Polar solvents, proton transfer reactions theory

We shall show below that a description of the solvent using the concept of the medium polarization itself does not signify the continuum approximation. Moreover, in a number of works, both the change of state of the solvent and that of the intramolecular degrees of freedom in the course of the reaction were taken into account. However, to distinguish these works from those of the first kind, we retain here for the former the name continuum theory. Below, a brief survey of the development of this (second) type of approach will be given. In order to present the modern development of this kind of theory, we shall consider the proton transfer reaction as an example. 

Useful information can be obtained from the experimental determination of the activation energy and preexponential factor separately, since they are determined by different factors. According to the quantum mechanical theory, the value of the preexponential factor in proton transfer reactions is determined mainly by the characteristics of the chemical bond to be broken, whereas the activation energy is determined by the Franck-Condon potential barrier associated with solvent polarization. Experimental investigation of the hydrogen evolution reaction from the ions and CHaCNH" in water and in... 

As might be expected, the results from both theory and experiment suggest that the solution is more than a simple spectator, and can participate in the surface physicochemical processes in a number of important ways [Cao et al., 2005]. It is well established from physical organic chemistry that the presence of a protic or polar solvent can act to stabilize charged intermediates and transition states. Most C—H, O—H, C—O, and C—C bond breaking processes that occur at the vapor/metal interface are carried out homolytically, whereas, in the presence of aqueous media, the hetero-lytic pathways tend to become more prevalent. Aqueous systems also present the opportunity for rapid proton transfer through the solution phase, which opens up other options in terms of reaction and diffusion. 

R. CoUepardo-Guevara, I.R. Craig, D.E. Manolopoulos, Proton transfer in a polar solvent from ring polymer reaction rate theory, J. Chem. Phys. 128 (2008) 144502. 

Dogonadze, Kuznetsov, and Levich " DKL) treat proton transfer in the same way (i.e., in terms of continuum solvent fluctuation) as that for simple electron transfer reactions, where no breakage or formation of bonds is involved. The reaction takes place only as a result of continuum solvent polarization fluctuation without taking into consideration the stretching of the H-OH2 bond and is not connected with the thermal heat sink, which is supposed to give the activated states in other theories. 

The multistate continuum theory for PCET provides a framework for the analysis of the effects of specific solute and solvent properties on the rates and mechanisms of PCET reactions. The properties of interest include the relative energies of the gas phase solute charge transfer states, the distance between the proton donor and acceptor, the distance between the electron donor and acceptor, and the solvent polarity. In Ref. [32], a comprehensive study of the effects of these physical properties on the rates, mechanisms, and kinetic isotope effects of PCET reactions is presented. Some of the predictions obtained from this study are discussed in this section. 

Calculations of KIEs derived from a classical reaction path (e.g. the MEP) in the presence of a solvent or polar environment typically add quantum corrections to that path [22]. Such a reaction path, however, includes classical motion of the proton, especially near the TS, and thus this technique exhibits no difference in quantum corrections between H and D at the TS for a symmetric reaction (AG])xn=0) [22b], in contrast to the present picture. In variational TS theory for gas phase H atom transfer, the TS significantly deviates from the MEP TS and is isotope-dependent [23]. This feature has been calculated for PT in an enzyme, where the KIE has been diminished because the TS position significantly differs between H and D even in a symmetric case ]22e[. 

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