A Computational Study Investigating the Energetics and Kinetics of the HNCO + (CH3)2NH Reaction Catalyzed by a Single Water Molecule.

Abstract

High-level ab initio calculations are used to explore the energetics and kinetics for the formation of 1,1-dimethyl urea via the reaction of isocyanic acid (HNCO) with dimethyl amine (DMA) catalyzed by a single water molecule. Compared to the uncatalyzed HNCO + DMA reaction, the presence of a water molecule lowers the reaction barrier, defined here as the energy difference between the separated HNCO + DMA + H2O reactants and the transition state (TS), by ∼26 kcal/mol. In addition to the HNCO + DMA + H2O reaction, the energetics of the analogous reactions involving, respectively, ammonia and methyl amine were also investigated. Comparing the barriers for these three amine addition reactions, which can be represented as HNCO + R-NH-R' + H2O with R and R' being either -CH3 or -H, we find that the reaction barrier decreases with the degree of methylation on the amine nitrogen atom. The effective rate constants for the bimolecular reaction pathways HNCO··H2O + DMA and HNCO··DMA + H2O were calculated using canonical variational TS theory coupled with both small curvature and zero-curvature tunneling corrections over the 200-300 K temperature range. For comparison, we also calculated the rate constant for the HNCO + OH reaction. Our results suggest that the HNCO + H2O + DMA reaction can make a non-negligible contribution to the gas-phase removal of atmospheric HNCO under conditions where the HNCO and water concentrations are high and the temperature is low.