Methyl radical bonding models

FIGURE 4 19 Bonding in methyl radical (a) If the structure of the CH3 radical IS planar then carbon is sp hybridized with an unpaired electron in 2p orbital (b) If CH3 IS pyramidal then car bon IS sp hybridized with an electron in sp orbital Model (a) IS more consistent with experimental observa tions... 

Of the two extremes, experimental studies indicate that the planar sp model describes the bonding in alkyl radicals better than the pyramidal sp model. Methyl radical is planar, and more highly substituted radicals such as ferf-butyl radical are flattened pyramids closer in shape to that expected for 5/) -hybridized carbon than for sp. ... 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

A detailed study of the CO insertion, or methyl migration, observing formation and decomposition of the transients, was performed so far only for one Cu(I) model system (93). It was reported that methyl radicals form transient complexes containing metal carbon -bonds with carbonmonoxide (n = 1, 3, 4) complexes of Cu(I). These complexes decompose yielding Cu(II) and acetaldehyde as final products via an copper acetyl intermediate formed by insertion of /migration of CH3 as described in Scheme 4. 

It has recently been reported that radical bond breakage may be initiated by carbon black of high surface area. The conversion of a model coal compound, 4-( I-naphthyl-methyl) bibenzyl was accelerated at 375°C without hydrogen pressure in either the presence or absence of hydrogen donors (41, 42). This suggests that polarity or surface radical content of carbon black may initiate the decomposition of phenyl-methylnaphthyl linkages. 

As a model study of methyl cobalamine (methyl transfer) in living bodies, a methyl radical, generated by the reduction of the /s(dimethylglyoximato)(pyridine)Co3+ complex to its Co1+ complex, reacts on the sulfur atom of thiolester via SH2 to generate an acyl radical and methyl sulfide. The formed methyl radical can be trapped by TEMPO or activated olefins [8-13]. As a radical character of real vitamin B12, the addition of zinc to a mixture of alkyl bromide (5) and dimethyl fumarate in the presence of real vitamin B12 at room temperature provides a C-C bonded product (6), through the initial reduction of Co3+ to Co1+ by zinc, reaction of Co1+ with alkyl bromide to form R-Co bond, its homolytic bond cleavage to form an alkyl radical, and finally the addition of the alkyl radical to diethyl fumarate, as shown in eq. 11.4 [14]. 

Transient intermediates arising by radical additions to the amide bond have been studied recently in an effort to explain the complex processes occurring in electron-capture dissociation of multiply charged peptide and protein cations [192]. The amino(hydroxy)methyl radical, HC(OH)NH2 (64), is a prototypical species that represents the simplest model for radical additions to peptide bonds. Radical 64 was generated from cation 64+ which in turn was prepared by... 

Bonding in methyl radical. Model (a) is more consistent with experimental observations. 

The simplest energetic differentiation question is the stereodifferentiation discussed in this chapter where bonding reorganization is nearly identical for each of the transition states of interest. More challenging questions involve significant electronic differentiation. The methyl radical plus ethylene regiochemical differentiation discussed in Section 7.2.1 demonstrates the level of uncertainty to be expected. Particularly important processes, where detailed model calibration... 

In this procedure, FSGO parameters are obtained from Frost model calculations on molecular fragments chosen to mimic various molecular environments . For example, a planar methyl radical would be used to mimic the environment of an sp carbon atom Fig. 1 a, where the locations of FSGOs containing the inner-shell and bonding electron pairs are indicated by the filled circles on the figure. The pair of spherical Gaussians, gj and g, located above and below the plane of the molecule are combined as... 

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