Characterizing the mechanism of the double proton transfer in the formamide dimer.

The double proton transfer in the formamide dimer is characterized computationally by combining density functional theory and ab initio methods. The intrinsic reaction coordinate (IRC) is obtained at the B3LYP level of theory. Energies of several points along the IRC are treated by the more rigorous focal point method to test the validity of the B3LYP functional. The reaction mechanism is examined in terms of the energy profile, the reaction force, the chemical potential, and the reaction electronic flux. The energy profile for the activation process of the formamide dimer to the imino ether product obtained with the B3LYP functional is in agreement with the results of the focal point method. Together with the reaction force analysis and the reaction electronic flux a precise assignment of the structural and electronic contributions to the activation barrier becomes possible. The results show that the reaction starts with a structural rearrangement, where the two dimers approach each other, and is followed by electronic changes before the system reaches the transition state. This electronic contribution to the activation barrier steers the activation process. After the transition state is reached, deviations of the B3LYP functional from the more accurate focal point energies become apparent, where the errors may be rationalized in terms of the treatment of exchange. The inconsistency could be assigned to the incapacity of the functional to describe delocalization effects over the whole system.