J. Phys. Chem. 1996, 100, 11355-11359
11355
Fluoride Elimination upon Reaction of Pentafluoroaniline with eaq , H, and OH Radicals in Aqueous Solution
Lian C. T. Shoute* and Jai P. Mittal*,† Chemistry DiVision, Bhabha Atomic Research Centre, Bombay 400 085, India
P. Neta Physical and Chemical Properties DiVision, National Institute for Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: March 22, 1996; In Final Form: April 18, 1996X
Reduction of pentafluoroaniline (PFA) leads to rapid fluoride elimination to form the aminotetrafluorophenyl radical. This radical undergoes rapid intramolecular electron transfer from the amino group to the phenyl radical site and protonates at the latter site to form the tetrafluoroaniline radical cation or its deprotonated • •form (pKa ) 2.3). Oxidizing radicals such as SO•4 , N3, and Cl2 oxidize PFA to the pentafluoroaniline radical cation. The pKa ) 1.9 of the pentafluoroaniline radical cation is 3.6 units lower than that expected from the substituent effects of fluorine atoms. Addition of OH to PFA is followed by rapid HF elimination to yield the aminotetrafluorophenoxyl radical. In acidic solution, however, the reaction of OH leads to formation of the pentafluoroaniline radical cation. Fluoride elimination upon reduction and upon OH addition to PFA was confirmed by determination of fluoride ion yield.
Introduction Reductive defluorination is a topic of current interest because of increaing concern for accumulation of fluorinated compounds in the environment1 and the increasing importance of electrochemical methods for their preparation.2 Controlled reductive defluorination of cyclic perfluoroalkanes has been reported to yield highly fluorinated aromatic compounds.3 With strong reducing agents, however, uncontrolled reduction takes place, leading to mineralization of all the fluorine.4 Due to the high C-F bond strength, the radical anions formed upon the addition of electrons to perfluorinated compounds in the gas phase5 and in organic matrices at low temperature6,7 are stable with respect to fluoride elimination. In aqueous solution at room temperature, due to the high solvation energy of fluoride ion, fluoride elimination is the dominant process in the reduction of some fluorinated compounds.8 Rapid fluoride elimination has been reported upon the addition of electrons to hexafluorobenzene9 and perfluorinated benzoate.10 The rate of fluoride elimination may depend on a number of factors. If the unpaired electron occupies an orbital which has significant σ character,6 a rapid rate is expected. A relatively slow rate is expected for a π radical as it requires an intramolecular electron transfer to the orthogonal σ orbital of the C-F bond. The presence of a substituent with high electron affinity would further reduce the rate.11 Recently we have shown that tetrafluorobenzosemiquinone, a π radical anion, does not lose fluoride.12 In contrast, rapid fluoride elimination was observed upon reduction of pentafluorophenol, which formed a σ radical, and the transient detected after fluoride elimination was the tetrafluorophenoxyl radical.13 In our effort7,12,13 to understand the factors affecting fluoride elimination from the radical anions of perfluorinated compounds and to elucidate the mechanism, we have investigated the †
Also affiliated with the Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore, India. ‡ The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the equipment or materials identified are necessarily the best available for the purpose. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00876-3 CCC: $12.00
transient formed upon addition of eaq, H, and OH to pentafluoroaniline (PFA) and determined the rate constants of their redox reactions. Experimental Section‡ The kinetic spectrophotometric pulse radiolysis apparatus and the associated detection and data-processing equipment have been described elsewhere.12,13 The bimolecular rate constants k for reaction of a reactant radical with a substrate (S) were obtained from the slope of a plot of kobs vs [S] using the equation kobs ) k0 + k[S], where kobs is the experimentally observed decay or formation rate constants of a transient absorption determined at different substrate concentrations [S]. The uncertainties in the values of the bimolecular rate constants reported here are estimated to be 10-20%.13 Fluoride ions produced upon γ radiolysis of PFA in aqueous solutions were determined with a Dionex Model DX-500 ion chromatograph using an Ionpac AS11 analytical column. PFA from PCR Research Chemical was used as received. Other chemicals used were either analytical or spectroscopic grade purity. Distilled water was further purified in a Millipore Milli-Q system. The pH of the solutions was adjusted by HClO4, KH2PO4, Na2HPO4, and NaOH. At acid concentrations higher than 0.5 mol L-1 HClO4, the Hammett acidity function H0 was used to represent the pH of the solution. Results and Discussion Reactions of PFA with Reducing Radicals. Pentafluoroaniline undergoes protonation only in strongly acidic solutions. The pKa for this equilibrium (eq 1) was determined by monitoring the absorbance at 275 nm as a function of pH and was found to be -0.3. PFA reacts with eaq very rapidly; a rate + C6F5NH+ 3 h C6F5NH2 + H
(1)
constant of k2 ) (8.8 ( 1.6) × 109 L mol-1 s-1 has been determined for this reaction by monitoring the eaq decay rate at 700 nm as a function of PFA concentration in deoxygenated solutions containing 0.2 mol L-1 2-methyl-2-propanol at pH 7. The rate constants for reactions of various radicals with PFA are summarized in Table 1. © 1996 American Chemical Society
11356 J. Phys. Chem., Vol. 100, No. 27, 1996
Shoute et al. SCHEME 1
, Figure 1. Transient absorption spectra of (b) aminotetrafluoroanilino radical (dose ) 29.3 Gy); (2) tetrafluoroaniline radical cation (32.2 Gy). (Inset) Effect of pH on the absorbance of the tetrafluoroaniline radical cation at 440 nm (40.5 Gy). (See text for details.)
TABLE 1: Rate Constants of Various Radicals radical eaq
H OH O•N•3 Cl•2 SO•4 C6HF4NH•+ 2 C6F5NH•+ 2 • C6HF4NH H2NC6F4O•
substrate
pH
k, L mol-1 s-1
C6F5NH2 C6F5NH2 C6F5NH2 C6F5NH2 C6F5NH2 C6F5NH2 C6F5NH2 AH2 AH2 AHA2-/AH-
7 0.6 7 13.3 7.2 2 1.1 0.3 0.3 7 11.3
(8.8 ( 1.6) × 109 (4.3 ( 0.8) × 108 (9.6 ( 2.0) × 109 (6.6 ( 1.2) × 108 (1.4 ( 0.3) × 108 (1.1 ( 0.2) × 108 (2.5 ( 0.5) × 109 (1.8 ( 0.4) × 108 (2.6 ( 0.5) × 108 (8.6 ( 1.6) × 107 (2.4 ( 0.4) × 107
The transient absorption spectrum recorded at 3 µs after pulse radiolysis of deoxygenated solutions containing 3.0 × 10-4 mol L-1 PFA and 1.0 mol L-1 2-methyl-2-propanol at pH 7 exhibits a peak at 395 nm (Figure 1). This spectrum cannot be ascribed to the initial electron adduct (reaction 2) or the phenyl radical produced by defluorination (reaction 3) but is attributed to the tetrafluoroanilino radical produced (reactions 4-6), in parallel with the previous observation on pentafluorophenol.13 Reactions 4 and 5 can be visualized as an intramolecular electron transfer from the amino group to the aromatic ring followed by protonation on a ring carbon. The spectrum of the tetrafluoroanilino radical is similar to that of the pentafluoroanilino radical obtained by one-electron oxidation of PFA (see below) and to the spectrum of the unsubstituted anilino radical reported previously.14 Due to very rapid fluoride elimination from the radical anion9,10 (reaction 3), cyclohexadienyl radical formed by protonation of the radical anion may not be a competitive route for the formation of the anilino radical (see below). This is further supported by a decrease of the anilino radical yield on increasing concentration of 2-propanol due to the hydrogen abstraction reaction of the phenyl radical (see below), a reaction in which the cyclohexadienyl radical is not reactive. •eaq + C6F5NH2 f C6F5NH2
(2)
• C6F5NH•2 f C6F4NH2 + F
(3)
C6F4NH2 f -C6F4NH•+ 2
(4)
•+ H2O + -C6F4NH•+ 2 f C6HF4NH2 + OH
(5)
•
C6HF4NH•+ 2
•
+
h C6HF4NH + H
(6)
The radical cation of aniline was reported to have more intense absorption with a double peak at 406 and 423 nm.14 It deprotonates to the neutral anilino radical with a pKa of 7.05. In our studies, the radicals derived from fluorinated aniline are found to have similar absorptions to those of the nonfluorinated analogs, but fluorine substitution is known to shift the pKa of the radical cation to much lower values due to the strong electron-withdrawing effect of the fluorine.4 Therefore, the tetrafluoroaniline radical cation can be observed only in strongly acidic solutions. The spectrum recorded at 8 µs after pulse radiolysis of deoxygenated solutions containing 6.0 × 10-4 mol L-1 PFA, 0.9 mol L-1 2-methyl-2-propanol, and 0.5 mol L-1 HClO4 (Figure 1) has peaks at 280 and 435 nm. However, under these conditions, eaq is rapidly converted into H atoms by reaction with protons. H atoms add to PFA at different ring positions to form isomers of a cyclohexadienyl type radical (eq 7), which is expected to absorb at 300-350 nm.14,15 Such absorption was
H + C6F5NH2 f •C6HF5NH2
(7)
not detected even at high concentrations of PFA, indicating that the H-adduct rapidly eliminates HF. HF elimination cannot form a phenyl type radical but must involve the amino group to form the anilino type radical as shown in Scheme 1 (for the para isomer). A rate constant of k7 ) (4.3 ( 0.8) × 108 L mol-1 s-1 was determined by following the formation at 430 nm as a function of PFA concentration in the pulse radiolysis of deoxygenated solutions containing 2.1 mol L-1 2-methyl-2propanol at pH 0.6. The acid dissociation constant of the tetrafluoroaniline radical cation (equilibrium 6) was determined from the pH dependence of the absorbance at 440 nm (inset of Figure 1) obtained upon pulse radiolysis of deoxygenated solutions of 2.2 × 10-3 mol L-1 PFA and 1.0 mol L-1 2-methyl-2-propanol. The value, pKa ) 2.3, obtained for the tetrafluoroaniline radical cation is close to that (pKa ) 1.9) for the pentafluoroaniline radical cation (see below). The tetrafluoroanilino radical produced upon deprotonation of the cation is a mild oxidant. It oxidizes ascorbate ion with a rate constant of k8 ) (8.6 ( 1.6) × 107 L mol-1 s-1 at pH 7. The rate constant was determined by monitoring ascorbate
C6HF4NH• + AH- f C6HF4NH2 + A•-
(8)
radical formation at 360 nm as a function of ascorbate concentration in the pulse radiolysis of deoxygenated solutions of 1.9 × 10-3 mol L-1 PFA and 2.0 mol L-1 2-methyl-2propanol. Oxidizing reactions of the tetrafluoroaniline radical cation provide additional evidence for its formation upon the addition of H atoms to PFA. The rate constant for reaction with ascorbic acid (AH2) was determined to be k9 ) (1.8 ( 0.4) × 108 L mol-1 s-1 at pH 0.3. The rate constant was determined from • + C6HF4NH•+ 2 + AH2 f C6HF4NH2 + AH + H
(9)
the dependence of the radical cation decay rate, monitored at
Fluoride Elimination upon Reaction of C6F5NH2
, Figure 2. Time-resolved spectra showing oxidation of ascorbic acid by the tetrafluoroaniline radical cation recorded at (b) 2.5 µs and (2) 25 µs after pulse radiolysis of a deoxygenated solution of 3.1 × 10-3 mol L-1 PFA, 1 mol L-1 2-methyl-2-propanol, and 0.49 mol L-1 HClO4 containing 6.1 × 10-4 mol L-1 ascorbic acid. Dose ) 32 Gy.
430 nm, on ascorbic acid concentration in the pulse radiolysis of deoxygenated solutions containing 2.1 × 10-3 mol L-1 PFA, 2.0 mol L-1 2-methyl-2-propanol, and 0.49 mol L-1 HClO4. The time-resolved spectra (Figure 2) show the absorption of the tetrafluoroaniline radical cation at 2.5 µs after the pulse, with a peak at 430 nm, and the absorption of the ascorbate radical at 25 µs after the pulse, with a peak at 360 nm.16 The decay at 430 nm was concomitant with the formation at 360 nm. The aminotetrafluorophenyl radical produced upon reduction of PFA (reaction 2) can abstract hydrogen from a suitable donor, e.g., 2-propanol (2PrOH), in competition with the protonation (reaction 5). Therefore, the rate constant for protonation, k5, can be estimated by competition kinetics using the hydrogen abstraction reaction as the reference. By monitoring the yield •
C6F4NH2 + (CH3)2CHOH f C6HF4NH2 + (CH3)2C•OH (10)
of the tetrafluoroanilino radical, i.e., the absorbance at 395 nm in the absence (A0) and the presence (A) of 2-propanol and plotting A0/A vs [2PrOH], we can calculate a rate constant k5 ) (7.7 ( 1.0) × 107 s-1 from the slope of the plot according to the relation A0/A - 1 ) k5/k10[2PrOH], assuming k10 ) 5.5 × 106 mol L-1 s-1 as in phenyl radicals.17 This value should be taken as a rough estimate because k10 for the tetrafluorophenyl radical is not known. Normally fluorination of carbon18 and oxygen19 centered radicals increases the rate constant for hydrogen abstraction. Defluorination upon reduction of PFA by hydrated electrons was also confirmed by determining the yield of fluoride in γ-irradiated solutions. Deoxygenated solutions containing 1 × 10-3 mol L-1 PFA and 2 × 10-3 mol L-1 carbonate buffer at pH 11 gave a radiolytic fluoride yield of G(F-) ) 10. The yield was similar also in aerated or N2Osaturated solutions, indicating that all the primary radicals of water radiolysis, eaq, H, or OH, lead to defluorination of PFA. The fact that the G value is higher than the total yield of radicals (ca. 6) suggests a short chain reaction as discussed previously.13 Reactions of PFA with Oxidizing Radicals. The results described above indicate that reduction of PFA leads to the formation of an oxidizing radical. To provide further evidence for the mechanism proposed, we studied the transients formed •on oxidation of PFA. Oxidation of PFA by SO•4 and Cl2 radicals in acidic solutions led to the formation of the pen-
J. Phys. Chem., Vol. 100, No. 27, 1996 11357
, Figure 3. Absorption spectra of (b) pentafluoroaniline radical cation (dose ) 12.6 Gy); (2) pentafluoroanilino radical (dose ) 27.8 Gy). (Inset) Effect of pH on the absorbance of the pentafluoroaniline radical cation at 430 nm (see text for details).
tafluoroaniline radical cation, whereas oxidation by SO•4 and N•3 radicals in neutral solutions produced the neutral pentafluoroanilino radical. The reaction of SO•4 was studied by pulse radiolysis of deoxygenated solutions containing 3.7 × 10-4 mol L-1 PFA, 0.05 mol L-1 K2S2O8, 0.09 mol L-1 2-methyl-2-propanol, and 0.13 mol L-1 HClO4. The spectrum recorded at 11 µs exhibits peaks at 285 and 430 nm (Figure 3). The rate constant for reaction 11 was determined by following the decay of the SO•4 absorption at 460 nm at various PFA concentrations and •+ 2SO•4 + C6F5NH2 f C6F5NH2 + SO4
(11)
was found to be k11 ) (2.5 ( 0.5) × 109 L mol-1 s-1 at pH was studied in N2O-saturated 1.1. The reaction of Cl•2 solutions containing 0.11 mol L-1 KCl. The spectrum observed at pH 1 also exhibited peaks at 285 and 430 nm. The rate constant for reaction 12 was determined atpH 2 by monitoring •+ Cl•2 + C6F5NH2 f C6F5NH2 + 2Cl
(12)
the decay of the Cl•2 absorption at 350 nm as a function of PFA concentration and was found to be k12 ) (1.1 ( 0.2) × 108 L mol-1 s-1. In agreement with some of the above results, oxidation of PFA in neutral solutions led to the formation of the neutral radical. Oxidation by SO•4 radicals was examined by pulse radiolysis of deoxygenated solutions containing 4.5 × 10-4 mol L-1 PFA, 0.05 mol L-1 K2S2O8, and 0.1 mol L-1 2-methyl-2propanol at pH 6.4. The spectrum of the transient species recorded at 5 µs after the pulse has peaks at 290 and 400 nm (Figure 3). Similarly, oxidation by N•3 at pH 7.2 produced a species absorbing at 290 and 400 nm. From the rate of buildup of this absorption monitored as a function of PFA concentration in the pulse radiolysis of N2O-saturated 0.4 mol L-1 NaN3 at pH 7.2, we derived a rate constant of k13 ) (1.4 ( 0.3) × 108 L mol-1 s-1 for the oxidation of PFA by N•3 radicals. + N3 + C6F5NH2 f C6F5NH• + N3 +H
(13)
To determine the pKa for the pentafluoroaniline radical cation, we used the SO•4 system since it provides a good oxidant throughout the pH range. The absorbance at 430 nm was monitored by pulse radiolysis of deoxygenated solutions containing 1.3 × 10-3 mol L-1 PFA, 0.1 mol L-1 K2S2O8, and 0.16 mol L-1 2-methyl-2-propanol at different pH values. The
11358 J. Phys. Chem., Vol. 100, No. 27, 1996
Shoute et al. 6
6
i)2
i)2
+ + + E0 ) 2.2 + 0.8(σ+ p1 + ∑σi ) + 0.4σp1∑σi
, Figure 4. Transient absorption spectrum of the aminotetrafluorophenoxyl radical recorded at 2 µs after pulse irradiation of a N2O-saturated solution of 2.2 × 10-4 L mol-1 PFA at pH 7. Dose ) 24.9 Gy.
SCHEME 2
pH dependence (inset of Figure 3) yields a pKa ) 1.9. pKa
C6F5NH•+ \z C6F5NH• + H+ 2 y
(14)
The radical cations of pentafluoroaniline and tetrafluoroaniline are ca. 5 orders of magnitude more acidic than the corresponding aniline radical cation.14,20 Jonsson et al.20 from the studies of the dependence of the pKa values of aniline radical cations on different substituents arrived at an empirical relation between the pKa and Brown substituent constants21 (σ+ i ), eq I, (where 6
pKa ) 7.09 - 3.17∑σi+
(I)
i)2
+ + + σ+ i is σo , σm, and σp for the substituents in positions 2-6). On the basis of this relation, the pKa of the pentafluoroaniline radical cation should be ca. 5.5, which is much higher than the experimentally observed value. This indicates that simple application of the substituent effect does not hold for polyfluorinated compounds. The strong decrease of the pKa value in the perfluoro compound may be explained by anionic hyperconjugation as discussed by Taft et al.22 These authors observed that (CF3)3CH is several orders of magnitude more acidic than the perfluorinated bridge-head C-H compound (1Hundecafluorobicyclo[2.2.1]heptane). Since these compounds have similar stabilizations of the anion by field/inductive effects, polarizability of the substituents, and destabilization due to repulsion of the anionic center and fluorine atoms, the difference in the pKa was ascribed to the difference in the geometry of the anions; whereas the bridge-head compound is rigid, (CF3)3C- can be stabilized by anionic hyperconjugation. For perfluoroaniline this effect can be represented as in Scheme 2: In contrast to the pKa, the empirical relationship of the substituent effect on the oxidation potential (E0)20 gave a good estimate. Equation II (where σ+ p1 is -1.3 for NH2 in aniline) yields E0(PFA•+/PFA) ) 1.3 V vs NHE. A rate constant k13 ) 1.4 × 108 L mol-1 s-1 for oxidation of PFA by N3 (1.3 V) is close to the expected rate for a thermoneutral reaction (see below).
(II)
The reaction of PFA with OH radicals was also studied at different pH values, but the results are more complex than those obtained with the above oxidizing species. The spectrum recorded by pulse radiolysis of a N2O-saturated solution containing PFA at pH 7 (Figure 4) has peaks at 295 and 450 nm. The latter peak is clearly different from the 400 nm peak observed upon oxidation in neutral solution and must be ascribed to a different species. Most probably the reaction of OH takes place via addition and HF elimination to yield the aminotetrafluorophenoxyl radical. If addition is assumed to occur mainly at the ortho and para positions with respect to the amino group, a mixture of 2-amino- and 4-aminophenoxyl radicals is expected to be formed. The nonfluorinated 4-aminophenoxyl has a peak
OH + C6F5NH2 f HO•C6F5NH2
(15)
OH•C6F5NH2 f •OC6F4NH2 + H+ + F-
(16)
at 440 nm.23 The peak observed at 450 nm can be ascribed to the fluorinated analog. The 2-aminophenoxyl radical probably absorbs only near 330 nm24 (by comparison with ortho and para semiquinones). A fraction of the OH radicals also may add at the carbon bearing the amino group. This adduct may undergo water elimination, to yield pentafluoroanilino radical, or elimination of ammonia, to yield pentafluorophenoxyl radical. Since both of these radicals absorb near 400 and not 450 nm, the observed spectrum indicated that they have minimal contributions. The rate constant k15 ) (9.6 ( 2.0) × 109 L mol-1 s-1 at pH 7 was determined by following the formation of the absorption at 450 nm as a function of PFA concentration in the pulse radiolysis of N2O-saturated solution. A similar rate constant was observed by competition kinetics using the reaction with KSCN as a reference. The rate of decay of the 450 nm absorption was accelerated in the presence of ascorbate and from the concentration dependence we derived a rate constant of k17 ) (2.4 ( 0.4) × 107 L mol-1 s-1 for oxidation of ascorbate dianion at pH 11.3. Oxidation of ascorbate monoanion at pH 7 was considerably slower and was not measurable under our conditions.
H2NC6F4O• + A2- f H2NC6F4O- + A•-
(17)
In acidic solution H2O elimination (k18) appears to compete with HF elimination (k16) in aminohydroxypentafluorocyclohexadienyl radical to yield the pentafluoroaniline radical cation.
H+ + HO•C6F5NH2 f C6F5NH•+ 2 + H2O
(18)
The spectrum recorded at 2 µs after pulse irradiation of O2saturated solution of 4.2 × 10-4 mol L-1 PFA and 0.15 mol L-1 HClO4 was similar to that displayed in Figure 3. The radical cation is an oxidizing agent; it oxidizes ascorbic acid with a rate constant of k19 ) (2.6 ( 0.5) × 108 L mol-1 s-1 as derived from the rate of the radical cation decay at 430 nm as a function of ascorbic acid concentration in the pulse radiolysis of a O2-saturated solution of 2.7 × 10-3 mol L-1 PFA containing 0.51 mol L-1 HClO4. • + C6F5NH•+ 2 + AH2 f C6F5NH2 + AH + H
(19)
Conclusion. Reduction of pentafluoroaniline by the hydrated electron leads to fluoride elimination to form a phenyl radical.
Fluoride Elimination upon Reaction of C6F5NH2 This radical undergoes rapid intramolecular electron transfer and protonation at a ring carbon to form the aminotetrafluoroaniline radical cation. The cation and its conjugate base, the tetrafluoroanilino radical, can oxidize ascorbic acid. Thus, reduction of pentfluoroaniline produces an oxidizing species. Addition of H atoms produces the same radical cation, by a different mechanism not involving a phenyl radical intermediate. A dramatic increase in the acidity of perfluoroaniline radical cation, several orders of magnitude higher than those expected from the substituent effects, is ascribed to the contribution of anionic hyperconjugation in the perfluoroanilino radical. Addition of hydroxyl radicals to pentafluoroaniline also leads to fluoride elimination and formation of phenoxyl radicals. It is noted that the initial adducts formed by the reactions of PFA with eaq, H, and OH have very short lifetimes and predominantly eliminate fluoride, but by different mechanisms. The present results indicate that the radiation chemistry of perfluorinated compounds can be very different from that of the nonfluorinated analogs due to the prevalent fluoride elimination process. Acknowledgment. This research was supported in part by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, and by the Indo-US collaborative program between BARC and NIST. References and Notes (1) Visscher, P. T.; Culbertson, C. W.; Oremland, R. S. Nature 1994, 369, 729. Lovley, D. R.; Woodward, J. C. EnViron. Sci. Technol. 1992, 26, 925. Oremland, R. S.; Miller, L. G.; Strohmaier, F. E. EnViron. Sci. Technol. 1994, 28, 514. Krone, U. E.; Thauer, R. K.; Hogenkamp, H. P. C.; Steinback, K. Biochemistry 1991, 30, 2713. (2) Fuchigami, T. Top. Curr. Chem. 1994, 170, 1. Iwasaki, T.; Yoshiyama, A.; Sato, N.; Fuchigami, T.; Nonaka, T. J. Electroanal. Chem. 1987, 238, 315. Sato, N.; Yoshiyama, A.; Cheng, P. C.; Nonaka, T.; Sasaki, M. J. Appl. Electrochem. 1992, 22, 1082. Kariv-Miller, E.; Vajtner, Z. J. Org. Chem. 1985, 50, 1394. (3) MacNicol, D. D.; Robertson, C. D. Nature 1988, 332, 59. Marsella, J. A.; Gilicinski, A. G.; Coughlin, A. M.; Pez, G. D. J. Org. Chem. 1992, 57, 2856. (4) Perry, R. In Fluorine-The First Hundred Years; Banks, R. E., Sharp, D. W. A., Tatlow, J. C., Eds.; Elsevier: New York, 1986; p 293. Sheppard, W. A.; Sharts, C. M. Organic Fluorine Chemistry; W. A. Benjamin: New York, 1969; p 450. Hudlicky, M. Chemistry of Organic Fluorine Compounds; Ellis Horwood Limited: Chichester, 1976; p 563. Oku, A.; Nishimura, J.; Nakagawa, S.; Yamada, K. Nippon Kagaku Kaishi 1985, 1963. Kavan, L.; Dousek, F. P. J. Fluorine Chem. 1988, 41, 383. (5) Gant, K. S.; Christophorou, L. G. J. Chem. Phys. 1976, 65, 2977. Lifshitz, C.; MacKenzie Peers, A.; Grajower, R.; Weiss, M. J. Chem. Phys. 1970, 53, 4605. Chowdhury, S.; Grimsrud, E. P.; Heinis, T.; Kebarle, P. J. Am. Chem. Soc. 1986, 108, 3630. Kebarle, P.; Chowdhury, S. Chem. ReV. 1987, 87, 513.
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