Mechanism of the water-catalysed addition between melamine and formaldehyde: a theoretical investigation

The addition reaction between melamine and formaldehyde in aqueous solution was investigated theoretically using the B3LYP and MP2 methods. The self-consistent reaction field method was employed with the polarisable continuum model to simulate the implicit solvent effects. The specific water molecules explicitly added in the computational models were found to play the role of catalyst that assists the proton transfer from amino nitrogen to carbonyl oxygen. In the absence of specific water molecules, the activation energy was calculated to be 134.2 kJ mol(-1) at the MP2/6-31+G** level, while in the presence of one and two water molecules the barriers were lowered to be 62.7 and 52.9 kJ mol(-1), respectively. The nucleophilic attack precedes the proton transfer, and the proton donation from water to carbonyl oxygen is ahead of the proton abstraction from the amino group. The potential energy profiles show that a zwitterionic-like intermediate previously found in the reactions of formaldehyde with ammonia and methylamine does not exist on the PES of the present reaction, indicating the reaction occurs with a concerted mechanism of asynchronous character.