Hydration of the carbonyl group. Further evidence for a cooperative mechanism from experimental and theoretical studies of the hydrations of formaldehyde, acetaldehyde, acetone, and cyclohexanone

The experimental pseudo-first-order rate constant for the neutral hydration of acetone by H 2 18 O has been determined by taking advantage of the 18 O isotope shift in the 13 C NMR spectrum of 2- 13 C-labelled acetone. With the inclusion of this result, a previously reported computational procedure has been extended, with some refinements, to the neutral hydrations of formaldehyde, acetaldehyde, acetone, and cyclohexanone by different-sized water clusters in water solvent. Excellent agreement has been achieved between the experimental free energies of activation and the free energies of activation calculated for a cyclic mechanism in which the principal reaction channel contains four nonspectator water molecules. The computational procedure predicts that the cyclic water tetramer and the cubic water octamer are the principal constituents of bulk water, in agreement with Benson's tetramer-octamer model.