Associative mechanism rate constant

Reaction of the newly prepared and characterized cation trans-[Co(en)2(S203)(OH2)], containing S-bonded thiosulfate, with nitrite or thiocyanate follows a second-order rate law. It is much more likely that this is to be explained by a dissociative interchange than by an associative mechanism. Rate constants and activation parameters (A// and A5 ) are reported. The S-thiosulfate ligand has a considerably smaller fran -labilizing effect than S-sulfite. The hydroxo complex tr(3n5 -[Co(en)2(S203)(OH)] is less reactive than the aquo complex. 

NEIGHBORING GROUP MECHANISM ASSOCIATION ASSOCIATION CONSTANT DISSOCIATION CONSTANT STABILITY CONSTANT Association/dissociation rate constants, determination,... 

Validation of the Mechanism. The process of matching the predictions of the mechanism to experimental smog chamber data is termed validation of the mechanism. The first step in a validation procedure is to establish values for the two major classes of parameters that appear in the mechanism—the reaction rate constants and the stoichiometric coeflBcients. Base values of the rate constants can be estimated from the chemical literature. However, with the sacrifice of chemical detail present in the new, simplified mechanism is a loss in the ability to associate the rate constant values with particular reactions. Therefore, the rate constants in the simplified mechanism are more a quantitative assessment of the relative rates of competing reactions than a reflection of the exact values for particular reactions. Base values for the parameters that appear in the kinetic mechanism are thus established on the basis of published rate constants. However, we must expect that final validation values will consist of those values which produce the best fit of the mechanism to actual smog chamber data. A recent summary of rate constants for specific hydrocarbon systems was made by Johnston et al. 40) from which rate constants for the Reactions in Table I can be estimated for a number of hydrocarbons. 

Models with increasing sophistication for the analysis of dynamic processes in supramolecular systems, notably micelles, as well as for the determination of other parameters have been developed over the past two decades. The basic conceptual framework has been described early on [59,60,95,96] and has been classifred into different cases which take into account the extent of quencher mobility and the mechanism of quenching [95]. Two of those cases lead to information about mobility and will be discussed. It is important to emphasize that this analysis is only applicable to self-assembled system such as micelles and vesicles it cannot be applied to host-guest complexes. This model assumes that the probe is exclusively bound to the supramolecular system and that no probe migration occurs during its excited state lifetime. The distribution of probe and quencher has been modeled by different statistical distributions, but in most cases, data are consistent with a Poisson distribution. The Poisson distribution implies that the quencher association/dissociation rate constants to/from the supramolecular system does not depend on how many... 

There are several sources of uncertainty in an atmospheric chemical mechanism. First, some important reactions may be completely unknown and therefore not included in the mechanism. Second, for those reactions included in the mechanism, rate constants are known to varying degrees of accuracy. Third, the products or the distribution of products for some reactions are in question. In addition, when a chemical mechanism is used for a non-steady-state simulation, initial concentrations must be supplied for all the species in the mechanism. Some of these initial concentrations may not be available for simulation, and those that are measured may have uncertainties associated with them. The first source... 

Kinetics and mechanism of water exchange at iron(III), in the form of Fe aq and of FeOtf aq, have been much studied recently. It is now clear that water exchange at the former takes place by an associative mechanism, at the latter by a dissociative mechanism. Rate constants, " activation enthalpies and entropies, and activation volumes have been obtained from variable-temperature and variable-pressure 0 nmr experiments. The activation volumes for water exchange at Fe Caq) and at FeOH (aq) are -5.4 and 4-7.0 cm moL respectively the former is almost exactly the same as that reported for the isoelectronic Mn (aq) ion. If acidic (HCIO4) aqueous solutions containing iron(III) are heated for a time, then water exchange at the polynuclear product is over a... 

This two-term form, normal for square-planar complexes, is extremely unusual for substitution at an octahedral complex. The tantalum(v) appears to be present in the reaction system solely as [TaF ], but of course [TaF ] is a stable anion so that parallel associative and dissociative paths for fluoride exchange represent a reasonable mechanism. Rate constants and activation parameters are listed in Table 9. The activation entropy for the ki term is entirely consistent with associative fluoride exchange via a... 

In writing Eqs. (7.1)-(7.4) we make the customary assumption that the kinetic constants are independent of the size of the radical and we indicate the concentration of all radicals, whatever their chain length, ending with the Mj repeat unit by the notation [Mj ], This formalism therefore assumes that only the nature of the radical chain end influences the rate constant for propagation. We refer to this as the terminal control mechanism. If we wished to consider the effect of the next-to-last repeat unit in the radical, each of these reactions and the associated rate laws would be replaced by two alternatives. Thus reaction (7. A) becomes... 

Aside from merely calculational difficulties, the existence of a low-temperature rate-constant limit poses a conceptual problem. In fact, one may question the actual meaning of the rate constant at r = 0, when the TST conditions listed above are not fulfilled. If the potential has a double-well shape, then quantum mechanics predicts coherent oscillations of probability between the wells, rather than the exponential decay towards equilibrium. These oscillations are associated with tunneling splitting measured spectroscopically, not with a chemical conversion. Therefore, a simple one-dimensional system has no rate constant at T = 0, unless it is a metastable potential without a bound final state. In practice, however, there are exchange chemical reactions, characterized by symmetric, or nearly symmetric double-well potentials, in which the rate constant is measured. To account for this, one has to admit the existence of some external mechanism whose role is to destroy the phase coherence. It is here that the need to introduce a heat bath arises. 

Some quantities associated with the rates and mechanism of a reaction are determined. They include the reaction rate under given conditions, the rate constant, and the activation enthalpy. Others are deduced reasonably directly from experimental data, such as the transition state composition and the nature of the rate-controlling step. Still others are inferred, on grounds whose soundness depends on the circumstances. Here we find certain features of the transition state, such as its polarity, its stereochemical arrangement of atoms, and the extent to which bond breaking and bond making have progressed. 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

AB A2 + 2 B (a) What rate law is predicted by this step (b) What units are associated with the rate constant for this rate law (c) Write additional steps that complete the mechanism. 

The rate constants associated with the conversion of the pyrrolo[ 1,2-c/Jindole hydroquinone to its quinone methide were fit to the rate law equation (7.1), see Fig. 7.17 for rate data and the fit. The solid line in Fig. 7.17 was generated with Eq. 7.1 where k0 = 0.09min-1 and k 1.5 x 105M min The mechanism consistent with the pH-rate profile is the spontaneous elimination of acetate (kf) process) and the proton assisted elimination of acetate (kx process) from the electron-rich hydroquinone. The k0 process is independent of pH and exhibits a zero slope while the kx process exhibits a — 1 slope consistent with acid catalysis. 

To distinguish between simple, reversible slow binding (scheme B) and an enzyme isomerization mechanism (scheme C), one can examine the dependence of kobs on inhibitor concentration. If the slow onset of inhibition merely reflects inherently slow binding and/or dissociation, then the term kobs in Equations (6.1) and (6.2) will depend only on the association and dissociation rate constants k3 and k4 as follows ... 

This is a linear equation, and we can thus expect kobs to track linearly with inhibitor concentration for an inhibitor conforming to the mechanism of scheme B. As illustrated in Figure 6.4, a replot of kobs as a function of [/] will yield a straight line with slope equal to k3 and y-intercept equal to k4. It should be noted that in such an experiment the measured value of k3 is an apparent value as this association rate constant may be affected by the concentration of substrate used in the experiment, depending on the inhibition modality of the compound (vide infra). Hence the apparent value of Ki (Kfw) for an inhibitor of this type can be calculated from the ratio of... 

The reactions of the bare sodium ion with all neutrals were determined to proceed via a three-body association mechanism and the rate constants measured cover a large range from a slow association reaction with NH3 to a near-collision rate with CH3OC2H4OCH3 (DMOE). The lifetimes of the intermediate complexes obtained using parameterized trajectory results and the experimental rates compare fairly well with predictions based on RRKM theory. The calculations also accounted for the large isotope effect observed for the more rapid clustering of ND3 than NH3 to Na+. 

The conclusions derived from the preceding experiments may be summarized with the aid of the reaction mechanism illustrated in Scheme II. The ester undergoes a rapid, reversible association with the cycloamylose, C—OH. An alkoxide ion derived from a secondary hydroxyl group of the cycloamylose may then react with an included ester molecule to liberate a phenolate ion and produce an acylated cycloamylose. This reaction is characterized by a rate constant, jfc2(lim), the maximal rate constant for the appearance of the phenolate ion from the fully complexed ester in the pH range where the cycloamylose is completely ionized. Limiting rates are seldom achieved, however, because of the high pK of cycloamylose. 

The rate constant for the k term equals that for reaction of [Ca(parv)] with cydta, consistent with rate-determining dissociation of [Ca(parv)] in both cases the k2 term may be assigned to an associative (adjunctive) process (497). This mechanism parallels that of parallel associative and dissociative pathways established for displacement of edta from Ca(edta)2 by Ttr+ (cf. Section II.D.3 (334)). 

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