Radicals methyl, electronic structure

Orbital diagram of the methyl radical. The structure of the methyl radical is like that of the methyl cation (Figure 4-13), except there is an additional electron. The odd electron is in the p orbital perpendicular to the plane of the three C—H bonds. 

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... 

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... 

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.

The rotational barrier about the C—O bond in the cyanomethoxymethyl radical, [35]/[36], constitutes a similar case, although the situation is somewhat more complicated (Beckwith and Brumby, 1987). As oxygen carries two lone pairs of electrons, the transition structure for rotation about the C—O bond can still be stabilized by conjugation. Compared to the methoxy-methyl radical, the barrier in the captodative-substituted radical is 1-2 kcal mol higher. 

Most radicals have a planar or nearly planar structure. Carbon is sp hybridized in the methyl radical, giving three a C-H bonds, and the single electron is held in... 

Some information has been obtained on the fragmentation patterns of 1,4-thiazine derivatives. A group of 2- and 3-substituted A-methylated benzothiazines with the general structure 2 (R = Me) lost a methyl radical in electron impact mass spectrometry at 70eV (Scheme 2) <1982J(P1)831>. 

In the case of the triphenylmethyl radical shown in Figure 4.86, it is possible to write many different resonance structures but in a small free radical such as the methyl radical there is only one possible structure. The reactivity of the radical decreases as the unpaired spin density at each site decreases, and the radical also becomes more stable because of the resonance energy. This resonance stabilization is zero for the phenyl radical, since the unpaired electron resides in an orbital which is orthogonal to the it system. By contrast, the methylphenyl radical has a resonance stabilization energy of some 10 kcalmol-1, and the larger methylnaphthyl radical is stabilized by about 15 kcalmol-1. These resonance stabilizations can have important consequences for the energy balance of photochemical reactions (see e.g. sections 4.4.2 and 4.4.4). 

ESR spectroscopy is perhaps the best method for the unequivocal detection and observation of free radicals, and ESR 13C hyperfine splitting (hfs) constants are considered to be a very useful indicator of a radical s geometry because non-planarity introduces s character into the orbital that contains the unpaired electron. The methyl radical s 13Ca value of 38 G is consistent with a planar structure. Fluoromethyl radicals exhibit increased 13Ca values, as shown in Table 1, thus indicating increasing non-planarity, with trifluoromethyl radical s value of 272 G lying close to that expected for its sp3 hybridization [4]. 

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