalpy of formation [∆H(298)] and Gibbs free energy (∆G) values, positive ∆G values (59 to 114 kJ mol-1) for fluoro-formamidetautomerism confirms the stability of fluoro-formamide at carbonyl.