Mechanistic discrimination

Perusal of Table 8 reveals that the problem of identifying the electroactive species is not solved for a number of reactions (nos. 3, 5, 23, 27, 28, 29) and that recent work utilizing modern electrochemical techniques has revived the discussion of some important ones (nos. 14, 15) and paved the way for renewed attempts at mechanistic discrimination. We shall deal with some of these reactions in the following discussion. 

The differences between platinum and palladium appear in a number of guises, but nowhere more clearly than in the reactions of neopentane. Although caution has constantly to be recommended in the use of values of activation energy for mechanistic discrimination, large differences may well betoken the intervention of alternative intermediates, differing in their degrees of dehydrogenation. Values... 

This section shows the versatility of the channel flow EPR cell for mechanistic discrimination. 

Cooper JA, Compton RG (1998) Photoelectrochemical reduction of p-bromonitrobenzene mechanistic discrimination via channel microband array voltammetry. Electroanalysis 10 (17) 1182-1187. doi 10.1002/(sici)1521-4109(199811)10 17<1182 aid-elanll82>3.0.co 2-k... 

The elucidation of the X-ray structure of chymotrypsin (Ref. 1) and in a later stage of subtilisin (Ref. 2) revealed an active site with three crucial groups (Fig. 7.1)-the active serine, a neighboring histidine, and a buried aspartic acid. These three residues are frequently called the catalytic triad, and are designated here as Aspc Hisc Serc (where c indicates a catalytic residue). The identification of the location of the active-site groups and intense biochemical studies led to several mechanistic proposals for the action of serine proteases (see, for example, Refs. 1 and 2). However, it appears that without some way of translating the structural information to reaction-potential surfaces it is hard to discriminate between different alternative mechanisms. Thus it is instructive to use the procedure introduced in previous chapters and to examine the feasibility of different... 

The main mechanistic difficulty involves Ce(IV) sulphate and V(V), which attack ketones at rates less than those of enolisation. The kinetics and relative oxidation rates are of little assistance in discriminating between attack on keto and enol forms. However, the solvent isotope effects in these oxidations are in strict accord with the measured acidity-dependences if attack had occurred on the enol then the change of solvent should have enhanced the expected koiolf HiO l y a factor of 2.5, because is favoured to such an extent in D2O as compared with H2O. No such enhancement is found and attack is, therefore, on the keto... 

The coupling between transport and phosphorylation of the sugar is more interesting from a mechanistic point of view. Two types of activities may be discriminated when transport and phosphorylation of the sugar would not be strictly coupled... 

The specific goal of the mechanistic studies of DcNOx reaction is to identify the key intermediates involved in the N—N and 0—0 bonds making, discriminate them from spectator species and ascertain the sequence and conditions of their appearance. To clarify the role of the mono- and dinitrosyl complexes as intermediates or spectators of the principal mechanistic reaction steps, it is necessary to develop a more in-depth insight into the structure-reactivity relationships for both adducts, and to understand the possible ways of attaching the second NO molecule to the mononitrosyl complex. 

An understanding of the recognition of chirality at a molecular level has become of interest in many fields of chemistry and biology. In the past decade, many attempts to clarify the mechanism of chiral recognition on CSPs for liquid chromatography have been made by means of chromatography, NMR spectroscopy,199 202 X-ray analysis, and computational methods.203 - 206 The successful studies have been mostly carried out for the small-molecule CSPs, especially cyclodextrin-based CSPs and Pirkle-type (brush-type) CSPs. In contrast, only a few mechanistic studies on chiral discrimination at the molecular... 

This mechanistic question is one of the examples of the success of density functional theory methods in organometallic chemistry. Earlier work on the reaction mechanism could not discriminate between the two alternatives. Analysis of the different orbitals based on extended Hiickel calculations came to the result that the [3+2] pathway is more likely, but could not exclude the possibility of a [2+2] pathway [13]. Similar conclusions where obtained from the results of Hartree-Fock calculations in combination with QCISD(T) single point calculations [21], Attempts to use Ru04 as a model for osmium tetraoxide indicated that the formation of an oxetane is less favorable compared to the [3+2] pathway, but still possible [22, 23],... 

The experimental mechanistic study of the anodic cyclization reactions requires values for the variation of the peak potential (Ep) in LSV with the sweep rate (v), the concentration of substrate (C) and the concentration of added base (B). The plots of dEp/dlogv, dEp/dlogC, and dEp/dlogB provide an effective tool for qualitative mechanistic analysis. The diagnostic criteria developed for discrimination between the various possible mechanisms [5] and adopted for oxidative cyclizations are presented in Table 1. 

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