Trifluoromethyl radical geometry

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2-methy 1-2-propyl radical, trifluoromethyl radical, trichloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a n-electron donor or as a Tt-electron acceptor (relative to hydrogen). Does replacement of the hydrogens by 7t-donor groups make the radical center more or less puckered Does replacement by Jt-acceptor groups make the radical center more or less puckered Justify your observations. 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

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]. 

Estimates of the geometry of substituted radicals can be obtained from analysis of species labeled with Fessenden and Schuler determined that the anglle F-C-F in the trifluoromethyl radical is 111.1° (very nearly sp hybridization) and that increasing fluorine substitution causes the radicals to go from planar, sp2 hybridization for CHj to nearly tetrahedral, sp hybridization for "CF3. This conclusion is consistent with results of INDO (intermediate neglect of differential overlap) calculations Figure 4.9 shows the geometries of fluoromethyl radicals calculated by this method. 

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