Kinetics of protonation

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

Ionisation processes in IMS occur in the gas phase through chemical reactions between sample molecules and a reservoir of reactive ions, i.e. the reactant ions. Formation of product ions in IMS bears resemblance to the chemistry in both APCI-MS and ECD technologies. Much yet needs to be learned about the kinetics of proton transfers and the structures of protonated gas-phase ions. Parallels have been drawn between IMS and CI-MS [277]. However, there are essential differences in ion identities between IMS, APCI-MS and CI-MS (see ref. [278]). The limited availability of IMS-MS (or IMMS) instruments during the last 35 years has impeded development of a comprehensive model for APCI. At the present time, the underlying basis of APCI and other ion-molecule events that occur in IMS remains vague. Rival techniques are MS and GC-MS. There are vast differences in the principles of ion separation in MS versus IMS. 

Kinetics of Proton Transfer in Molecule-Cluster Ion Interactions... 

Mallik R, Udgaonkar JB, Krishnamoorthy G (2003) Kinetics of proton transfer in a green fluorescent protein a laser-induced pH jump study. Proc Indian Acad Sci-Chem Sci 115 307-317... 

Some theoretical aspects of thiophene reactivity and structure have also been discussed, for example the kinetics of proton transfer from 2,3-dihydrobenzo[6]thiophenc-2-onc <06JOC8203>, the configuration of imines derived from thiophenecarbaldehydes <06JOC7165>, and the relative stability of benzo[c]thiophene <06T12204>. The kinetics of nucleophilic aromatic substitution of some 2-substituted-5-nitrothiophenes in room temperature ionic liquids have also been investigated <06JOC5144>. 

The kinetics of proton transfer from protonated 1,8-bis(dimethyl-amino)-2,7-dimethoxynaphthalene to substituted phenolate ions (69) were studied in 70% (v/v) Me2SO—H20 using the temperature-jump technique with spectrophotometric detection to follow reactions with half-lives in the range 1-100 ms (Hibbert and Robbins, 1978). A limited... 

Two parameters that are accessible to the experimentalist have been the effect of the interchange of a deuteron for a proton upon the dynamics of transfer and the effect of temperature variation upon the kinetics of proton-deuteron transfer [49]. As previously mentioned, the semiclassical model has been employed in the rationalization of kinetic deuteron isotope effects that exceed the factor of 7.0, the maximum predicted by the classical model [5]. However, the full quantum model also allows for the wide range in the kinetic deuteron isotope effect, the range of which overlaps that predicted by the semiclassical model [53]. Thus, the kinetic deuteron isotope effect in and of itself cannot be used to distinguish between the two models. 

Statement number 6 has to do with carbon acids and is supported by reference (7). There are, in fact, other references that suggest solvent plays a much more direct role in the kinetics of protonating carbanions than statement number 6 would imply. For example, there is evidence that nuclear reorganization and rehybridization of the carbon atom are too rapid to have much kinetic importance when compared with solvent reorientation. The strong dependence of carbanion protonation rates on the solvent supports this view. These rates are typically much faster in organic solvents, such as DMSO, than in water. A particular reaction that was studied in different solvents (17) is... 

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.

Results of a study of polar, steric and structural influences on the kinetics of proton transfer (ylide formation) from phosphonium ions to electrogenerated bases have been interpreted with caution. ... 

Tab. 10.3 Enthalpies of adsorption and kinetics of proton adsorption and desorption on iron oxides.

In summary, although the BH model predicts an inverted region for the kinetics of proton in the nonadiabatic regime, the BH model is only in qualitative accord with the data derived from the proton transfer within the benzophenone-N, A -dimethylaniline contact radical ion pairs. The failure of the model lies in its ID nature as it does not take into account the degrees of freedom for the vibrations associated with the proton-transfer mode. By incorporating these vibrations into the BH model, the LH model provides an excellent account of the parameters serving to control the kinetics of nonadiabatic proton transfer. A more rigorous test for the LH model will come when the kinetic deuterium isotope effects for benzophenone-A, A -dimethylaniline contact radical ions are examined as well as the temperature dependence of these processes are measured. 

The apparent kinetics of proton-coupled ReV/in electron transfer also are dependent on 0X0 group geometry here (ks,h)cis/(ks,h)trans 100 [36]. It is proposed that the apparent kinetics is controlled by the thermodynamic accessibility of the intermediate Re (IV) state, whose effective potential is modulated by protonation, 0X0 group geometry, and pyridyl ligand substituents. 

The kinetics of proton transfer in aqueous DMSO from benzoylnitromethane (353) and l,2-diphenyl-2-nitroethanone (354) to various bases has recently been examined.318... 

The kinetics of proton transfer from ethyl bis(4-nitrophenyl)acetate to N-bases with guanidine-like character, in acetonitrile, are determined by basicity and steric hindrance in die vicinity of the reacting site of the N-base and also by different distributions of positive charge in protonated A-bases.147... 

Deeble DJ, von Sonntag C (1985) TheUV absorption spectra of theC(5) and C(6) OH adduct radicals of uracil and thymine derivatives. A pulse radiolysis study. Z Naturforsch 40c 925-928 Deeble DJ, von Sonntag C (1987) Radioprotection of pyrimidines by oxygen and sensitization by phosphate a feature of their electron adducts. Int J Radiat Biol 51 791-796 Deeble DJ, Das S, von Sonntag C (1985) Uracil derivatives sites and kinetics of protonation of the radical anions and the UV spectra of theC(5) and C(6) H-atom adducts. J Phys Chem 89 5784-5788... 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

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