Synergism of catalysis and reaction center rehybridization in nucleophilic additions to cumulenes: the one-, two-, three-water hydrolyses of carbodiimide and methyleneimine.

The results of a theoretical study of the one-, two- and three-water hydrolyses of carbodiimide and the one- and two-water hydrolyses of methyleneimine are presented. All structures were optimized and characterized at the MP2(full)/6-31G* level of theory. Energies for the one-water hydrolysis of carbodiimide were determined at numerous higher levels of theory, up to the QCISD(T)(fc)/6-311+G(3df,2p)//MP2(full)/6-31G* level. The Delta E0(Delta G298) activation barriers for the rate-determining steps of the one-, two- and three-water hydrolyses of carbodiimide, respectively, are 44.8 (46.3), 29.3 (32.3) and 22.9 (26.2) kcal mol(-1) at the MP2(full)/6-31G* level. The consideration of a second water molecule catalyzes the hydrolysis by 15.5 kcal mol(-1) on the E0 surface and by 14.0 kcal mol(-1) on the G298 surface with respect to the one-water hydrolysis. Placement of a third water molecule opposite the site of proton transfer catalyzes the reaction by an additional 6.4 kcal mol(-1) on the E0 surface and by 6.1 kcal mol(-1) on the G298 surface. The catalytic effect of the third water molecule results from the synergistic effects of rehybridization and charge relaxation in the transition state. The charge relaxation in the transition state is illustrated through natural population analysis calculations on the pre-coordination complexes and the transition state structures. We also consider the placement of the third water molecule in the proton transfer chain and we show this to be of little catalytic relevance. The activation barriers determined for the one- and two-water hydrolyses of methyleneimine are Delta G298=51.9 and Delta G298=35.5 kcal mol(-1), respectively, and they are larger than for carbodiimide. The results are compared with the hydrolyses of carbon dioxide and formaldehyde.