Proton-transfer reactions kinetics

In analyzing the behavior of these types of tetrahedral intermediates, it should be kept in mind that proton-transfer reactions are usually fast relative to other steps. This circumstance permits the possibility that a minor species in equilibrium with the major species may be the major intermediate. Detailed studies of kinetics, solvent isotope effects, and the nature of catalysis are the best tools for investigating the various possibilities. 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

Gao J, Wong K-Y, Major DT (2008) Combined QM/MM and path integral simulations of kinetic isotope effects in the proton transfer reaction between nitroethane and acetate ion in water. J Comput... 

German ED, Kuznetsov AM, Dogonadze RR (1980) Theory of the kinetic isotope effect in proton transfer reactions in a polar medium. J Chem Soc, Faraday Trans 2 76 1128-1146... 

As an example, consider an early calculation of isotope effects on enzyme kinetics by Hwang and Warshel [31]. This study examines isotope effects on the catalytic reaction of carbonic anhydrase. The expected rate-limiting step is a proton transfer reaction from a zinc-bound water molecule to a neighboring water. The TST expression for the rate constant k is... 

ApA < 1. In Fig. 2 the region of curvature is much broader and extends beyond — 4 < ApA < + 4. One explanation for the poor agreement between the predictions in Fig. 3 and the behaviour observed for ionisation of acetic acid is that in the region around ApA = 0, the proton-transfer step in mechanism (8) is kinetically significant. In order to test this hypothesis and attempt to fit (9) and (10) to experimental data, it is necessary to assume values for the rate coefficients for the formation and breakdown of the hydrogen-bonded complexes in mechanism (8) and to propose a suitable relationship between the rate coefficients of the proton-transfer step and the equilibrium constant for the reaction. There are various ways in which the latter can be achieved. Experimental data for proton-transfer reactions are usually fitted quite well by the Bronsted relation (17). In (17), GB is a... 

Fig. 5 Variation in kinetic solvent isotope effect, k(H20)/A (D20), for the normal proton-transfer reaction (28)...

The reduction of dioxygen to its fully reduced form, H20, requires the transfer of 4 electrons, and the transfer may proceed via a series of intermediate oxidation states, such as 02 /H00, HOO /HOOH, 0 /OH. These reduced forms of oxygen exhibit different redox properties and in the presence of substrate(s) and/or catalyst(s) may open different reaction paths for the electron transfer process. Fast proton transfer reactions between the corresponding acid-base pairs can introduce composite pH dependencies into the kinetic and stoichiometric characteristics of these systems. 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

The reaction of ammonia and hydrogen chloride in the gas phase has been the subject of several studies in the last 30 years [56-65], The interest in this system is mainly that it represents a simple model for proton transfer reactions, which are important for many chemical and biological processes. Moreover, in the field of atmospheric sciences, this reaction has been considered as a prototype system for investigation of particle formation from volatile species [66,67], Finally, it is the reaction chosen as a benchmark on the ability, of quantum chemical computer simulations, to realistically simulate a chemical process, its reaction path and, eventually, its kinetics. 

Eight generalizations are given arising from world-wide studies of proton transfer reactions in aqueous media carried out over the past twenty-five years. Future directions of research on proton transfer kinetics are predicted, and recent kinetic studies by the authors on proton transfer in nonaqueous media (methanol, acetonitrile, and benzonitrile) are reviewed. 

Recently, some attempts were nndertaken to uncover the intimate mechanism of cation-radical deprotonation. Thns, the reaction of the 9-methyl-lO-phenylanthracene cation-radical with 2,6-Intidine (a base) was stndied (Ln et al. 2001). The reaction proceeds through two steps that involve the intermediary formation of a cation-radical/base complex before unimolecular proton transfer and separation of prodncts. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tnnneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves n bonding between the unshared electron pair on nitrogen of the Intidine base with the electron-deficient n system of the cation-radical. Nonclassical cation-radicals wonld also be interesting reactants for snch a reaction. The cation-radical of the nonclassical natnre are known see Ikeda et al. (2005) and references cited therein. 

Chain transfer of a P-proton to other basic substances in the reaction mixture is also possible. The various P-proton transfer reactions limit polymer molecular weight, but do not terminate the kinetic chain. 

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