DFT Study of the Proton Transfer in the Urethane Formation between 2,4-Diisocyanatotoluene and Methanol

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<jats:title>Abstract</jats:title> <jats:p>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.</jats:p>

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