Proton transfer reactions in water

Table 4-1. Rate Constants for Proton Transfer Reactions in Water ...

Loewenstein and Szoke (1962) measured activation energies for proton transfer reactions in water over the range 293°-353°K. The Arrhenius activation energies are 2-6 + 0-3 kcal mole-1 in the acidic range and 4-8 + 0-5 kcal mole-1 in the basic range. In these two ranges the intermediate must be the hydronium and hydroxide ion respectively. 

Acids provide protons, H, in proton transfer reactions in water, while bases take up protons in proton transfer reactions in water, s When both acids and bases are present in the same solution, they will transfer protons between themselves. 

Equilibrium Constants of Proton-transfer Reactions in Water (Section 1, Table 1-10)... 

The activation energies of proton transfer reactions in water. J. Am. Chem. Soc.,... 

The centroid PMFs from PI-FEP/UM simulations for the nitroethane deprotonation reaction in NAO and in water (Scheme 21.2) are shown in Figure 21.2. The reaction coordinate is defined as the difference between the breaking (donor-proton) and forming (acceptor-proton) bond distances (Scheme 21.2). The computed free energies of activation are 15.9 and 24.4kcalmol for the enzymatic and the uncatalyzed proton transfer reaction in water, respectively, in accord with experimental results (14.0 and 24.8kcalmol- ). ... 

Figure 8.4 Free-energy changes in bonding and solvation for a proton-transfer reaction in water (schematic). See text. Only the proton-transfer step (pt in Figure 8.3) is shown.The motions of the solvent molecules and of the proton are assumed to be uncoupled.

Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. The acid-base properties of monoanions of polyprotic acids are complicated, however, because the monoanion is simultaneously the conjugate base of the parent acid and an acid in its own right. For example, hydrogen carbonate anions undergo two proton-transfer reactions with water ... 

C17-0125. When ammonium acetate dissolves in water, both the resulting ions undergo proton transfer reactions with water, but the net reaction can be written without using water ... 

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

Note that A is called the conjugate base of HA and BH+ the conjugate acid of B. Proton transfer reactions as described by Eq. 8-1 are usually very fast and reversible. It makes sense then that we treat such reactions as equilibrium processes, and that we are interested in the equilibrium distribution of the species involved in the reaction. In this chapter we confine our discussion to proton transfer reactions in aqueous solution, although in some cases, such reactions may also be important in nonaqueous media. Our major concern will be the speciation of an organic acid or base (neutral versus ionic species) in water under given conditions. Before we get to that, however, we have to recall some basic thermodynamic aspects that we need to describe acid-base reactions in aqueous solution. 

SM2/AM1 and SM3/PM3 calculations in water as well as SM4/AM1 and SM4/PM3 calculations were performed on cyclohexane to study proton transfer reactions in 1-methylindene with two bases, ammonia and trimethylamine. The calculations confirmed predictions that the proton moves relatively freely over the indene ring once it is abstracted from the original location by the base [130]. 

Bronsted acid-base theory — In 1923, Bron-sted and, independently of him, Lowry published essentially the same theory of acids and bases which can be applied not only to water as a solvent but also to all other - protic solvents, as well as to proton transfer reactions in gases. An acid is defined as a proton donor, i.e.,... 

A second advantage of water is that in addition to being able to dissolve electrolytes by the physical forces involved in solvation, it is also able to undergo chemical proton-transfer reactions with potential electrolytes and produce ionic solutions. Water is able to donate protons to, and to receive protons from, molecules of potential electrolytes. Thus, water can function as both a source and a sink for protons and consequently can enter into ion-forming reactions with a particularly large range of substances. This is why potential electrolytes often react best with water as a partner in the proton-transfer reactions. Finally, water is stable both chemically and physically at ambient temperature, unlike many organic solvents which tend to evaporate (Table 4.24) or decompose slowly with time. 

Dickson, A. G. (1984) pH-Scales and Proton Transfer Reactions in Saline Media Such as Sea-water, Geochim. Cosmochim. Acta 48, 2299-2308. 

NMR has been used extensively to study the kineties of proton transfer reaetions in water and other solvents. In the ease of the protolysis reaction... 

Data for some protolysis and hydrolysis reactions in water and methanol are summarized in table 7.8. The rate constants for the backward reactions are all very fast, whereas those for the forward reactions depend very much on the nature of the reaction. When these data are combined with those in table 7.2 a more complete description of proton transfer reactions in protic solvents is available. It is clear that the solvent plays an important role in these processes. 

Dibenzosuberene derivatives have been shown to undergo proton-transfer reactions to water from their excited singlet states. Biphenyl derivatives are also reactive under these conditions. A full report of the excited state proton transfer in phenols has been published. °... 

Figure 7.2 (A) Schematic representation of photoinduced proton-transfer reaction in the enol form (E ) and twisting motion in the keto-type (K ) structure to generate the keto-rotamer (KR ) ofexcited l -hydroxy,2 -acenaphthone (HAN). (B) Illustration of HAN molecule in water and complexed with one or two CD nanocavities.

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