Proton transfer model

In addition, there is a large number of studies involving aromatic alcohols such as phenol [166] or naphthol, which have in part been reviewed before [21], These include time-resolved studies [21], proton transfer models [181], and intermolecular vibrations via dispersed fluorescence [182]. Such doubleresonance and more recently even triple-resonance studies [183] provide important frequency- and time-domain insights into the dynamics of aromatic alcohols, which are not yet possible for aliphatic alcohols. 

In order to confirm the proton transfer mechanism proposed previously [160], the results of IQNS on terephthalic acid were reported [164]. The jump distance is calculated to be 0.7 A for the proton transfer model and 2.1 A for the 180° rotation model - the latter process was ruled out on the basis of the experimental IQNS results, leading to the conclusion that the mechanism of the proton dynamics is indeed a double proton exchange. IQNS results for terephthalic acid and acetylene dicarboxylic acid have also been reported [165]. For both samples, the jump distance was found to be less than 1 A. For acetylene dicarboxylic acid, single crystal measurements yielded a jump distance of 0.73 A. The Q-depen-dence was found to be in excellent agreement with the 2-site jump model. From these results, the 180° rotation model can be ruled out in favour of the proton transfer model. 

Although we have only discussed the partial-proton-transfer model for one of the B 12-dependent carbon-skeleton mutases [69], we can expect that there also exists a continuum between no protonation and full protonation of the substrate in the other reactions. Analogously, partial hydride removal from the substrate of glutamate mutase may serve to facilitate this rearrangement. 

Nonlinear behavior is also observed in the wide-range (0.1-2.5 GPa) pressure dependence of the ESPT rate of DCN2 in alcohols [44[. At low pressure, the proto-lytic photodissociation rate slightly increases, reaching the maximum value. With further pressure increase this rate decreases below the initial value at atmospheric pressure (Fig. 13.11). To explain the unique nonexponential dependence of ESPT rate constants on pressure, as well as temperature, Huppert et al. have developed an approximate stepwise two coordinate proton-transfer model that bridges the high-temperature nonadiabatic proton tunneling limit with the rate constant... 

Figure 7.10. Global fit of a stepwise double proton transfer model to racemization progress curves at pH 8.9. Dashed lines, experimental data solid lines, fitted curves. Positive and negative CD signals correspond to L- and D-alanine, respectively. Alanine...

Calculations using conventional TST and the Bigeleisen-Wolfsberg [16] treatment for isotope effects have demonstrated that thd = 1-44 is a useful benchmark for primary hydrogen isotope effects. Using empirical harmonic force fields and various reactant-state and transition-state geometries, More O Ferrall and Kouba [30] found, for proton-transfer models, that the exponents were within 2% of the 1.44 value, and similar computational approaches gave Xhd = 1.43-1.45 (343 K)... 

Figure 18. PI-QTST activation free-energy curves as a function of the proton asymmetric stretch coordinate for a A-H-A proton transfer model (see Ref. 77). The solid line depicts the classical free-energy curve for the solute in isolation with a rigid A-A distanee, while the dotted line is the quantum free energy for the rigid, isolated solute with a fully quantized proton. The long-dashed line is the quantum free-energy curve for the isolated solute in which the A-A distance is allowed to fluctuate. The dot-dashed and short-dashed lines depict the quantum free-energy curves for the rigid and flexible solutes, in the polar solvent.

Biological proton transfers models are generally based on the expression of the rate constant given by the classical formulation of transition state theory (TST) ... 

The aim of this section is to introduce methods used in the applications described in the following sections and, more generally, to help the non-specialist reader to understand the literature about biological proton transfer models. 

The function g x), called the potential of mean foree (PMF), is the key quantity of proton transfer models. Indeed, the standard free-energy ehange is given by ... 

Following these lines, it is hoped to interpret important chemical reactions first by cyclic proton transfer models in terms of RP calculations. 

Figure 14. Comparison of free-energy surfaces for a symmetrical electron transfer reaction and for a proton transfer reaction. The simplified proton transfer model is treated in equation 34. The term solvation includes the approach of reactants inside the solvent cage.

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