Methyl radical hybrid orbitals

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... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

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. 

There is also a third type of reactive species that we shall discuss in detail in Chapter 9, namely radicals. Briefly, radicals are uncharged entities that carry an unpaired electron. A methyl radical CH3 results from the fission of a C-H bond in methane so that each atom retains one of the electrons. In the methyl radical, carbon is sp hybridized and forms three CT C-H bonds, whilst a single unpaired electron is held in a 2/ orbital oriented at right angles to the plane containing the ct bonds. The unpaired electron is always shown as a dot. The simplest of the radical species is the other fission product, a hydrogen atom. 

Structures. The methyl radical is planar and has D symmetry. Probably all other carbon-centerd free radicals with alkyl or heteroatom substituents are best described as shallow pyramids, driven by the necessity to stabilize the SOMO by hybridization or to align the SOMO for more efficient pi-type overlap with adjacent bonds. The planarity of the methyl radical has been attributed to steric repulsion between the H atoms [138]. The C center may be treated as planar for the purpose of constructing orbital interaction diagrams. 

Spin Polarization. The methyl radical H3C is a planar species with C3v symmetry. The unpaired electron resides in a carbon 2p orbital and the C-H bonds are sp2 hybrids. The unpaired electron has zero probability in the plane of the hydrogen nuclei, yet the protons display an isotropic splitting of 2.3 mT, so we must rationalize this experimental observation. 

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]. 

Like carbocations, free radicals are sp2 hybridized and planar (or nearly planar). Unlike carbocations, however, the p orbital perpendicular to the plane of the C—H bonds of the radical is not empty it contains the odd electron. Figure 4-15 shows the structure of the methyl radical. 

ESR tells us that the methyl radical is planar the carbon atom must therefore be sp2 hybridized, with the unpaired electron in a p orbital. We can represent this in an energy level diagram. 

Ethylene has the well-known classical >2/1 structure with a barrier to rotation. The next in complexity of the simple hydrides is the methyl radical CH3. The obvious (sp2) planar arrangement can only accommodate six of the seven valence electrons. The electronic configuration of this molecule can therefore not be described in terms of either atomic wave functions or hybrid orbitals. An alternative approach is to view the structure of the methyl radical as a reduced-symmetry form, derived from the structure of methane, to be considered next. 

We saw earlier (Sec. 2.21) that the methyl radical may not be quite flat that hybridization of carbon may be intermediate between sp- and sp For the allyl radical, on the other hand— and for many other free radicals—flatness is clearly required to permit the overlap of p orbitals that leads to stabilization of the radical. 

The carbon atom in the methyl radical is also sp hybridized. The methyl radical differs by one unpaired electron from the methyl cation. That electron is in the p orbital. Notice the similarity in the ball-and-stick models of the methyl cation and the methyl radical. The potential maps, however, are quite different because of the additional electron in the methyl radical. 

Spectroscopic data can be used to distinguish between planar and nonplanar rapidly inverting radical centers. The hyperfine coupling constant a in the methyl radical is 23.0 G, which is a typical value for the splitting of an EPR signal by protons attached to a radical center. Theoretical analysis of the spectrum suggested that the methyl radical is probably flat, although a deviation from planarity of 10-15° could not be ruled out There is also spectroscopic evidence that the methyl radical in the gas phase is essentially planar. Thus, the methyl radical is conveniently described by sp hybridization with the unpaired electron located primarily in the p orbital. 

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