Theoretical Modelling of Specific Solvation Effects upon Carbonyl Addition

Geometries have been fully optimized at the HF/3-21G and AMI levels of theory for reagent, activated, and product complexes for ammonia addition to formaldehyde catalyzed by 0, 1, or 2 molecules of water. One water molecule reduces the 3-21G reaction barrier by 113 kJ mol^-1by virtue of increased hydrogen bonding with the zwitterion-like ammonia-formaldehyde moiety in the cyclic six-membered activated complex. The second water molecule reduces the barrier by a further 33 kJ mol^-1in 3-21G by permitting less-bent hydrogen bonds in the cyclic eight-membered activated complex. Hydrogen bond strengths are overestimated at the 3-21G and MP2/6-31G*//3-21G levels but are underestimated by AMI, particularly for hydrogen bonds involving charged species; AMI consistently prefers bifurcated to linear hydrogen bonds. Relative Gibbs free energies of activation estimated for the addition reactions predict the two-water-catalyzed process to be preferred over the one-water-catalyzed process by ~10 kJ mol^-1in the gaseous and aqueous phases and by ~25 kJ mol^-1in dioxan. The reaction-coordinate vibrational modes are dominated by motions of the transferring protons; proton donation is more advanced than proton abstraction in the activated complexes.