Kinetics of proton transfer

2 Theoretical Prediction of Proton Tranter Rate Coefficients 

There are a number of theoretical prescriptions for determining rate coefficients of exothermic ion-molecule reactions. The simplest originates from concepts developed more than a century ago by Langevin and applies to a reaction between a point ion and a spherical (non-polar) molecule [20]. The dominant long-range attractive force in this case is the ion-induced dipole interaction, which takes the form 

Incorporation of this attractive potential into a simple collision model, which we shall not present here, allows the prediction of the reaction cross section and, ultimately, the reaction rate coefficient. The Langevin rate coefficient, as it is often known [21], is given by 

Notice that in this case the potential energy is a function of both the atom-molecule separation, r, and the angle, 0, between a line joining the ion to the centre of charge in the neutral molecule and the direction of the permanent dipole moment vector. Notice that the dependence on r is now rather than r so on this basis alone if the Langevin model was used it wiU underestimate the rate coefficient for a reaction between a point ion and a polar molecule. 

A more realistic description was provided by Su and Bowers and is known as the average dipole orientation (ADO) theory [22,23]. The ADO rate coefficient is given by 

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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... 

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. ... 

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... 

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. 

In porphyrin H2P (71, also known as porphine) the tautomerism is degenerate i.e., tautomers 71a and 71b are identical. Also, in the anion HP-72, the four tautomers are all degenerate the kinetics of proton transfer have been studied. In chlorins, bacteriochlorins, and isobacteriochlorins, all the tautomers are different, but all the evidence indicates that the most stable tautomers are those with the inner protons in a trans disposition, represented by 73, 74, and 75, all having an 18-rr-delocalized system. Gossauer s penta-phyrin, a 22-77-electron pentapyrrole macrocycle with two inner hydrogen atoms, should also be prone to annular tautomerism [83JCS(CC)275]. 

Kinetics of proton transfer to and from C2[Pg.1258]

The Kinetics of Proton Transfer Processes, conference reports, Angew. Chem., Int. Ed. Engl, 4, 791 (1965). 

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. In diglyme or 2-hydroxyethanol, perfluoropropyl vinyl ether [355 R = F(CF2)30] reacts with sodium carbonate to give (356 same R) which on heating at 113-132 °C gives rise to (357 same R) and (358 same R) via parallel mechanisms. ... 

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