Activation proton jumps

Besides a normal activated proton jump mechanism with a rate which obeys an Arrhenius type law... 

It is interesting to note that, for malonic acid (which is structurally related to DMMA), the activation energy measured from XH NMR Tx measurements [170] is 5.6 kj mol-1, which is significantly lower than in DMMA and is assigned to proton jumps between the two minima of an asymmetric double well potential. This emphasises the importance of the effect of the crystal packing on the asymmetry of the potential function, which defines the mechanism of the proton dynamics in carboxylic acid dimers. 

Figure 47. As oxygen ions move towards each other on account of lattice vibrations, die activation energy for proton jump is lowered, and die proton changes partner. According to Ref..192. (Reprinted from K. D. Kreuer, W. Munch, U. Traub and J. Maier, On Proton Transport in Perovskite-Type Oxides and Plastic Hydroxides , Ber. Bunsenges. Phys Chem. 102, 552-559. Copyright 1998 with...

A combination of N CPMAS NMR, N quadrupole double resonance, and x-ray studies of solid 3,5-dimethylpyrazole (19) between 270 K and 350 K has been necessary to discuss the intermolecular proton transfer in a structure like (55) <89JA7304> the experimental evidence (species HHH and DDD) is consistent with a correlated triple hydrogen jump with an activation energy of 11 kcal mol". The cases of 3,5-diphenyl-4-bromopyrazole (a dimer like (54)) and 3,5-diphenyl-pyrazole (a tetramer like (56)) were subsequently studied <92JA9657> here also proton jumps were found with activation energies similar to those found for (19). The dynamic behavior of pyrazole... 

Proton exchange among the three possible protonation sites in mesitylene (the —C—H sites) appeared to occur without intervention of an acid molecule to carry the proton. The activation energy of this process has been measured as 10 kcal mole-1. In hexamethylbenzene the C-methyl positions are protonated and the proton jump process has the same activation energy. In pentamethylbenzene the proton is always located on the —C—H site. Later results of MacLean and Mackor (1962) are more comprehensive. Intermolecular proton transfer via a solvent molecule has been established for mesitylene, anisole and m-xylene, but in hexamethylbenzene the transfer is always intramolecular. High activation energies of at least 8 kcal mole-1 were measured for proton transfers from the carbonium ion and this was associated with a weak interaction between... 

Ab initio calculations 15-0-01 Activation energy, proton jumps 15-0-01... 

Clearly the charges and local softnesses contain different information since they sometimes change in the opposite direction. Even without proton jump, the sensitivity coefficients of the pyridine molecule are drastically affected. In this model, the role of the active surface is explicitly taken into account and can help us to better understand its influence on the activation of substrates. 

Proton diffusion in glass induces cracks in the same way as occurs on substitution of Na and by Li ions which implies that large H30 ions are not formed. In fact, 0 and diffusion measurements show that proton diffuses 100 times faster than oxygen . This supports the assumption that the diffusion mechanism consists also of proton jumps and not only of OH jumps. The corresponding activation energy has... 

The free transport is the principal mode of transport of protons in oxides, and in this mechanism protons jump from one oxygen ion to a neighbouring one. After each jump the proton in the hydroxide rotates such that the proton reorients in the electron cloud and becomes aligned for the next jump. This is illustrated schematically in Fig.5.11. The rotation and reorientation is believed to involve a small activation energy and the jump itself is considered to be the ratedetermining step. 

Studies with ab initio types of hybrid potential include the early work of Weiner et al. on the nature of catalysis in trypsin and the studies of the catalytic activity of phospholipase A2 by Hillier et al. Investigations with semiempirical hybrid potentials are more extensive and include calculations of the reactions in triosephosphate isomerase by Bash et al. and in chorismate mutase by Lyne et al. and a study of the proton jump in the catalytic triad of human neutrophil elastase. The study of the chorismate mutase reaction was especially interesting because the enzyme is the only known one that catalyzes a pericyclic reaction that also occurs readily in solution. The results of the hybrid study were particularly lucid in this case because the enzyme works, not by chemically catalyzing the reaction, but by preferentially binding a distorted form of the substrate and stabilizing the transition state. 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

The His35, with coordinated His46 in close proximity, has frequently been suggested as a site for electron transfer reactivity of azurin. Two processes have been detected in a temperature-jump study on the equihbration of azurin with cytochrome C551, its physiological partner [57]. The fast process is assigned to electron transfer, and the slower process to a conversion between inactive and active forms of reduced azurin. It has been concluded that the active form is protonated. A second H-bonded form of His35 is believed to result from the protonation [2]. 

Different models determine A in different ways. Nation exhibits a water-uptake isotherm as shown in Figure 7. The dashed line in the figure shows the effects of Schroeder s paradox, where there is a discontinuous jump in the value of A. Furthermore, the transport properties have different values and functional forms at that point. Most models used correlate A with the water-vapor activity, since it is an easily calculated quantity. An exception to this is the model of Siegel et al., ° which assumes a simple mass-transfer relationship. There are also models that model the isotherm either by Flory—Huggins theory" or equilibrium between water and hydrated protons in the membrane and water vapor... 

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