HYDRATION OF THE CARBONYL GROUP - A THEORETICAL-STUDY OF THE COOPERATIVE MECHANISM

The thermochemical parameters, vibrational frequencies, solvent isotope effects, and proton inventories for the neutral hydration of formaldehyde by water and by clusters containing two, three, and four water molecules have been calculated at 298 K for the gas phase, and also for water solvent, using abinitio molecular orbital theory at the MP2/6-31G* level and the self-consistent reaction field method. All of the stationary points required for an examination of a cyclic (cooperative) mechanism, first proposed by Eigen, have been found. Basis set superposition error has been taken into account, and this has allowed the calculation of the free energy changes associated with the different ways in which CH2O and (H2O)(n) (n = 1, 2, 3, 4, 8) can reach transition states containing different numbers of water molecules. In the gas phase, a major reaction channel involves the formation of a complex containing three water molecules, which then proceeds to the product. In water solvent, when concentrations and entropic effects associated with the loss of translational and rotational motion are taken into account, 99.9% of the reaction proceeds via this complex, and the experimental pseudo-first-order rate constant for the hydration of formaldehyde in water is reproduced. These findings are consistent with the results of R. P. Bell and co-workers, who concluded that uncatalyzed hydration of a carbonyl group proceeds via a cyclic transition state containing two extra water molecules. Although the process is disfavored entropically, the entropy loss is almost exactly balanced by the gain in enthalpy resulting from more favorable O ... H ... O hydrogen bonding in an 8-membered ring containing three water molecules than in a 6-membered ring containing two water molecules, as suggested by Gandour. A similar, favorable, hydrogen bonding geometry is present in the cyclic water tetramer. The enthalpy change and solvent isotope effect calculated for the conversion of the complex of formaldehyde with three water molecules to methanediol solvated by two water molecules are in good agreement with the experimental results in water solvent. The different active hydrons of the water molecules of this complex make different normal contributions to the solvent isotope effect, with proton transfer to the adjacent water molecule from the water molecule that forms the C-O bond significantly more advanced than proton transfer to the carbonyl oxygen. Nevertheless, the process is characterized by a non-linear (dome-shaped) proton inventory.