Abstract
Abstract The results of DFT study of the substituent effects on molecular structures, relative stabilities, and mechanisms of intramolecular proton transfer in isolated acetyl and carbamoyl drivatives, for both neutral molecules and their radical cations, are reported. The reaction in the presence of one and two water molecules is also investigated. Furthermore, the effect of bulk water is considered for the system with two water molecules using the PCM continuum models. All of geometries of the local minima and transition states were optimized without symmetry restrictions at the B3YLP level of theory with D95++(d,p) basis sets and were verified by second derivative calculations. The DFT theory predicts the substituents with π-donating ability tend to reduce the energy barriers of enolization and ketonization in both drivatives and their radical cations. The relative stabilities of acetyl drivative tautomers exhibit that enolization is endothermic in the neutral molecules but exothermic in the radical cations. However, it shows that only endothermic process can be observed in carbamoyl drivative. The influence of water molecules largely reduce the barrier of the proton transfer by about 30 kcal/mol in striking contrast to the minor changes in the relative stability of keto and enol forms. In addition, the present of water molecules has the transfer of C-hydrogen or N-hydrogen onto the water-oxygen advance than the transfer of the hydrogen from this water to the oxygen of the acetaldehyde or formamide.