Hydrogen abstraction, description

One of the simplest chemical reactions involving a barrier, H2 + H —> [H—H—H] —> II + H2, has been investigated in some detail in a number of publications. The theoretical description of this hydrogen abstraction sequence turns out to be quite involved for post-Hartree-Fock methods and is anything but a trivial task for density functional theory approaches. Table 13-7 shows results reported by Johnson et al., 1994, and Csonka and Johnson, 1998, for computed classical barrier heights (without consideration of zero-point vibrational corrections or tunneling effects) obtained with various methods. The CCSD(T) result of 9.9 kcal/mol is probably very accurate and serves as a reference (the experimental barrier, which of course includes zero-point energy contributions, amounts to 9.7 kcal/mol). 

Hydrogen abstraction by RH02 could also participate in the process of initiating a chain of thermal oxidation reactions (pathy). In aqueous systems, cations will further react by solvolysis, and superoxide anion will readily disproportionate to yield H202 (path i). This is in contrast to the fate of superoxide anions in ozonation advanced oxidation processes (AOPs), where they react primarily with ozone to produce hydroxyl radical. This description of the chemical pathways of UV/H202 oxidation of organics illustrates that, when oxygen is present, the major paths directly or indirectly create more... 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

In reaction with the active heme complex, compound 1, it is often assumed that the first stage is homolytic hydrogen abstraction, resulting in radical formation. This is certainly likely to occur with aliphatic hydroxylation. Ideally a calculation of the transition state should be carried out, but this is difficult in practice. Relative radical stabilities have therefore been used as an approximation. In a typical calculation one generates all possible radicals for a substrate, optimizes them, and determines their relative stabilities. They are then docked into the 3D protein structure using constraints between the heme and the sites predicted from the radical calculations. Early descriptions of this approach made use of homology models, but the same techniques can obviously be used with crystal structures. 

Under normal conditions, excited carbonyl compounds isomerize to cyclobutanols (see Section 7. A.1.1.5.) via intramolecular y-hydrogen abstraction and closure of the intermediate 1,4-biradical. Nevertheless, a similar, formally /i-hydrogen abstraction, step leading to cyclopropanols occurs with several A-functionalized ketones (see Houben-Weyl, Vol. 4/5 b, p 797, literature up to 1971, and Vol. 6/la, part 2, pp 788-799, literature up to 1975 for a general mechanistic description of the formation of alcohols by photochemical reduction of carbonyl compounds, see Vol. 6/1 b, p432). 

Fan and Ziegler concluded [69] that non-local corrections according to Becke and Perdew are essential for an accurate description of hydrogen abstraction reactions 3.1.1a and 3.1.1b. The reaction 3.1.1c will be discussed separately below. 

Chapter 52 presents a thorough description of the general properties and behavior of the hydroxy-biradicals formed by n,n hydrogen abstraction as well as the history of their detection. This chapter focuses on how biradicals of different sizes differ in behavior and how these differences affect our general understanding of biradical behavior. 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

Abstract. A phase equilibriums in intermetallic compounds hydrides in the area of disordered a-, (3-phase in the framework of the model of non-ideal lattice gas are description. LaNi5 hydride was chosen as the subject for the model verification. Position of the critical point of the P—w.-transition in the LaNi5-hydrogen system was definite. 

The oxidation of -butane to maleic anhydride is a 14-electron oxidation. It involves the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and a multi-step polyfunctional reaction mechanism that occurs entirely on the adsorbed phase. No intermediates have been observed under standard continuous flow conditions, although mechanisms for this process have been proposed based on a variety of experimental and theoretical findings. The description of the active site is linked to the mechanism and is the subject of considerable debate in the literature. The mechanisms are linked to the researchers hypotheses of the active site, which will be discussed in a separate section in this chapter. It is widely accepted that the (100) plane of vanadyl pyrophosphate, (VO)2P207, (referred to as the (020) plane by certain authors) plays an important role in the selective oxidation of butane. 

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