No abstract

No abstract

Proton exchange between P-fluorinated ethanols and ethoxide ions has been studied using the MNDO SCF-MO method. Calculations were performed on reactions of ethoxide ion with ethanols substituted in the P-position with 0, 1, 2, and 3 fluorine atoms as well as on reactions where both the ethanol and the ethoxide ion were substituted with the same number (1, 2, 3) of fluorine atoms in the P-position. The energies obtained for the ion-molecule reactant complexes and the transition states from these reactions have been analyzed using the Marcus equation. Through the calculated force-constant matrices of reactants and transition states we also calculated the kinetic isotope effects for the proton-transfer reactions. The semiclassical isotopic rate constant ratios ( k~l k~)~ were found to be of rather normal magnitude and showed a variation with the energy of reaction. The calculated ratios of tunnel correction factors, QM/ QID, proved to be unrealistically high. These factors were also calculated with the frequencies scaled down by 10% and this was found to reduce the QtH/QtD ratios to more realistic values. Cnergies obtenues pour les complexes ion-molCcule qui rCagit ainsi que pour les Ctats de transition de ces rkactions. A l'aide des matrices des constantes de force calculCes pour les rCactifs et pour les Ctats de transition, on a aussi calculC les effets isotopiques cinktiques pour les rkactions de transfert de protons. On a trouvt que les rapports (kH/kD) des constantes de vitesse isotopiques semi-classiques sont d'une amplitude assez normale et on a trouvC qu'elles varient avec I'tnergie de la rkaction. I1 appert que les rapports calculCs pour les facteurs de correction pour l'effet tunnel, QtH/Q,D, sont trop tlevCs et non rtalistes. On a aussi calculC ces facteurs en se basant sur les frtquences rCduites par 10% et on a ainsi trouvC que les valeurs des rapports QtH/QtD sont beaucoup plus rkalistes.[Traduit par la revue]Proton-transfer reactions have been extensively studied from an experimental point of view (1). In such investigations isotope effects and linear free-energy relationships have often been employed. These methods have generated parameters that the investigators have interpreted in terms of the transition-state theory. This approach has proved successful in gaining knowledge usable in predicting the behaviour of many reactions. However, in order to use the transition-state theory one has to make assumptions regarding the reaction step studied, not always easily done in an unambiguous way.Modem quantum-mechanical methods and powerful computers have made it possible to calculate the stationary pointson the potential-energy surface for simple reactions, and thus calculated quantities, e.g., isotope effects, can be compared with experimental data. Even though other assumptions are introduced by the quantum-mechanical methods the two approaches may complement each other. A successful application of this to proton-transfer reactions would contribute to our understanding of such reactions.In an earlier st...

No abstract

No abstract