Proton transfer parameter

None of the complexes pairing an amine with a hydrogen halide were computed to be of the pure ion pair variety with a minimal basis set ° . However, there were some that were close. More specifically, since the equilibrium position of the proton need not shift precipitously from one atom to the other, but rather can move gradually as the acidity and basicity increase, a proton-transfer parameter was devised to indicate the degree of transfer of the proton from the acid to the base. The quantity p was defined as... 

The values calculated for this proton-transfer parameter are shown in Fig. 6.20 as a function of the acidity of A and the basicity of B. More precisely, the horizontal scale is a nor-... 

Figure 6.20 Proton transfer parameter, p, computed for various methylated amines paired with hydrogen halides. The numerical label n on each data point refers to the number of methyl substitutes on the amine N(CH3) Hj. The horizontal scale is a normalized difference in proton affinity between the amine and the halide .

Two pairs of proton-transfer parameters Srd=da/dClexp/ T,[Pg.1618]

This study is particularly noteworthy in the evolution of QM-MM studies of enzyme reactions in that a number of technical features have enhanced the accuracy of the technique. First, the authors explicitly optimized the semiempirical parameters for this specific reaction based on extensive studies of model reactions. This approach had also been used with considerable success in QM-MM simultation of the proton transfer between methanol and imidazole in solution. 

Rate parameters [(da/df), A, E measured for dehydroxylations are frequently sensitive to the availability of water vapour in the vicinity of the reactant and this accounts for the apparent variations in kinetic data sometimes found between different reports concerned with the same reaction. Water adsorbed on product adjoining the reaction interface could be expected to participate in the reversible proton transfer step, the precursor to water elimination. Despite this influence of PH2o on reaction rate, we are aware of no reported instance of S—T behaviour in dehydroxylations. 

The detritiation of [3H]-2,4,6-trimethoxybenzene by aqueous perchloric acid was also studied, the second-order rate coefficients (107/c2) being determined as 5.44, 62.0, and 190 at 0, 24.6, and 36.8 °C, respectively, whilst with phosphate buffers, values were 3.75, 13.8, and 42.1 at 24.6, 39.9, and 55.4 °C, respectively. The summarised kinetic parameters for these studies are given in Table 134, and notable among the values are the more negative entropies of activation obtained in catalysis by the more negative acids. This has been rationalised in terms of proton transfer... 

The approach presented above is referred to as the empirical valence bond (EVB) method (Ref. 6). This approach exploits the simple physical picture of the VB model which allows for a convenient representation of the diagonal matrix elements by classical force fields and convenient incorporation of realistic solvent models in the solute Hamiltonian. A key point about the EVB method is its unique calibration using well-defined experimental information. That is, after evaluating the free-energy surface with the initial parameter a , we can use conveniently the fact that the free energy of the proton transfer reaction is given by... 

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

Figure 4. Proton affinities (PA) of carbenes from Table 7 (circles) and Table 8 (squares) as a function of Taft parameters for proton transfer equilibria.110,150 The coefficients for Ecra, Ectf, and Ectr+ were optimized, with PA( CH2) = 206 kcal/mol constant (r2 = 0.897).

Far less data are available from other diaqua Pt(II) compounds. Comparison of the diaqua derivatives of cis- and trans-DDP has shown that their complexation with inosine derivatives is mechanistically similar, but the rate parameters for various steps show considerable differences [40,41], For example, for isomeric [Pt(NH3)2(H20)2]2+ ions kx cis) = 10 k trans), whereas for the [Pt(0H)(NH3)2(H20)]+ ions the difference is k cis) 6 kyitrans) in the formation of 1 1 complexes. The ability of isomeric 1 1 complexes to bind the second nucleobase is, however, very similar in both cases, also by taking proton transfer formally from inosine... 

Because solvent viscosity experiments indicated that the rate-determining step in the PLCBc reaction was likely to be a chemical one, deuterium isotope effects were measured to probe whether proton transfer might be occurring in this step. Toward this end, the kinetic parameters for the PLCBc catalyzed hydrolysis of the soluble substrate C6PC were determined in D20, and a normal primary deuterium isotope effect of 1.9 on kcat/Km was observed for the reaction [34]. A primary isotope effect of magnitude of 1.9 is commonly seen in enzymatic reactions in which proton transfer is rate-limiting, although effects of up to 4.0 have been recorded [107-110]. 

With the RPA model it has been possible to simulate many sets of CPA/alumina data mentioned in the literature [18], with the same set of unadjusted parameters (PZC, K and K2, OH density). Since pH shifts in the presence and the absence of CPA adsorption on alumina [23] and PTA adsorption on silica [19] are similar, it can be concluded that metal and proton transfer are independent in these systems. Thus the pH shift model can be used in concert with the RPA model not only to predict metal uptake, but also to compute final pH from the initial pH of the contacting solutions [18,28],... 

As discussed in Section 2.5.1, aryl radicals are easily reduced at the potential where they are generated. This reduction that can take place at the electrode surface (ECE) or in the solution (DISP) opposes the substitution process. This three-cornered competition between substitution (SUBST) electron + proton transfer (ECE or DISP) depends on two competition parameters that are closely similar to the HAT-ECE-DISP parameters described in the preceding section ... 

The following discussion begins by presenting an in-depth view of the mechanism for the photochemical reduction of benzophenone by N, iV-dimethyl-aniline. This discussion is followed by a presentation of the theoretical models describing the parameters controlling the dynamics of proton-transfer processes. A survey of our experimental studies is then presented, followed by a discussion of these results within the context of other proton-transfer studies. 

In 1989, Borgis and Hynes proposed a theory for nonadiabatic proton transfer that includes all the parameters contained with the DKL model. In addition, they addressed the important issue of low-frequency vibrations serving as promoting modes in proton tunneling [11]. For nonadiabatic proton transfer, the distance dependence of the tunneling coupling, C(Q), has the analytical form [13]... 

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