A Computational Study Investigating the Energetics and Kinetics of the HNCO + (CH 3 ) 2 NH Reaction Catalyzed by a Single Water Molecule.

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 + H2 O reactants and the transition state (TS), by ∼26 kcal/mol. In addition to the HNCO + DMA + H2 O 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' + H2 O 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··H2 O + DMA and HNCO··DMA + H2 O 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 + H2 O + 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.