Theoretical models for solvation and catalysis in carbonyl addition
Autor: | Richard L. Schowen, Ian H. Williams, Douglas A. Femec, Dale P. Spangler, Gerald M. Maggiora |
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Rok vydání: | 1983 |
Předmět: | |
Zdroj: | Journal of the American Chemical Society. 105:31-40 |
ISSN: | 1520-5126 0002-7863 |
Popis: | Alternative modes of catalysis of formaldehyde hydration by a single ancillary water molecule are investigated by ab initio calculations at the STO-3G level. A cyclic transition state (FW2 double dagger) involving formaldehyde with two molecules of water is characterized and is only 0.8 kcal mol/sup -1/ higher in energy than the isolated reactants. The results indicate that gas-phase formaldehyde hydration probably proceeds via FW2 double dagger in a concerted mechanism with a Gibbs free energy of activation of 27 kcal mol/sup -1/. Addition of a water dimer to formaldehyde via FW2 double dagger is predicted to occur in water with a Gibbs free energy of activation of 16 kcal mol/sup -1/, in agreement with experiment. Empirical extrapolation to the liquid phase of entropies of activation calculated for reaction of one or two waters in the gas phase suggests that a mechanism involving three water molecules would be consistent with experiments for dioxan solution. Specific solvation by four water molecules is predicted to stabilize the zwitterionic adduct H/sub 2/O/sup +/CH/sub 2/O/sup -/ (an unbound state in the gas phase) by 111 kcal mol/sup -1/ relative to H/sub 2/O/sup +/CH/sub 2/O/sup -/ and 4H/sub 2/O or by 37 kcal mol/sup -1/ relativemore » to (H/sub 2/O)/sub 3/ and CH/sub 2/O.2H/sub 2/O. Thus a stepwise mechanism for formaldehyde hydration also may be feasible, although the energy barrier to formation of a solvated zwitterionic intermediate has not yet been calculated.« less |
Databáze: | OpenAIRE |
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