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Bending and folding of hydrocarbon chains has been probed by using fluorescence methods since the discovery by Hirayama*

The pure electrophilic nature and unique reactivity characteristics and perfluoro-n-alkyl radicals have made the quantitative determination of their reactivity a matter of fundamental significance. Initial insight into the behavior of such radicals was obtained in our recently reported study which resulted in determination of absolute rate constants for their addition to a broad spectrum of alkenes.1.2 In these LFP experiments it was found that perfluorinated radicals were much more reactive than their hydrocarbon counterparts in additions to normal, electronrich alkenes, such as 1-hexene (40 000 times as reactive).In this paper we report the use of this LFP data in conjunction with a series of bimolecular competition experiments to determine approximate absolute rate constants for some hydrogen atom transfer reactions of the perfluoro-n-heptyl radical with substituted silanes and thiophenol, and then we demonstrate how these data can be used to determine the rates of cyclization of some fluorinated alkenyl radicals, which as such comprise the first reported fluorinated 'clock" reactions.Recently we reported that the rate of hydrogen atom transfer to n-C7FIS* by tributyltin hydride was -75 times that to n-alkyl radicals at 30 O C . 2 In the present work even more greatly enhanced rates of hydrogen atom transfer to Rf* (vs R') by three common silane reducing agents are observed. For example, in contrast to its relatively low reactivity with hydrocarbon radicals, where it has been found to be virtually useless as an effective chain-sustaining hydrogen atom transfer agent,3 triethylsilane was observed to reduce perfluoroalkyl bromides and iodides efficiently under photochemical initiation, transferring hydrogen atoms to the intermediate perfluoro-n-alkyl radicals at a rate which should make Et3SiH a very useful competitive reducing agent for studying chain processes which involve fluorinated radicals. As seen in Table 1, Et3SiH was >850 times more reactive with Rf' than with R' at 30 OC. In a similar manner, the even more reactive reducing agents (TMS)ZSiHMe and (TMS),SiH were shown to transfer hydrogen atoms -440 and -110 times Contribution No. 6795. (1) Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Dolbier, W. R., Jr.; Pan, H.-Q.( 2 ) Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Dolbier, W. R., Jr.; Pan, H.-Q.; (3) Lusztyk, J.; Maillard, B.; Ingold, K. U.Table 1. Rate Constants for Hydrogen Atom Abstractions a t 30 OC rate of reduction (k~/(106 M-I s-')) (TMS)2-(TMS)y n-Bus-Radical PhSH EtoSiH SiMeH SiH SnH R-CH2'" 150 0.OOO 85 0.037 0.46 2.76 n-C7F15' 0.28 (0.03) 0.75 (0.07) 16.3 (1.5) 51 (5) 203 (14) Reference 7. Reference 2.more rapidly, respectively, to perfluoro-n-alkyl than to the analogous hydrocarbon radicals.& The observed enhanced rates of Rf* with the silanes and n-Bu,-SnH are consistent with the greater exothermicity of such reductions ( R r H BDE = 107 kcal/molvs 98 kcal/mol for R-H).* However, this certainly cannot be the whole story since it was found that PhS-H, a very efficient H atom donor to hydrocarbo...

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The structures and energies of several conformers of 2-fluoroethanol and 2-fluoroacetaldehyde enol (I-hydroxy-2-fluoroethylene) have been calculated by using ab initio molecular orbital theory with a triple-{ basis set augmented by polarization functions on all atoms. Correlation and zero-point energy corrections were included. The gauche hydrogen-bonded conformer (GG) is the ground-state structure for CH2FCH20H, in agreement with experiment. At the MP-2 level, the gauche rotamer with no hydrogen bonding (GT) is 1.9 kcal/mol higher in energy, and the all-trans conformer TT is 2.0 kcal/mol higher than GG. The greater stability of gauche-CH2FCH20H therefore is almost entirely due to hydrogen bonding, and the "gauche effect" contributes only about 0.1 kcal/mol. For CHF=CH(OH) at the MP-2 level, the cis-syn hydrogen-bonded isomer is 3.2 kcal/mol more stable than the cis-anti isomer, and 4.1 kcal/mol more stable than the trans-syn isomer, which is the lowest energy trans conformer. In contrast to the results for CH2FCH20H, the "cis effect" in CHF==CH(OH) (0.9 kcal/mol) is nearly identical with that found in CHF=CHF (1.0 kcal/mol), even though the internal hydrogen bond in the enol is considerably stronger than that in CH2FCH20H.