Structure methyl radical

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... 

Precise description of the pyramidal structures would also require that the bond angles be specified. The EPR spectrum of the methyl radical leads to the conclusion that its structure could be either planar or a veiy shallow pyramid. The IR spectrum of the methyl radical has been recorded at very low tempertures in frozen argon. This IR study puts a maximum of 5° on the deviation from planarity. A microwave study has also indicated... 

Step through the sequence of structures corresponding to combination of two methyl radicals to give ethane (methyl radical combination). 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

Equations 12 and 13. The loss of a methyl radical from the ion at m/e 88 is prominent in Figure 3, but not in Figures 1 and 2, making it characteristic of the 5-deoxy furanoid structure. 

We shall discuss in detail only one example, and for the sake of simplicity we shall take the (actually undissociated) sym. diphenylethane. Let us first consider what happens when the ethane dissociates. In the first step, the C —C bond breaks and there are formed two phenyl-methyl radicals, which however can resonate between only the structures A and B of Fig. 3. 

However, a comparison of the line shape of the observed spectra with spectra of methyl radicals (Fig. lib) clearly proves that the species present here are not methyl radicals. The EPR spectrum of a methyl radical is a quartet of lines. However, the observed spectrum, though dominated by a quartet structure, shows a couple of additional lines pointing to additional interactions of the unpaired electron. By comparing the observed line shape to other alkyl radicals it turned out that the present spectrum can be attributed to ethyl radicals. Figure 11c shows the EPR spectrum of ethyl radicals created in an ethylchloride matrix generated by photolysis for comparison [121]. 

The occurrence of a 5a-C-centered tocopherol-derived radical 10, often called chromanol methide radical or chromanol methyl radical, had been postulated in literature dating back to the early days of vitamin E research,12 19 which have been cited or supposedly reconfirmed later (Fig. 6.5).8,20-22 In some accounts, radical structure 10 has been described in the literature as being a resonance form (canonic structure) of the tocopheroxyl radical, which of course is inaccurate. If indeed existing, radical 10 represents a tautomer of tocopheroxyl radical 2, being formed by achemical reaction, namely, a 1,4-shift of one 5a-proton to the 6-oxygen, but not just by a shift of electrons as in the case of resonance structures (Fig. 6.5). In all accounts mentioning... 

Hybrids of the type sp3 are unjustified for disilane. An important conclusion from the above hybridization statement No. 4 is concerned with the contrasting structures of the radicals SiH3 and CH3. The planar geometry of the methyl radical can readily be explained by the (bond-strengthening) sp2-hy-bridization, while the pyramidal silyl radical is thought to be stabilized (with respect to the planar arrangement) through the s-admixture to the lone electron orbital. 

The o1-structure is less well known, but it seems probable that, after electron loss, a distortion that leads to a o1-type radical is a necessary step in dissociation. For example, the Me3Sn-CH3+ cation, formed at 77 K, readily gives methyl radicals on annealing [18] (12,13). Examples of such species include the... 

After several years of effort it has been possible to reproduce Schmidbaur fs addition of methyliodide W The problem appears to be that the methyliodide product is extremely photosensitive in solution. It decomposes with the production of methyl radicals and the ultimate formation of a mixture of the Au(I), and Au(II) diiodide products. The X-ray crystal structure of the methyliodide product is presented in Figure 5. Two features are to be noted. Firstly, the metal-metal distance lengthens only a small amount, 0.04A, but the Au iodide distance lengthens by approximately 0.20. The structural trans effect caused by the methyl group is not attenuated by the Au-Au bond. 

Fig. 25 B3LYP/6-31G structures for (a) methoxyformamidyl 102e, (b) transition state and (c) 103e from methyl radical reaction with iV-methoxyformamidyl.

To select between these two alternative structures it was necessary to synthesize a labeled analog. Three hydrogen atoms of the methyl moiety of the ester group were substituted for deuterium. One of the principal pathways of fragmentation of [M N2]+ ions involves the loss of CH3 radical. Since all R substitutes in diazo ketones 4-1 were also methyls it was important to detect what group exactly is eliminated from the [M N2]+ ion. The spectrum of deuterated sample has confirmed that the methyl radical of the ester moiety leaves the parent ion. As a result the cyclic structure 4-2 was selected as the most probable. The ketene structure 4-3 is hardly able to trigger this process, while for heterocyclic ion 4-2 it is highly favorable (Scheme 5.22). 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

Table 6.12 Comparison of experimental radical stabilization energies at 0 K (kJ/mol) of substituted methyl radicals with those calculated3 with wavefunction-based electronic structure methods.

Table 6.26 Comparison of experimental reaction enthalpies at 0 K (kj/mol) for the addition of methyl radical to alkenes CH2=CXY with those calculated8 with the DFT-based electronic structure methods.

---