Density functional theory study of formamide–formamidic acid tautomerization

Abstract

AbstractThe mechanism of tautomerization reaction of formamide–formamidic acid in gas phase and in water has been completely investigated in the present study using, density functional theory (DFT) and the second‐order Møller–Plesset perturbation (MP2) method. Large basis sets 6‐311++G(d,p) have been employed to determine the equilibrium structure and vibrational frequencies. Activation energy of the non‐water F → FA reaction is 202.9 kJ/mol at B3LYP/6‐311++G(d,p) level. Extensions to situations in which tautomerization is assisted by neighboring solvent molecule(s) are considered. If a single H2O molecule is considered, three stable structures and transition states of formamide–water complexes are found. If the water locates between the HN group and the CO moiety, the activation energy decreases to 94.5 kJ/mol. If the water locates in other positions, it can cause the activation energy to increase to 2–4 kJ/mol. When two intimate H2O molecules locate between the HN group and the CO moiety, the activation energy decreases to 89.3 kJ/mol. Intimate solvent participation is thus found much more strongly affecting the activation energy than the overall thermodynamics in this case. Finally, the solution phase studies are also carried out using the Onsager reaction field model, with a range of dielectric constants from 2 to 80 at the B3LYP/6‐311++G(d,p) level. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004