Theory proton transfer

Br0nsted-Lowry acid-base theory, proton-transfer reaction, proton source, proton remover, amphoteric... 

Bam S Correlation Consistent Sets DNA Bases and Base Pairs Ab Initio Calculations Intermolecular Interactions by Perturbation Theory Proton Transfers Involving Anions and Dianions Solvation Modeling Water Clusters,... 

Density Functional Applications G2 Theory M0l-ler-Plesset Perturbation Theory Proton Transfers Involving Anions and Dianions. 

A more general theory of acids and bases was devised independently by Johannes Br0n sted (Denmark) and Thomas M Lowry (England) m 1923 In the Brpnsted-Lowry approach an acid is a proton donor, and a base is a proton acceptor The reaction that occurs between an acid and a base is proton transfer... 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

Equation (5-69) is an important result. It was first obtained by Marcus " in the context of electron-transfer reactions. Marcus derivation is completely different from the one given here. In electron transfer from one molecule (or ion) to another, no bonds are broken or formed, so the transition state theory does not seem to be applicable. Marcus assumed negligible orbital overlap in the electron-transfer transition state, but he later obtained the same equation for group transfer reactions requiring significant overlap. Many applications have been made to proton transfers and nucleophilic displacements. ... 

Different Types of Proton Transfers. Molecular Ions. The Electrostatic Energy. The ZwiUertons of Amino Acids. Aviopro-tolysis of the Solvent. The Dissociation Constant of a Weak Acid. Variation of the Equilibrium Constant with Temperature. Proton Transfers of Class I. Proton Transfers of Classes II, III, and IV. The Temperature at Which In Kx Passes through Its Maximum. Comparison between Theory and Experiment. A Chart of Occupied and Vacant Proton Levels. 

Proton Transfers of Classes II, III, and IV. Although, as Table 9 shows, the value of K may decrease with rise of temperature, or may increase, or may pass through a maximum, the tentative theory proposed in Sec. 64 leads to the conclusion that for proton transfers of classes II, III, and IV the value of — kT In Kz should in no case decrease or pass through a maximum or a minimum but in every case should increase steadily with rise of temperature. According to (132) the quantity — kT In Kx is equal to J, which consists of a part J,nv increasing with temperature, and a part Jnm independent of temperature. 

The Variation of J with Temperature. Although each proton transfer has its own characteristic value of 0, the variation of K near the maximum shows a marked degree of uniformity, as already mentioned in Sec. 64. If a parabola of the form p(T — 0)2 is fitted to the experimental results, a single value of p, namely 5 X 10-6, reproduces the variation of log K, not only for proton transfers of class III, but also for those of class II and class IV. If we accept (140) as providing a qualitative theory of the phenomena, we have at once a physical explanation of the observed uniformity. Whether we are concerned with the... 

In the proton transfer view of acid-base reactions, an acid and a base react to form another acid and another base. Let us see how this theory encompasses the elementary reaction between U+(aq) and OH (aq) and the reaction of disso-... 

In (59) the acid H30+ transfers a proton to the base OH-, forming an acid, H20, and a base, H20. We see that within the proton transfer theory, the molecule H20 must be assigned the properties of an acid and, as well, those of a base. 

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

Bronsted-Lowry theory A theory of acids and bases involving proton transfer from one species to another. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

Paradoxically, all these significant recent contributions to the theory of the ORR, together with most recent experimental efforts to characterize the ORR at a fuel cell cathode catalyst, have not led at aU to a consensus on either the mechanism of the ORR at Pt catalysts in acid electrolytes or even on how to properly determine this mechanism with available experimental tools. To elucidate the present mismatch of central pieces in the ORR puzzle, one can start from the identification of the slow step in the ORR sequence. With the 02-to-HOOads-to-HOads route appearing from recent DFT calculations to be the likely mechanism for the ORR at a Pt metal catalyst surface in acid electrolyte, the first electron and proton transfer to dioxygen, according to the reaction... 

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. 

Rates of addition to carbonyls (or expulsion to regenerate a carbonyl) can be estimated by appropriate forms of Marcus Theory. " These reactions are often subject to general acid/base catalysis, so that it is commonly necessary to use Multidimensional Marcus Theory (MMT) - to allow for the variable importance of different proton transfer modes. This approach treats a concerted reaction as the result of several orthogonal processes, each of which has its own reaction coordinate and its own intrinsic barrier independent of the other coordinates. If an intrinsic barrier for the simple addition process is available then this is a satisfactory procedure. Intrinsic barriers are generally insensitive to the reactivity of the species, although for very reactive carbonyl compounds one finds that the intrinsic barrier becomes variable. ... 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

Zhang, Q., Bell, R., Truong, T. N., 1994, Ab Initio and Density Funcional Theory Studies of Proton Transfer Reactions in Multiple Hydrogen Bond Systems , J. Phys. Chem., 99, 592. 

Marcus, R. A., Similarities and differences between electron and proton transfers at electrodes and in solution, Theory of hydrogen evolution reaction, Proc. Electrochem. Soc., 80-3, 1 (1979). 

The development of the theory of the processes of proton transfer has taken more than 50 years and the description of earlier approaches may be found in review articles cited previously.1 5 Some points of earlier models continue to be of interest. However, methods have been developed in recent years which enable us to take into account a number of new physical effects playing certain roles in these processes. 

It should be noted that recent work has not been devoted only to the application of the theory to new processes and phenomena but has also been concerned with the basis of the theory. Therefore, new important results have been obtained also for processes which have been under theoretical investigation for many years, in particular, for electron and proton transfer reactions. These results open new directions for further investigations. 

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... 

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