Hydrogen atom abstraction model

ACS Symposium Series American Chemical Society Washington, DC, 1975. 

When the tunneling correction Is combined with the rest of the rate constant ratio, (fca/kj))dags resultant form of InAq 

Is almost Identical with that for tunneling only. This Is because ln(kjj/kjj) has almost exactly the Arrhenius form, with 

At lower temperatures, InAq exceeds the upper limit of 0.35 predicted by Bell in the absence of tunneling (Indicated by the upper dashed line). This is contrary to what had been expected before our calculations were made, because earlier calculations concentrated on the region to the left of the Inflection point in Fig. 6. 

The value of Aq does not correlate simply with the extent 

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CW Hoganson, N Lydakis-Simantiris, XS Tang, C Tommos, K Warnke, G Babcock, BA Diner, J McCracken and S Styring (1995) A hydrogen-atom abstraction model for the function ofYz in photosynthetic oxygen evolution. Photosynthesis Res 46 177-184... 

Using the propagation steps for chlorination of methane as a model, write analogous steps for ethane. In step 1, a chlorine atom abstracts a hydrogen ... 

The principal photochemical reaction mechanisms of interest are hydrogen atom abstraction from hydrogen donors and exciplex formation with electron donors. For these mechanisms quite interesting effects have been observed by using cis-azoalkanes as chromophores. Of course, electron and energy transfer processes involving n,jt -excited states are also relevant, but can be treated with elassical models [158-161]. 

The peroxide decomposes at elevated temperatures to give free radicals, which then abstract a hydrogen atom from the methyl group. The radicals formed then combine to form a hydrocarbon linkage. Results obtained by reacting model systems with benzoyl peroxide and analysing the reaction products are consistent with this type of mechanism. ... 

Spin density surface for the most stable radical formed by hydrogen atom abstraction from a model of a-tocopherol shows delocalization of the unpaired electron. 

Even if atomic oxygen-carbon cluster reactions are rapid, as is assumed in the new neutral-neutral model, the synthetic power of this model can be recovered if exothermic hydrogen atom abstraction reactions of the sort,... 

The second mechanism is realized when organic or inorganic compounds are reduced by endogenous reductants (for example, by NADH or NADPH and the other components of mitochondrial or microsomal respiratory chains). The typical compounds are anthracycline antibiotics and carbon tetrachloride. CC14 is easily reduced by microsomes to the free radical CC13 , which is able to abstract a hydrogen atom from unsaturated lipids and initiate lipid peroxidation. Because of this, the CCl4-initiated lipid peroxidation is a reliable and frequently applied model system for the study of in vitro iron-independent lipid peroxidation and the effects of antioxidants (see for example Ref. [54]). 

Jones JP, Mysinger M, Korzekwa KR. Computational models for cytochrome P450 a predictive electronic model for aromatic oxidation and hydrogen atom abstraction. Drug Metab Dispos 2002 30(1) 7-12. 

Korzekwa KR, Jones JP, Gillette JR. Theoretical-studies on cytochrome-P-450 mediated hydroxylation—a predictive model for hydrogen-atom abstractions. J Am Chem Soc 1990 112(19) 7042-7046. 

About 20% of the HO radicals interact with the sugar phosphate by H-atom abstraction and about 80% react by addition to the nucleobases. In model sugar compounds, the H abstraction would occur evenly between the hydrogens on Cl, C2, C3, C4, and C5. In DNA, H abstraction occurs mainly at C4 since the C4 -H is in the minor groove and to some extent with the C5 -H2. 

Lastly, the ability to model systems from the real world of chemistry means that atomistic simulations are a perfect complement to the abstract models (harmonic oscillator, hydrogen atom, ideal gas, etc.) that are the traditional focus of physical chemistry textbooks. Students are left with a more realistic idea, and greater appreciation, of science. 

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

FIGURE 10.13 Typical model system for studying metalloenzyme reaction mechanisms. This is a model of the TS for hydrogen atom abstraction from substrate by the Fe(III)OH center of lipoxygenase. 

Absolute measurements 124.251 of the rate coefficient for the N03/DMS reaction are in reasonably good agreement. A temperature-independent value of 1 x 10 12 cm3 molecule 1 s 1 has been adopted in the model. The products of the reaction have yet to be established. On the basis that Tyndall et al. (24) saw no evidence for DMSO production, hydrogen-atom abstraction has been assumed to occur (reaction 4). 

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