Bimolecular Proton Transfer in Solution

Ultrafast UV/vis spectroscopy has until now been used extensively in the case of excited state intramolecular hydrogen or proton transfer (ESIHT/ESIPT) [25-27], where donor and acceptor groups, linked by the hydrogen or proton, are part of the same molecule [28, 29]. The dynamics and microscopic mechanisms of 

Hydrogen-Transfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. limbach, and R. L Schowen Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

ESIHT or ESIPT have been studied in fine detail, see also Chapters 9, 14 and 20, ivithout interference from any additional processes ivhere translation or reorientation of donor and acceptor groups are required to facilitate the hydrogen or proton transfer reaction. 

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According to general views on bimolecular proton-transfer in solution, the rate of proton transfer to solvent is closely coupled with solvent relaxation, and the measured dissociation constants (k J are assumed to follow an Arrhenius relation ... 

The first step is a bimolecular reaction leading to the formation of a hydrogen bond the second step is the breaking of the hydrogen bond such that the protonated species H B+ is formed the third step is the dissociation reaction to form the products. In aqueous solutions, the bimolecular reaction proceeds much faster than would be predicted from gas phase kinetic studies, and this underscores the complexity of proton transfer in solvents with extensive hydrogen-bonding networks capable of creating parallel pathways for the first step. In their au-... 

Ab initio and DFT studies of keto-enol equilibria of deltic acid (2,3-dihydroxycyclo-prop-2-en-l-one) in gas phase and aqueous solution have deflned a bimolecular proton transfer mechanism. Effects of ring size on the tautomerism and ionization reaction of cyclic 2-nitroalkanones in cyclohexane have been studied by H NMR and DFT calculations. " ... 

The resultant charged species can then take part in hydride, proton, and electron transfer equilibria in the solution. Intermediate oxidation products, which have not had time to engage in the bimolecular equilibria, thus have shorter lengths of conjugation. The behavior of oxidized polyfurfuryl in solution may prove to be a useful model for the conductivity of insoluble conductive polymers. 

Akinetic study was also performed in a variety of vesicular solutions (DDAB, DODAB, DODAC [NaOH] = 2.25mM, 25 °C). Interestingly, the vesicles possess stronger catalytic reaction environments than the micelles. The rate-determining proton transfer from carbon to the hydroxide ion was accelerated up to 850 fold in di- -dodecyldimethylam-monium bromide (DDAB) vesicles. This is evidence that the reaction sides are less aqueous than those in micelles, as anticipated. Application of the pseudophase model afforded the bimolecular rate constants in the vesicles (kves). For the different vesicles, ves is significantly higher (ca. 12 times for DODAB) than the second-order rate constant in water. This shows that the catalysis is due to both a medium effect and a concentration effect. It was assumed that there was a fast equilibrium for substrate binding to the inner and outer leaflets of the bilayer, in accord with the fact that no two-phase kinetics were found. 

Since proton-transfers are essentially bimolecular processes, the velocity constants which correspond to the time ranges in Table 15 will depend upon the reactant concentrations which can conveniently be used k l/at, where a is the larger of the concentrations of the two reacting species) and will therefore vary with the nature of the system and the instrumental set-up employed. However, more than half of the methods listed in Table 15 can, under favourable circumstances be used to measure second-order velocity constants up to 10 °-10 Mm mol s" which will be shown later to represent the maximum attainable value for solution reactions. 

The presence of two metal centres opens up the possibility to smdy electronic coupling, and for multielectron photoredox processes to take place, whilst relatively long lifetimes of the excited states (see below) makes possible bimolecular reactions in solution. Accordingly, quadruply-bonded di-Re complexes have been reported to engage in bimolecular electron-transfer reactions, whereas the di-Mo and di-W complexes participate in oxidative addition and two-electron redox reactions. For example, UV irradiation of phosphate-supported M2 dinuclear complexes under acidic conditions leads to one- or two-electron oxidation of the metal-metal core accompanied by production of H2 gas by reduction of two protons. 

Rate constants were determined for a number of bimolecular photoreactions of hydroxy- and aminoaromatic compounds and heteroaromatic compounds with carboxylate anions and organic acids, respectively, in various micelles. Such bimolecular photoreactions were found to improve the efficiency of forward proton transfer reactions and to reduce reverse recombination of the primary products ( up to 2 orders of magnitude ) comparatively to aqueous solutions. 

In aqueous solution is of the order oflO M" s for molecules and slightly more or less for ions of opposite or the same charge. This value constitutes the limit for rate coefficients of bimolecular processes, which are then diffusion-controlled and have an activation energy of 3 to 5 kcal mole Many proton-transfer reactions are of this type. 

In aspirin, the acid catalysis is enhanced by the presence of an intra-molecular hydrogen bond. The H-bond in the salicylate anion is worth 17-21 kJ mol and raises the of the phenolic OH to 13. Other molecular models showed that EMs for intra-molecular general acid-base catalysis can be at least 10" M, if favourable intra-molecular H-bonds are formed [10]. This is also the factor that separates the typical pre-exponential factor of a bimolecular reaction in solution, 10 M sec from that of an intra-molecular reaction between the hydrogen-bonded reactants, 10 sec The basis for this change has been discussed in the context of the intersecting-state model (ISM) applications to proton transfers... 

Superoxide is believed to react as an effective hydrogen atom acceptor. Thermodynamically, it is one of the weakest oxidants in Table 9.10, but the electron transfer alternative is highly disfavored by the inaccessibility of 022 in aqueous solution Taube estimates that H02- has a pXa value of 21.110 Thus, a HAT mechanism is inferred for the oxidations of [Con(sep)]2 +, [Fen(tacn)2]2 +, [Run(tacn)2]2 +, and [Run(sar)]2 + nl in these reactions the ligands are the hydrogen atom donors, but it is the metal centers that are oxidized. The bimolecular disproportionation of H02 is another likely example of hydrogen atom transfer this mechanism may also apply to the disproportionation via the reaction of H02 with 02, 27 Curiously, the reaction of 02- with the hydrated electron is fast and seems not to be pH dependent. This latter reaction may yield genuine 022- as an intermediate, or it may be a form of PCET where the solvent donates a proton during the addition of the electron. 

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