Non-dissociative pathway

Akeypart in the analysis and proposal was electron accountancy - on the basis of the usual propensity for the adoption of a noble gas outer shell configuration - and all of the proposed intermediates have 16- or 18-electron configurations except the metallocyclobutane (26) in pathway II which has 14. On this basis, one might expect that pathway I, the non-dissociative pathway, would predominate over pathway II. In the presence of a large excess of Cy3P, which was used only to simplify the kinetic analysis, this is clearly the case. However, under the... 

Many studies have been carried out to determine the detailed pathway of rearrangement. In many cases experimental evidence54,55 eliminates the possibility that intermolecular exchange, either by unimolecular dissociation or by some other more complex path, is responsible for the site exchange. Unimolecular, non-dissociative pathways must therefore be considered, and the four most obvious ones are shown in Fig. 23-19. Pathway (1),... 

An electron-transfer reduction of RX can be dissociative (no intermediate anion-radical RX" ) or nott-dissodative (intermediate RX ). Where the non-dissociative pathway is well documented (e.g., aryl halides), only two reactions of the intermediate RX" are generally posited, dissociation and electron-transfer oxidation [17,18], However, product evidence suggests that R-X may undergo other reactions, including reduction. 

The non-cleavage pathway would remove most cohesin during prophase/ pro-metaphase by an as yet obscure mechanism. This pathway could involve phosphorylation of a cohesin subunit by mitotic protein kinases, because vertebrate cohesins rebind to chromatin in telophase when mitotic kinases are inactivated and chromosomes decondense (Losada et al 1998). The dissociation of cohesin from chromatin during prophase coincides with, but does not depend on, the association of condensin with chromosomes. This first phase of cohesin removal may be crucial (possibly along with the arrival of... 

Dissociation of the neutral acid in water necessitates modifications for air-sea exchange in the model, which is based on Henry s law. Other possible pathways, e.g. sea spray, are neglected. Henry s law is restricted to concentrations of physically solved, non dissociated substances. Since only the non-dissociated acid is volatile, it is important to correct the air-water partition coefficient as to reflect the relative proportions of volatile and non-volatile components. The corrected parameter is the effective Henry s law coefficient, which is related to the Henry s law coefficient as a function of pH (modified Henderson-Hasselbalch equation) ... 

Operating within the framework of the Chauvin mechanism, the main consideration for the reaction mechanism is the order of events in terms of addition, loss and substitution of ligands around the ruthenium alkylidene centre. Additionally, there is a need for two pathways (see above), both being first order in diene, one with a first-order dependence on [Ru] and the other (which is inhibited by added Cy3P) with a half-order dependence on [Ru]. From the analysis of the reaction kinetics and the empirical rate equation thus derived, the sequence of elementary steps via two pathways was proposed, one non-dissociative (I) and the other dissociative (II), as shown in Scheme 12.20. The mechanism-derived rate equation is also shown in the scheme and it can thus be seen how the constants A and B relate to elementary forward rate constants and equilibria in the proposed mechanism. 

The solvent-influenced synjanti dichotomy for bimolecular eliminations of acyclic and medium-ring bromides, tosylates, and onium salts has been reviewed [395, 693] and will be mentioned only briefly. As a rule, the s jn-elimination pathway gains importance in non-dissociating solvents, while dissociating solvents facilitate the more common anti-elimination reaction. The more unusual s jn-elimination is favoured in non-dissociating solvents because of ion-pair association, which favours a cycHc six-membered activated eomplex as shown in Eq. (5-151a) see reference [395]. 

The role of CO, H2 or hydrocarbon is to scavenge atomic oxygen resulting from the NO dissociation. The observed increase in the selectivity towards N2 is a consequence of increased NO dissociation, i.e. a decreased amount of molecular NO, and an increased amount of atomic N on the surface. Both factors vours reaction (6) over reaction (7). This dissociative mechanism is the generally accepted pathway under ultra high vacuum conditions [18]. However, a recent study by Klein et al [19] has questioned the validity of the dissociative mechanism under atmospheric pressure conditions in favour of a non-dissociative mechanism. A particular difficulty with the non-dissociative mechanism is that it cannot readily account for... 

An additional advantage of using primary HsO" " ions, besides that they do not react with the natural compounds of air, is that many of their proton transfer processes are non-dissociative, so that only one product ion species occurs for each neutral reactant with a mass one Dalton greater than the neutral. Table 1 (updated from Ref. 12) shows a variety of such cases. There are, however, compounds where dissociation does occur. These quite often follow a straightforward pattern like in the cases of the reactions of H3O with alcohols, where proton transfer followed by the ejection of an H2O molecule is the predominant reaction pathway. ... 

It is believed that the Catalytica process involves three steps C-H activation, oxidation, and functionalization with ligand dissociation first to form a T complex (see Fig. 12). There could be other sequence of the first two steps in a (non) dissociative way. Xu et al. [83] have examined thermodynamics of four possible reaction pathways, involving activation-after (or -before) -oxidation pathways, and dissociative versus associative pathways for both the diammine and the bpym systems. As depicted in Fig. 15, pathways 1 and 2 are activation-before-oxidation processes, while pathways 3 and 4 are activation-after-oxidation processes. Pathways 1 and 3 may be considered as associative pathways, where the departure of ligand CP is accompanied by the coordination of CEU, whereas pathways 2 and 4 are dissociative pathways, involving the formation of a T-complex as an intermediate [27]. The calculated reaction free energies are summarized in Table 5. 

The carbon dioxide cation plays an important role in the dynamics of plasma discharges and chemistry of planetary atmospheres [126]. In general, two possible pathways can be distinguished in single ionization of CO2. The first pathway (non-dissociative ionization see also above), affords C02 ion, possibly in an electronically and/or rovibrationally excited state (1.19). The vibronic structures of 02" in the various states have been studied by means of different experimental... 

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