Methyl radical nonplanar

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. 

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 very shallow pyramid. The IR spectrum of methyl radical has been recorded at very low temperatures in frozen inert gas. ° Under these conditions, a relatively high concentration of reactive species can be obtained, since chemical reactions are prevented by the inertness of the surrounding matrix. The IR spectrum puts a maximum of 5° on the deviation from planarity. Similar studies have suggested a planar arrangement for the mono-chloromethyl radical, although the trichloromethyl radical is found to be pyramidal. Both EPR and IR studies lead to the conclusion that the trifluoromethyl radical is nonplanar. ... 

Two simple a, P-unsaturated acylsilanes, l-trimethylsilyl-2-propen-l-one (III) and l-trimethylsilyl-2-methyl-2-propen-l-one (IV) were chosen for polymerization studies. The polymerization of the carbon analogues of these a,p-unsaturated acylsilanes, that is, 4,4-dimethyl-2-propen-3-one (vinyl tert-butyl ketone, V) and 2,4,4-trimethyl-2-propen-3-one (isopropenyl tert-hutyl ketone, VI) has been studied by Willson et al. 16, IT), These authors reported that whereas V readily polymerizes under free-radical-polymerization conditions, VI undergoes polymerization only under anionic-initiation conditions in the presence of a crown ether as a complexing reagent. On the basis of UV and NMR spectroscopic data, Willson et al. (i6, 17) ascribed the difference in polymerization behavior to the nonplanar, unconjugated structure of ketone VI brought about by steric hindrance caused by the methyl group at C-2. 

In the case of the bicyclo[6.1.0] nonatriene system, substituents at C-9 also have an influence on the rate of the cleavage of the central cyclopropane bond between C-l and C-8 (91). fThe additional methyl group at C-l in the bicyclopentene systems should facilitate the opening of the bond between C-l and C-5 by about 2 kcal/mol (40). The homolytic cleavage of a cyclopropane bond is expected to have a smaller activation barrier in norcaradiene than in bicyclopentene (by about 5.3 kcal/mol) due to the different stabilization of the radical site at C-l, pentadienyl resonance vs. allyl resonance (92). The nonplanar geometry of the cyclooctatriene ring does not permit any prediction for the bicyclo[6.1.0) nonatriene system. 

---