DFT study of the proton transfer in the urethane formation between 2,4-diisocyanatotoluene and methanol

The proton transfer effects on the urethane formation between 2,4-diisocyanatotoluene (2,4-TDI) and methanol was investigated by using density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level. The calculations show that the direct addition of methanol to 2,4-TDI follows a concerted path with an energy barrier of 31.71 kcal mol-1 and an activation free energy of 39.32 kcal mol-1. With the methanol, diphenylamine, and methyl N-methylcarbamate molecule serving as a proton transporter or a reactive catalyst, the energy barriers were significantly reduced to 12.28, 17.93, and 20.13 kcal mol-1 respectively, and the free energies of activation were lowered to 25.22, 32.54, and 36.86 kcal mol-1 correspondingly. The results indicate that the labile hydrogen-containing compounds play a key role in accelerating the reaction rate and the proton transfer. In the case of one extra methanol molecule serving as a proton transporter, the energy barrier was estimated to be 12.28 kcal mol -1, which is in good agreement with the experimental activation energy between 8.6 and 12.0 kcal mol-1. The urethane formation between 2,4-TDI and methanol is more likely a third-order reaction with respect to the initial reactants. © 2013 The Chemical Society of Japan.