Reactive Fragments

The validity of these macroscale arguments is questionable, however, when microquantities are involved, or when abnormally reactive fragments may be present. 

A set of oxygen donor atoms, providing both a and tt donation to a metal center, is not appropriate to stabilize any low oxidation state of a metal.19 This is, however, a synthetic advantage since very reactive, unstable, low-valent metalla-calix[4]arenes can be generated in situ and intercepted by an appropriate substrate. In the absence of a suitable substrate, the reactive fragment, however, can collapse to form metal-metal bonded dimers. The formation of metal-metal bonds has been, however, so far observed in the case of Group V and VI metals only. The most complete sequence so far reported has been for tungsten, molybdenum, and niobium. 

A most striking result from the work described above is that the composition of the bottoms product and residues from the dissolution reaction did not depend on the chemical structure of the original coal material only their relative quantities differed. This supports the view of a mechanism involving the stabilisation of reactive fragments rather than an asphaltene-intermediate mechanism. The formation of a carbon-rich condensed material as a residue of the reaction and the fact that hydrogen transfer occurred largely to specific parts of the coal further supports this view. 

The rearrangement is thought to go through dissociation of the initial ozonide into reactive fragments that recombine to give the ozonide. 

The second approach consists of synthesizing first the complex MLra 1(L X) with the desired ratio (L )/(M) this complex bears the reactive fragment X which then reacts with the surface of the silica. This method is of limited interest, because the synthesis and isolation of these functionalized complexes is not straightforward. One of the successful examples concerns the synthesis of nickel carbonyl complexes anchored to the surface via two bonds in an attempt to increase the stability through a sort of chelate effect. Initial attempts to achieve this by the methods described in Equation(5) (initial functionalization of silica) and Equation(6) (initial functionalization of complex) failed, as demonstrated by 29Si and 31P CP MAS NMR spectroscopies.51... 

First, all these classes of compounds are 1,3-dipoles, that is, they serve as the starting reagents in 1,3-dipolar cycloaddition reactions, which can be considered as a modem powerful method for the synthesis of various heterocyclic and polyfunctional compounds (5). All three dipoles have the common reactive fragment ... 

We can calculate an enthalpy of reaction with bond enthalpies by assuming the reaction consists of two steps first, bonds break, and then different bonds form. This approach can be simplified further if we consider the reaction consists only of reactive fragments, and the products form from these fragments. The majority of the molecule can remain completely unchanged, e.g. we only need to consider the hydroxyl of the alcohol and the carboxyl of the acid during a simple esterification reaction. 

AN+- (Reitstoen and Parker, 1991). In other words, the triad of reactive fragments produced in (63) in the charge-transfer excitation of the EDA complex with A-nitropyridinium ion is susceptible to mutual (pairwise) annihilations leading to the Wheland intermediate W and the nucleophilic adduct N (Scheme 12), so that the observed second-order rate constant ku for the spectral decay of ArH+- in Table 3 actually represents a composite of k2 and k2. A similar competition between the homolytic and nucleophilic reactivity of aromatic cation radicals is observed in the reaction triad (55)... 

Balance of Activatlon/DetoxifIcatlon Reactions. The activated intermediates or reactive fragments appear to be carbamoyl sulfoxides or mono-, dl- and trlchloroacroleins, all of which are relatively unstable compounds. The carbamoyl sulfoxides are rapidly detoxified by reaction with GSH, involving catalysis by a GSH -transferase in the case of -alkyl and -benzyl thiocarba-mate sulfoxides ( -5, 24) but probably not with -chloro-... 

Within polymer solids, volatile, reactive fragments are trapped and often rereact, forming rearranged structures. If the rearranged structures exhibit markedly better stability, excessive char results. Thus solid ldpe decomposes with little char, whereas polyacrylonitrile (PAN) gives excessive char because of the formation of thermally stable rearranged products. 

Even when conditions can he found which result in a distribution of isotopes in upper states, different from that in AIIg a reaction is still required which fixes this upper state distribution as stable compounds of mercury. Therefore, a developer reaction is required, which usually involves substrate molecules which contain Cl, O, OH, etc. Such reactive fragments, however, react not only with excited, but also with ground-state mercury atoms. Isotopic fractionation will therefore occur only in an isotopieally-specific primary process of the type ... 

Under the usual commercial hydrodesulfurization conditions (elevated temperatures and pressures, high hydrogen-to-feedstock ratios, and the presence of a catalyst), the various reactions that result in the removal of sulfur from the organic feedstock (Table 4-3) occur. Thus, thiols as well as open chain and cyclic sulfides are converted to saturated and/or aromatic compounds depending, of course, on the nature of the particular sulfur compound involved. Benzothio-phenes are converted to alkyl aromatics, while dibenzothiophenes are usually converted to biphenyl derivatives. In fact, the major reactions that occur as part of the hydrodesulfurization process involve carbon-sulfur bond rupture and saturation of the reactive fragments (as well as saturation of olefins). 

The dissociation of gas molecules in a plasma involves the formation of intermediate, highly reactive fragments, which are very unstable under normal circumstances. Therefore, PECVD allows the deposition of quite unique materials with very unusual properties. 

There are a number of cases in which these extended collision times can have kinetic consequences. One of them concerns the primary dissociation of a molecule into reactive fragments, while the second concerns the recombination of active radicals to form an inactive molecule. If we consider first the hypothetical species A-B decomposing in solution to form active species (e.g., radicals) A + B, we can formulate the kinetic scheme by... 

Loss of a ligand from saturated metal complexes often generates highly reactive fragments. In the absence of other reaction pathways, these fragments can condense to metal-metal bonded dimers or higher-order clusters. The best example of this reaction is loss of carbon monoxide from metal carbonyls to yield dimers and clusters. For example, photogenerated Fe(CO)4 species readily react with Fe(CO)5 to yield Fe2(CO)9 (equation 61). 

Low pressure reduces the condensation of reactive fragments forming coke and heavies... 

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