For many decades, the concept of a "rate-determining step" has been of central importance in understanding chemical kinetics in multistep reaction mechanisms and using that understanding to advantage. Yet a rigorous method for identifying the rate-determining step in a reaction mechanism was only recently introduced, via the "degree of rate control" of elementary steps. By extending that idea, we argue that even more useful than identifying the rate-determining step is identifying the rate-controlling transition states and the rate-controlling intermediates. These identify a few distinct chemical species whose relative energies we could adjust to achieve a faster or slower net reaction rate. Their relative energies could be adjusted by a variety of practical approaches, such as adding or modifying a catalyst, modifying the solvent, or simply modifying a reactant's molecular structure to affect electronic or steric control on the relative energies of the key species. Since these key species are the ones whose relative energies most strongly influence the net reaction rate, they also identify the species whose energetics must be most accurately measured or calculated to achieve an accurate kinetic model for any reaction mechanism. Thus, it is very important to identify these rate-controlling transition states and rate-controlling intermediates for both applied and basic research. Here, we present a method for doing that.

Page 4191. The absolute stereochemistry of compounds in Table 2 has been revised on the basis of an empirical model proposed previously and by analogy to related π-allyl reactions. 13 For entry 7, the absolute stereochemistry of the sulfamate product was determined following its conversion to the known (R)-tert-butyl-1-hydroxybut-3-en-2-ylcarbamate; the stereochemical result is in accordance with the empirical model. Structures depicted in the Supporting Information should reflect the changes made in the corrected version of In this paper, we extended Campbell's "degree of rate control of each elementary step" to similarly define the "degree of rate control of each intermediate" for the general kinetic analysis of any multistep reaction mechanism. We did not realize that a very similar extension had already been made by Kozuch and Shaik, 1 who introduced the "degree of turnover frequency control of each intermediate" for analyzing multistep catalytic mechanisms. Kozuch and Shaik also had previously presented many useful applications of this concept, and ideas that evolve from it, 1,2 some of which are similar in some ways to points made in our paper. We sincerely apologize for not having cited their two pioneering papers in these very important respects. Table 2. Pd-Catalyzed Asymmetric Allylic Amination a Reactions were performed in THF using 2.5 mol % Pd 2 (dba) 3 · CHCl 3 , 7.5 mol % (S,S)-L 1 at 0.2 M; yields are based on limiting amounts of nucleophile 2. Literature Citedb Reaction conducted using (R,R)-L 2 . c Reaction conducted in dioxane.

The possible existence of a compensation effect, i.e. concurrent changes in activation energy and prefactor, is investigated for the hydrogenation and dehydrogenation kinetics of metal hydrides, by analyzing a series of reported kinetic studies on Mg and LaNi(5) based hydrides. For these systems, we find a clear linear relation between apparent prefactors and apparent activation energies, as obtained from an Arrhenius analysis, indicating the existence of a compensation effect. Large changes in apparent activation energies in the case of Mg based hydrides are rationalized in terms of a dependency of observed apparent activation energy on the degree of surface oxidation, i.e., a physical effect. On the other hand, we find the large concurrent changes in apparent prefactors to be a direct result of the Arrhenius analysis. Thus, we find the observed compensation effect to be an artifact of the data analysis rather than a physical phenomenon. In the case of LaNi(5) based hydrides, observed scatter in reported apparent activation energies is less pronounced supporting the general experience that LaNi(5) is less sensitive toward surface contamination.

The mechanism of SO2 absorption in seawater is treated. Emphasis is on applications of scrubbing of marine engine exhaust gas containing SO2. The formulated model is used to predict the influence of various parameters on SO2 absorption efficiency, e.g., seawater temperature, partial pressure of SO2, seawater salinity, and seawater alkalinity. It is found that the absorption capacity of standard seawater is approximately twice that of brackish water with close to zero salinity. The absorption capacity decreases with both decreasing salinity and alkalinity. Different scenarios in which the required water supply rate for a given SO2 cleaning efficiency is calculated. It is found that a 66% cleaning efficiency, corresponding to meeting the limits of SO x emission control areas (SECA) when operating on a fuel containing 4.5% w/w sulfur, requires a minimun water supply rate of 40–63 kg/(kW h) depending on the seawater composition in terms of salinity and alkalinity. Such data are essential in judging the operating cost of seawater scrubbing compared to alternative methods.

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