Proton dissociation rate determination

The advantage of Eqs. (12.13)-(12.15) and (12.17) was that they allowed the direct determination of the excited-state equilibrium constant by a single kinetic measurement. The proton dissociation rate constant and hence also the proton recombination rate constant may also be found from the same measurement. Although this method has been applied successfully in only a few cases [60, 61], the values thus found have been in very good agreement with values independently estimated from the Forster cycle or by steady-state titrations. 

Steps 2 and 4 are proton transfer reactions and are very fast Nucleophilic addi tion to the carbonyl group has a higher activation energy than dissociation of the tetra hedral intermediate step 1 is rate determining... 

Reaction conditions can be modified to accelerate the rate of lithiation when necessary. Addition of tertiary amines, especially TMEDA, facilitates lithiation53 by coordination at the lithium and promoting dissociation of aggregated structures. Kinetic and spectroscopic evidence indicates that in the presence of TMEDA lithiation of methoxybenzene involves the solvated dimeric species (BuLi)2(TMEDA)2.54 The reaction shows an isotope effect for the o-hydrogcn, establishing that proton abstraction is rate determining.55 It is likely that there is a precomplexation between the methoxybenzene and organometallic dimer. 

The dioxo complexes of W(IV) and Mo(IV), having high pKa values (Table II), are formed via hydrolysis as the rate-determining step (Scheme 4) and the observed rate constants for the inversion along the O-M-O axis for the W(IV) and the Mo(IV) complexes are therefore defined by Eq. (18). These were calculated as a function of pH, using the proton exchange rate constants (Table IV) and the acid dissociation constants (Table II)... 

As indicated in Fig. 13.10, the slowest, and therefore rate-determining, reaction step is then the nucleophilic attack of a water molecule at the carbonyl carbon of the protonated species. This carbonyl is much more susceptible to nucleophilic attack than in the neutral ester. Since the dissociation of the (protonated) leaving group (HO-R2) is fast (forward portion of reaction 4 in Fig. 13.10), the rate of ester disappearance through acid-catalyzed hydrolysis is given by ... 

It has been concluded from deuterium exchange experiments, using ethylene and heavy water, that the addition of an adsorbed proton to adsorbed ethylene is the actual rate-determining step. It can be seen that the two schemes differ, mainly in that the latter includes dissociative adsorption of water on the surface of the catalyst and does not specify the adsorption of ethylene, but they are consistent in that they assume the formation of a carbonium ion as the rate-determining step. 

Studies on the complexing of Cd2+ by Ar-carboxymethyliminobis(ethylenenitrilo-iV, N -diacetic acid) (H3L) in the pH range 7-2 are consistent with stepwise protonation of the species [CdL]3 to give finally [CdH2L] . The proton-assisted dissociation is interpreted as a stepwise unwrapping from one end of the molecule, with Cd—N fissions corresponding to the rate-determining steps.1213... 

The rates of dissociation of Gd3+ chelates used or proposed as CAs are generally low at pH = 7.4. The complexes dissociate much faster in acidic solutions, when the proton-assisted dissociation predominates. In order to compare the kinetic stabilities of complexes, some authors suggest use of the first-order rate constants (kobs) obtained in 0.1 M HCl or HC104 solution. Wedeking et al. compared the acid-assisted dissociation rates (kobs) of several acyclic and cyclic Gd3+ complexes with the long term (14 days) deposition of Gd in the whole body of mice [ 15]. They found an inverse proportionality between the dissociation rates (kobs) and the total residual Gd, i.e. the lower the dissociation rate, the less the residual Gd found in the bodies of the mice [15]. The (kobs) values determined by Wedeking et al. and other authors for several Gd3+ and Y3+ complexes are shown in Table 1. 

Fast atom bombardment mass spectrometry has been utilized for the quantitative determination of ionic species, in glycerol/water solutions, which are produced by chemical and enzymic reactions. It is shown that reaction constants can be determined in this manner and that they can be accurately related to those determined by other methods used in the analysis of aqueous solutions. The reactions studied include proton dissociation constants for organic acids, an enzyme equilibrium constant, and enzyme rate constants using natural substrates. 

The O-H bond dissociates with the liberation of a proton this bondbreaking is readily accomplished with the aid of a base, usually the solvent. Thus, the breaking of the C-H bond is generally the slow (rate-determining) step of the reaction. 

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