Cooperative effect of solvent in the neutral hydration of ketenimine: an ab initio study using the hybrid...

The detailed reaction pathways for the hydration of ketenimine by water and water clusters containing up to five explicit water molecules (CH(2) =C=NH + (n + 1)H(2)O --> CH(3)CONH(2) + nH(2)O, n = 0-4) in the gas phase have been investigated by the MP2 method in conjunction with the 6-31+G* and 6-311++G** basis sets, and the effects of bulk solvent are taken into account according to the conductor-like polarizable continuum model (COSMO). In the hybrid cluster/continuum model, apart from one directly attacking water molecule, four explicit water molecules participating in the water-assisted hydrolysis of ketenimine are divided into two groups, one involving in the proton relay and the other near the nonreactive nitrogen or carbon atom. Two possible reaction channels, water addition across the C=C bond or across the C horizontal lineN bond of ketenimine, are discussed. Our results indicate that the kinetically favorable mechanism involves an eight-membered ring transition state structure formed by a proton transfer chain of three water molecules. Meanwhile, two additional cooperative water molecules near the nonreactive region assist the hydration by engaging in hydrogen bonding to the substrate; such an interaction is found to be important in the hydration of ketenimine and other cumulenes. The lowest rate-determining activation barriers of C=C and C=N addition are 98.9 and 95.1 kJ/mol, respectively, suggesting that the two channels are competitive when more water molecules take part in the hydration. COSMO solution models do not modify the calculated energy barriers in a significant way.