Marcus theory of proton transfer

Figure 13. Exaggerated free energy surface showing two volcanoes with contours for a concerted proton transfer. At all values of the C-O distance there is a barrier to proton transfer. The Marcus theory of proton transfer predicts that a = since the saddle occurs at the value of x where the thermodynamics driving the proton transfer is in balance (QRS). The surface has been drawn for a symmetrical case, but this conclusion still holds for an unsymmetrical case. The strip cartoon shows the mechanism for the hydration dehydration reaction of a ketone.

Modern theoretical developments in the theory of proton-transfer reactions suggest that such linear Bronsted plots are only a first approximation when the range of the P-KHA-values is narrow. When a wider range of bases is used, the curve obtained should be such that the Gibbs free energy of activation AG fits the Marcus eqn (6). This equation was derived (Cohen and Marcus, 1968 ... 

Application of Marcus theory to proton-transfer reactions in soiution... 

The contribution of the Marcus treatment to the theory of proton transfer... 

The Marcus theory of electron-transfer reactions between ions resembles in some ways the theory of proton transfer outlined above, but there are important differences, which are... 

MARCUS THEORY OF PROTON AND ELECTRON TRANSFER IN LIQUIDS... 

It has been reported that rates of proton transfer from carbon acids to water or hydroxide ion can be predicted by application of multi-dimensional Marcus theory to a model whereby diffusion of the base to the carbon acid is followed by simple proton transfer to give a pyramidal anion, planarization of the carbon, and adjustment of the bond lengths to those found in the final anion.124 The intrinsic barriers can be estimated without input of kinetic information. The method has been illustrated by application to a range of carbon acids having considerable variation in apparent intrinsic barrier. 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

Application of Marcus rate theory to proton transfer in enzyme-catalyzed reactions was discussed by Kresge and Silverman, 1999. Relationships of log KIE and kinetics of the enzyme catalysis (kcat) and parameters of the reaction driving force were found to be in agreement with the Marcus model. 

Kresge, AJ. and Silverman, D.N. (1999) Application of Marcus rate theory to proton transfer in enzymy atalyzed reactions, in Schramm, V. L. and Purich, D. L. (eds.), Methods in Enzymology 308, Enzyme kinetics and Mechanism, Part E, Academic Press, San Diego, pp. 276- 297. 

In the early 1990s, in their studies of proton transfer in solution using Marcus rate theory Equation (5), Hynes and coworkers16 17 noticed the following limitation. If Q is the tunneling distance, it can be shown that the tunneling matrix element that appears in Equation (5) has the form A e- e. For typical electron transfer reactions... 

S.S. Kristjansddttir and J.R. Norton, Agreement of proton transfer cross reaction rates between transition metals with those predicted by Marcus theory, J. Am. Chem. Soc., 113 (1991) 4366. 

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