Kinetics multistep mechanisms

The influence of Pb + ions on the kinetics of zinc electrodeposition on Zn electrode in acidic sulfate electrolyte was discussed [217] in terms of a reaction model involving hydrogen adsorption and evolution, a multistep mechanism for zinc deposition and the overall reaction for zinc dissolution. The strongly adsorbed Pbads inhibited all the reactions taking place on the zinc electrode. 

Seibig S, van Eldik R (1997) Kinetics of F e (11) EDTA oxidation by molecular oxygen revisited. New evidence for a multistep mechanism. Inorg Chem 36 4115-4120 SimandanT, Sun J, DixTA(1998) Oxidation of DNA bases,deoxyribonudeosides and homopolymers by peroxyl radicals. Biochem J 335 233-240 Spinks JWT, Woods RJ (1990) Introduction to radiation chemistry. Wiley, New York Strohmeier Gort A, Imlay JA (1998) Balance between endogeneous superoxide stress and antioxidant defenses. J Bacteriol 180 1402-1410... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

Sebig S, van Eldik R. Kinetics of [Fen(edta)] oxidation by molecular oxygen revisited, new evidence for a multistep mechanism. Inorg Chem 1997 36 4115-20. 

In a multistep mechanism, a reaction can occur no faster than its rate-determining step. Oniy the concentrations of the reactants affecting the rate-determining step appear in the rate equation. In this example, the rate depends on the concentration of A—B only, because only A—B appears in the rate-determining step. A reaction involving only one reactant is said to be unimo-lecular. Because there is only one concentration term (raised to the first power), the rate equation is first order (the reaction follows first-order kinetics). 

Kinetic studies of COX inhibition reveal that many NSAIDs interact with COX through a multistep mechanism in which a rapid, reversible step is followed by one or more slow steps that may be poorly reversible (Eq. 2) (9, 10) ... 

The kinetics of 8 and 9 binding and inhibition with COX-1 reveal an interesting profile. Pre-steady state kinetic measurements indicate that binding of 8 and 9 proceeds by a multistep mechanism like that of INDO (Eq. 2). Although Aj, the ratio of k i to ki, is similar for 8, 9, and INDO, formation of E—I proceeds at a much faster rate (10-fold) for 8 and 9 than for... 

Armstrong and Firman ° analyzed a mechanism that included two successive electron-transfer reactions. A general approach to multistep mechanisms involving soluble species in semi-infinite diffusion was presented recently hy Harrington. It allows determination of the number of breakpoint frequencies on the Bode magnitude plot for an arbitrary mechanism and, in consequence, for the determination of the reaction mechanism and kinetics. 

Using information from items 1-5 above, the overall kinetics of the PTC cycle in a liquid-liquid system can be characterized. However, due to the complex multistep mechanism of PTC, various approaches to LLPTC modeling have been taken (Bhattacharya, 1996 Chen et al., 1991 Wu, 1993 Melville and Goddard, 1988 Evans and Palmer, 1981). However, though each of these models has its own merits, we believe that a comprehensive general model for aU LLPTC reactions is yet to be developed. 

Of course, this is a required result if the kinetic model has any pretense to validity, and it is important that the B V model attains it for the limit of / = 0, not only for the simple one-step, one-electron process, but also in the context of an arbitrary multistep mechanism. The derivation here was carried out for a mechanism in which the prereactions and postreactions involve net charge transfer however the same outcome can be obtained by a similar method for any reaction sequence, as long as it is chemically reversible and a true equilibrium can be established. 

The point of these results is to illustrate some of the difficulties in dealing with a multistep mechanism involving an embedded RDS. No longer is the potential dependence of the rate constant expressible in two parameters, one of which is interpretable as a measure of intrinsic kinetic facility. Instead, becomes obscured by the first exponential factors in (3.5.25) and (3.5.26), which express thermodynamic relationships in the mechanism. One must have ways to find out the individual values of ri rds before... 

Shortly after the work of Koehler et al. on Si(lll) appeared, Sinniah et al. [33] reported the unexpected result that desorption from Si(lOO) followed first-order kinetics. These experiments used an isothermal technique like that of Koehler et al. However, the measurements of Sinniah et al. were sensitive to coverages as low as 0.006 ML, allowing them to use initial coverages as low as 0.06 ML. This result demonstrates that the mechanism of recombi-native desorption on Si(lOO) is qualitatively different from that on metal surfaces, where kinetics are second order [34]. It is easy to rationalize second-order desorption kinetics by a mechanism in which two independently diffusing H atoms must approach one another to recombine. A mechanism that yields first-order kinetics must imply either (a) some interaction between the H atoms (so that their positions are correlated) or (b) a multistep mechanism where the rate-limiting step involves motion of only one H atom. 

In the first edition and in much of the literature, one finds used as the n value of the rate-determining step. As a consequence appears in many kinetic expressions. Since is probably always 1, it is a redundant symbol and has been dropped in this edition. The current-potential characteristic for a multistep process has often been expressed as . This is rarely, if ever, an accurate form of the i—E characteristic for multistep mechanisms. ... 

A computational study of the kinetics and mechanism of the gas-phase pyrolysis of allylmethylamine has been performed. Two mechanisms were investigated, one involving a six-membered cyclic TS and the other being a multistep, free radical mechanism. Both DFT and high-level calculations show that propene and imine formation are in accordance with a concerted cyclic mechanism. 

The interpration of the ultrasonic relaxation spectrum in terms of the multistep mechanism is complicated but gives detailed kinetic information. It requires, however, a basic knowledge of the equilibrium distribution. In practice the ultrasonic technique is often used to the investigate ion of systems for which the equilibrium properties cannot be measured by other experimental means and hence the detailed equilibrium description is not known. In those situations it is still possible to obtain some information about the rates involved by using the two state consideration (13). It must be stated, that the rate constants obtained by this procedure are sort of mean values that may differ somewhat from the individual true rate constants. 

Most unimolecular processes are apparently observed in the first-order (high-pressure or high-concentration) region, and we will continue to assume first-order kinetics for unimolecular steps in multistep mechanisms. Most proposed mechanisms do not include unimolecular steps, and there are not very many reactions known to have a one-step unimolecular mechanism. The first such reaction discovered was the isomerization of cyclopropane to propene. Others are the dissociation of molecular bromine and the decomposition of sulfuryl chloride. ... 

While we are still self-constrained to limit our treatment to what we believe is essential to physical chemistry, we have added further examples to the Chapter 7 treatment of reaction kinetics, which include some aspects of multistep mechanisms and introduced the steady-state approximation. The steady-state concept was then extended to the Eyring transition-state concept and used again for the critical step in the Michaelis-Menten treatment of enzyme kinetics. This has been a fast tour of some complicated algebra but in our experience students who learn the derivations have a deeper appreciation for the concepts. Casual interviews of students from past classes have revealed that the Michaehs-Menten derivations have been the most useful aspect of this chapter. 

A kinetic study of the arylpyrrolidino amido thiourea-catalysed 5 reaction of meio-diphenyl episulfonium ion by indole derivatives showed that the rate and enantioselectivity-determining step were the substitution reaction by C(3) of the indole on C of the episulfonium ion, and a secondary a-deuterium KIE of 0.93 found using 3-deutereoindole indicates that this step is an 5 2 process. " Spectroscopic and product studies supported this mechanism for the substitution. Yields are usually high (54 to >99%) with a high ee (79-95%). A proposed multistep mechanism involving the catalyst and a transition state for the 5 2 reaction are given. 

Identification of the intermediates in a multistep reaction is a major objective of studies of reaction mechanisms. When the nature of each intermediate is fairly well understood, a great deal is known about the reaction mechanism. The amount of an intermediate present in a reacting system at any instant of time will depend on the rates of the steps by which it is formed and the rate of its subsequent reaction. A qualitative indication of the relationship between intermediate concentration and the kinetics of the reaction can be gained by considering a simple two-step reaction mechanism ... 

In Chapter 1 we distinguished between elementary (one-step) and complex (multistep reactions). The set of elementary reactions constituting a proposed mechanism is called a kinetic scheme. Chapter 2 treated differential rate equations of the form V = IccaCb -., which we called simple rate equations. Chapter 3 deals with many examples of complicated rate equations, namely, those that are not simple. Note that this distinction is being made on the basis of the form of the differential rate equation. 

---