Proton transfer kinetics

Strandjord AJG, Barbara PF (1985) Proton-transfer kinetics of 3-Hydroxyflavone - solvent effects. J Phys Chem 89 2355-2361... 

Nelly R.N., Schulman S.G., Proton-Transfer Kinetics of Electronically Excited Acids and Bases, in Molecular Luminescence Spectroscopy Methods and Applications, part 2, Schulman S.G. (ed.), Wiley-Interscience, New York, 1988 pp 461-510. 

In summary, it is clear from the above-discussed aspects that it was possible by multinuclear NMR (oxygen-17, nitrogen-15, carbon-13, and technetium-99) to successfully study the very slow cyanide exchange and the slow intermolecular oxygen exchange in these oxocy-ano complexes and correlate them both with the proton-transfer kinetics. Furthermore, the interdependence between the proton transfer and the actual dynamic inversion of the metal center was clearly demonstrated. 

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. 

In the following amplification of these generalizations, some attention will be given to controversial aspects of these statements. It is interesting that an area of scientific study such as proton transfer kinetics could be an active one for over 25 years, particularly because of relaxation techniques, and still be one for which it is difficult to make many generalizations that workers in the field can endorse without major reservations. 

Table III suggests some of the proton transfer kinetic studies one is likely to hear most about in the near future. The very first entry, colloidal suspensions, is one that Professor Langford mentioned earlier in these proceedings. In the relaxation field, one of the comparatively new developments has been the measurement of kinetics of ion transfer to and from colloidal suspensions. Yasunaga at Hiroshima University is a pioneer in this type of study (20, 21, 22). His students take materials such as iron oxides that form colloidal suspensions that do not precipitate rapidly and measure the kinetics of proton transfer to the colloidal particles using relaxation techniques such as the pressure-jump method.

Table III Eight Areas for Future Research in the Study of Proton Transfer Kinetics ...

Table IV is an attempt to summarize the results of these proton transfer studies in nonaqueous solvents. There is no systematic trend in what seems to be the rate limiting step in contrast to the attractive Eigen-Wilkins generalization for the mechanism of metal ion complexation. Obviously, many more proton transfer kinetic studies in nonaqueous solutions are needed for beautiful generalizations to emerge. Whether investigators will have the patience to carry them out or not is the only uncertainty.

DR. PATEL One reason for much of the interest which prevails in this area right now, especially with iron ll), has to do with the corrosion of steel in industry and also in nuclear reactors. Normally one thinks of forming precipitates or particles by adding base to a solution and cooling it down. If iron(III) solutions are made more acidic and if you raise the temperature, these conditions lead to the formation of very, very well-defined particles. A very important event in this is the proton transfer kinetics that lead to the formation of the hydrolysis of many of these trivalent ions. 

JSTjpp is termed a kinetic pKa, since it does not represent a real ionization but is composed of the ratios of rate constants that are not for proton transfers. Kinetic p/fa s occur whenever there is a change of rate-determining step with pH. 

The interest in proton transfer to and from carbon arises partly because this process occurs as an elementary step in the mechanisms of a number of important reactions. Acid and base catalysed reactions often occur through intermediate carbonium ions or carbanions which are produced by reactions (1) and (2). A knowledge of the acid—base properties of carbonium ions or carbanions may also help in understanding reactions in which these species are present as reactive intermediates, even when they are generated by processes other than proton transfer. Kinetic studies of simple reactions such as proton transfer are also important in the development of theories of kinetics. Since both rates and equilibrium constants can often be measured for (1) and (2) these reactions have been useful in the investigation of correlations between rate coefficients and equilibrium constants (linear free energy relations). 

Many more details can be found about the kinetics of proton transfer reactions [8]. These often involve specific chemical features of the reactants. Since much of the focus in this chapter is on experimental methods, these reactions are not discussed further. However, the use of NMR spectroscopy to study proton transfer kinetics is considered in section 7.9. 

Adiabatic Proton Transfer Kinetic Isotope Effects... 

Recently, the usefulness of fs-resolved mid-IR measurements of some vibrational markers of the photoacid and the photobase was demonstrated by Nibber-ing et al. [97-100]. Direct mid-IR absorption spectroscopy has thus proved to be an additional tool for directly monitoring the proton-transfer kinetics of photoacids while in the excited state. 

R. Kelly and S. Schulman, Proton transfer kinetics of electronically excited acids and bases, in Molecular Luminescence Spectroscopy—Methods and Applications Part 2 (ed. S. Schulman), 1st edn., Wiley-Interscience, New York, 1988, pp. 461-510. 

McLuckey SA, Stephenson JL Jr, Asono KG. 1998. lon/ion proton-transfer kinetics implications for analysis of ions derived from electrospray of protein mixtures. Analytical Chemistry 70 1198-1202. 

Molecular Modelling of Proton Transfer Kinetics in Biological Systems... 

Since the transition state involves an associative mechanism accompanied by a proton transfer, kinetics are governed by the extent of coordination unsaturation of the metal, N - z, and the transfer ability of the proton. Larger values of (N - z) and greater acidities of the protons reduce the associated activation barriers and enhance the kinetics. The thermodynamics of olation depend on the strength of the entering nucleophile and the electrophilicity of the metal. The kinetics of olation are systematically... 

The picture of prototropic trjinsformations of the nucleic acid base tautomers will never be completed without a knowledge of inter- and intramolecular proton transfer kinetics. The most general data describing the kinetics of proton transfer are the set of temperature dependent rate constants. These data for nucleic acid bases are not yet available from either experimental or theoretical studies except the very recent paper [ 134] where the authors attempt to estimate the water assisted proton transfer rate constant for adenine. However, the calculated values of proton transfer barrier for both non-water assisted and water assisted pathways are available for the adenine, guanine and eytosine [119, 123, 134]. These data are collected in Tables 12 - 16, where, for convenience, we have defined as forward reaction the proton transfer process from the normal (canonical) to the hydroxo- (imino-) form. 

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