Fluoroethyl radicals

The first step in this pyrolysis consists of the fission of the Cl bond, with formation of a tertiary radical. The collapse of this radical into perfluoro(2-niethylpropene) and a per-fluoroethyl radical, which recombines with the iodide, gives pentafluoroiodoethane. The homolytic cleavage of the C-C bond seems to be rate determining. 

The epr spectra of many relatively simple fluoromethyl and fluoroethyl radicals and related substances have been examined in solution. Interest in this chemistry has been particularly keen since the spectroscopic results yield important clues concerning the structures of the radicals, rotation and inversion barriers, and the factors governing spin delocalization. Representative results which have been selected to portray the variations in experimental results and the small, but important influence of temperature on the a values are presented in Table 12. 

Temperature Dependence of the Coupling Constants for Fluoroethyl Radicals ... 

The analyses of the data for the /3-fluoroethyl radicals have generally been based on the idea that Equation 2 is applicable for the description of the angular dependence of the constants. Approximate molecular orbital theory suggests,... 

Clearly, these approximate values depend importantly on the structure of the radical. Quite different values are observed for fluoroalkyl radicals, ni-troxide radicals, nitrobenzene anion radicals, ketyls, and so forth. This conclusion would not be altered by the consideration of the angle independent term or by the reconsideration of the estimated spin densities. Furthermore, the values of are not the same for closely related radicals such as the mono-, di-, and tri-fluoroethyl radicals or the mono-, di-, and trifluoromethylnitrobenzene anion radicals as already discussed. The Bi value estimated for monofluoromethyl-... 

The most widely studied F atom reaction system is that of F plus ethylene, which was initially studied for the purpose of using ethylene as a scavenger in CF4 systems, and more recently wifc interest in the excited fluoroethyl radical intermediate. Studies using widely different sample preparation procedures and irradiation conditions are mutually consistent, ( , jj) and have demonstrated that the kinetics of the F atom reactions under study are independent of their irradiation history. 

Table II. Energetics and Yields for Decomposition Pathways for Fluoroethyl Radicals Formed by Addition of F to Olefins (6). ... 

Table X. Extrapolated Pressures for Half-Stabilization for Fluoroethyl Radicals in NF3, CF4, SFg and C2F6 Bath Gases.

The effects of substituents in the -position to the radical center are mostly inductive in nature. Comparison of the RSE values for the ethyl radical (- 13.8 kj/mol) with those of the propyl, 2-hydroxyethyl, 2-fluoroethyl, and 2-chloroethyl radicals with RSE values of - 11.9, - 8.5, - 5.5, and - 6.7 kj/mol also indicates that electronegative substituents in the -position uniformly destabilize the radical center, the effect being larger for more electronegative substituents. Comparison of the RSE values of the 2-fluoro, 2,2-difluoro, and 2,2,2-trifluoroethyl radicals of - 5.5, + 3.0, and + 8.1 kj/mol also indicate that these effects can accumulate to yield overall destabilized radicals relative to the methyl radical. Even less favorable RSE values are found for positively charged substituents directly attached to the radical center such as - NH3+ (+ 18.3 kj/mol) or-SH2+ (+ 12.8 kj/mol) (Table 1). 

A second model of the transition state having an early structure was based on the SCF m.o. geometry published by Hoyland (1971). Twelve of the transition-state vibrations were set equal to those of the reactant fluoroethyl-ene, and six equal to those of the attacking radical. Of the remaining six, one forms the reaction co-ordinate, the torsion mode between the attacking radical and the alkene was treated as a free rotation, and the others were set at of their values in the corresponding fluoropropanes. 

Line-broadening effects are discernible in the epr spectra of many radicals with a- and / -fluorine atom substituents as shown by the results in Table 14. Fluorine atoms at the radical site cause an increase in the barrier to rotation. Line-broadening effects have been observed for the 1,1-difluoroethyl, and 1,1,2,2,2-pentafluoroethyl radicals and other perfluoroalkyl radicals (67,68,94). No line-broadening effects have been detected, however, for the 1-fluoroethyl or the 1,2,2,2-tetrafluoroethyl radicals (68). A detailed analysis of the epr observations for the 1,1-difluoroethyl radical indicates that rotation of the methyl group is apparently responsible for the line broadening rather than the inversion of the carbon atom center (68,94). Meakin and Krusic also concluded that ro-... 

Edge and Kochi and Biddles and his co-workers also examined the temperature dependence of a 3 in the 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-triflu-oroethyl, and 3-fluoro-2-propyl radicals (69,71). They established that for the planar 2,2,2-trifluoroethyl radical is independent of temperature over a broad range of temperature. However, rotation may be restricted in the solid at 4.2 K (61). The values of the other fluoroethyl derivatives exhibit a novel temper-... 

All these factors make it especially difficult to deduce the structures of radicals from the coupling constants. Not long ago several groups argued that the unusually small values of a" for 2-fluoroethyl and 2-chloroethyl radical indicated that these radicals possessed distorted, bridged structures, XLII (69,70,92). The reduced values were related to the increased distance between... 

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