Formation mechanisms pathways

Pathways (la) and (lb) are both part of product channel (1), but their formation mechanisms are different. In this example, path (la) represents 90% of channel (1) and may be formed through abstraction by atom Aa of atom Ab from the triatomic reactant, whereas in path (lb) Aa could add to B followed by elimination to form AaB + AbAc. This example demonstrates why experimental detection of pathway branching is usually difficult pathways (la) and (lb) lead to chemically identical products, even though they represent dramatically different chemical mechanisms. 

The oxidation of a-tocopherol (1) to dimers29,50 and trimers15,51 has been reported already in the early days of vitamin E chemistry, including standard procedures for near-quantitative preparation of these compounds. The formation generally proceeds via orf/zo-quinone methide 3 as the key intermediate. The dimerization of 3 into spiro dimer 9 is one of the most frequently occurring reactions in tocopherol chemistry, being almost ubiquitous as side reaction as soon as the o-QM 3 occurs as reaction intermediate. Early accounts proposed numerous incorrect structures,52 which found entry into review articles and thus survived in the literature until today.22 Also several different proposals as to the formation mechanisms of these compounds existed. Only recently, a consistent model of their formation pathways and interconversions as well as a complete NMR assignment of the different diastereomers was achieved.28... 

The mechanism of action of inert barrier pigments is commonly stated to be to Increase the diffusion pathway to the substrate it is also possible that pigments may tend to block or prevent the formation of pathways for direct ionic conduction to the substrate. 

Because the way in which a mesostructure is prepared affects the structure, morphology and surface chemistry of the product obtained, the mesostructure formation mechanism is also expected to impact the preparation of functionalized derivatives. Mesostructure synthesis pathways can be subdivided into two general categories electrostatic and non-electrostatic pathways. 

Figure 6.3 shows catalase transformation under the substrate (ROOH) effect in complex II to be the predominant pathway. For neutral substrates, which are hydroperoxides, the rate of complex II formation is independent of pH and is usually described by the second-order equation [103, 104], Complex II is the general intermediate for catalase and peroxidase reactions with the only difference that for catalase it is colored green (unpaired electron is localized on heme) and for peroxidase it is red (unpaired electron is localized on distal amino-acid fragment). Complex III is also colored red for peroxidase. However, the formation mechanism is different. Complexes II, III and IV are typical of peroxidases, whereas for catalase only complex II is formed. At the stage of complex II formation, the general properties and distinctive features of catalase and peroxidase were determined. 

Our analysis of PCDD/F formation mechanisms and results from parametric trials on bench, pilot and full scale plant tends to reinforce rather than supplant existing strategies for reduction and control of PCDD/F emissions. Mechanistic considerations supply an underlying rationale for the requirements of Good Combustion Practice, many components of which were formulated before the reaction pathways were elucidated in laboratory experiments. The key to the... 

Ionization reactions can occur under vacuum conditions at any time during this process but the origin of ions produced in MALDI is still not fully understood [27,28], Among the chemical and physical ionization pathways suggested for MALDI are gas-phase photoionization, excited state proton transfer, ion-molecule reactions, desorption of preformed ions, and so on. The most widely accepted ion formation mechanism involves proton transfer in the solid phase before desorption or gas-phase proton transfer in the expanding plume from photoionized matrix molecules. The ions in the gas phase are then accelerated by an electrostatic field towards the analyser. Figure 1.15 shows a diagram of the MALDI desorption ionization process. 

Hybrid silica materials were prepared via a sol-gel pathway at pH 9. The influence of anionic surfactant (SDS) was studied by comparing templated materials (TbSn series) with hybrid materials obtained without surfactant (Tbn series). Two mechanisms of mesostructure formation can be considered as represented on Fig. 2. The pka of aminopropyl chain is about 10.6 in the reaction mixture propyl-amines are partially protonated. Electrostatic interactions between dodecylsulfate anion and NH and sodium cation neutralization may then occur, resulting in the condensation of the silica structure around surfactant micelles and aminopropyl groups at the surface of the pores. The other mechanism is SDS chains complex-ation by P-CD cavity, which wonld result in P-CD gronps located at the surface of the pores and aminopropyl less accessible, due to steric hindrance caused by P-CD bulky groups. A complete characterization of the prodncts and adsorption capacities will help nnderstanding the formation mechanism of mesoporons hybrid silica. 

Figure 1. Schematic pathway for preparing surfactant-templated mesoporous silicas, illustrating a formation mechanism based on preformed liquid crystal (LC) mesophase (route A) or a cooperative process (route B). Reprinted from [20], Copyright (2008) WILEY-VCH Verlag GmbH Co.

The structure of this paper is as follows in section 2 we briefly describe the formation mechanism of the Bjerrum defects used in this study and their possible ideal diffusion pathways, and in section 3 we introduce jump rates and diffusion coefficients in solids. In... 

The fluorescence pattern that grew from one metallization line to the next under low frequency excitation is believed to represent an aqueous pathway. Although the formation mechanism has not been identified, in light of the AC experiment, a chemical attack of the polymer leading to delamination at the interface may create an area for water to aggregate. The observed growth from one metallization line to another of opposite bias is consistent with chemical attack related to a local extreme in pH arising from an electrochemical reaction at a metallization defect. 

Figure 6.17 Alternative mechanisms of osazone formation. In pathway A all the labelled nitrogen appears as ammonia, and in pathway B less (in the case of symmetrical systems such as benzoin, half).

Two possible mechanism pathways may be involved for this reaction, either a common pathway for all reactants passing through the alkene formation followed by a standard hydroxycarbonylation of the substrate with CO formed in situ by decomposition from formic acid, or through oxidative insertion of iridium to an iodoakyl intermediate, corresponding to the carbonylation mechanism of an alcohol. These two possibilities are depicted in Scheme 4. 

A variety of tetrahydropyridines have been prepared with complete regiocontrol by the reaction of 4-(trimethylsilyl)-3-butenylamines with aldehydes and acid (Scheme 43). This reaction, although ostensibly an iminium ion-vinylsilane cyclization, is believed to occur by the pathway illustrated in equation (11), in which ring formation ensues from the allylsilane sigmatropic isomer. Consistent with this mechanism pathway, either the ( )- or (Z)-vinylsilane amine stereoisomer can be employed. 

Research on persistent radicals in the environment including characterization of their structure, mechanisms of formation, mechanisms of stabilization, and pathways of biological redox cycling... 

Since coke is the terminal product of the aromatic pyrolysis pathway, it is of interest to explore the formation mechanism. Insight into this process in the range 800° to 1100°C is provided by the benzene pyrolysis data of Kiney and Delbel (5) in a flow reactor. The diphenyl concentration vs. time behavior reported is characteristic of an intermediate in a sequential reaction A B C where A (benzene) decreases and C... 

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