Effects of Quaternary Ammonium Salts on Reactions of Aromatic

Chem. 1984, 88, 2368-2372 ... The measured u' = 0-1 vibronic separation of 1450 i ... The effect of Bu4N+ on the reactions of the radical anions is di...
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2368

J . Phys. Chem. 1984, 88, 2368-2372

idence suggests that the pyramidal CF3 radical is extremely reactive under these high-pressure conditions.

Conclusions The fluorine atom/methyl fluoride reaction provides sufficient CHzF radicals for observation of its first photoelectron band at 9.04 f 0.01 eV adiabatic and 9.22 f 0.01 eV vertical ionization energies. The measured u' = 0-1 vibronic separation of 1450 i 30 cm-' is due to the C-F stretching fundamental of the CHzF+ ground state; this vibronic interval shifts to 1530 f 30 cm-I for CDzF+due to interaction with the scissor bending mode. Considerable increases in the C-F stretching fundamentals of the cation as compared to the case of the neutral radicals are explained by increased net C-F bonding in the cations. Vinyl fluoride, fluoroacetylene, and the C F radical were also observed in this

reaction system; although the C H F intermediate was not detected directly, its production is indicated by the observation of HCCF and CF. This study demonstrates the usefulness of a multidetector photoelectron spectrometer to characterize the reactive and stable products of a reaction system; this method, in principle, can identify all species with sufficient lifetime to reach the ionization region. Acknowledgment. We gratefully acknowledge financial support for this research from the S.E.R.C. (U.K.). L.A. acknowledges a Sesquicentennial Associateship from the University of Virginia, a Visiting Fellowship from the S.E.R.C., and a Fulbright Senior Research Fellowship. Registry No. F, 14762-94-8; CH3F, 593-53-3; CH2F, 3744-29-4; CDIF, 89578-59-6.

Effects of Quaternary Ammonium Salts on Reactions of Aromatic Radical Anions Formed in Tetrahydrofuran by Pulse Radiolysis Yukio Yamamoto,* Shoichi Nishida, Katsuyoshi Yabe, Koichiro Hayashi, Seishi Takeda, and Kunihiko Tsumori The Institute of Scientific and Industrial Research, Osaka University, 8- 1 Mihogaoka, Ibaraki, Osaka 567, Japan (Received: August 22, 1983)

Pulse radiolysis of biphenyl (BP) and pyrene (Py) in tetrahydrofuran (THF) solution was carried out in the presence of various kinds of quaternary ammonium salts, such as Bu4NPF6,Bu4NBF4,Bu4NI, BzMe3NPF6,and PhMe3NPF6 (Bu, butyl; Me, methyl; Bz, benzyl; and Ph, phenyl). The decay behaviors of the radical anions, BP-- and Py-a, are significantly affected by the addition of the salts. The anions of the salts, PF6-, BF4-, and I-, are considered to form ion pairs with the solvent counterions, THF(H+), resulting in a retardation of the neutralization reactions. The rate constants for the neutralization reactions have been determined in the absence and presence of Bu4NPF6. The addition of BzMe3NPF6 (or PhMe3NPF6) accelerates the decay of BP-. and retards the decay of Py-., depending on the rates of reactions of the radical anions with BzMe3N+(or PhMe,N+). The reactivity of BP-. in the electron transfer to Py seems to be reduced in the presence of Bu4NPF6. The effect of Bu4N+ on the reactions of the radical anions is discussed.

Introduction Numerous studies have been reported for the salt effects on ionic reactions in radiation chemistry. According to the Bronsted-Bjerrum theory the rates of reactions between charged species depend on the ionic strength if other ions are present in the solution.' Such an effect has been studied on the reactions of solvated electrons with various kinds of inorganic ions in water" and methan01.~ On the other hand, in solvents of low polarity where ion-pair formation is important the reactivities of charged species are affected by the addition of salts through ion-pair formation with the counterions from the salts. The salt effect has been studied on reactions of negatively charged species such as solvated electrons, aromatic radical anions, and carbanions in tetrahydrofuran (THF) by pulse radiolysis methods.613 The salts (1) See, for example, Davies, C. W. "Progress in Reaction Kinetics"; Porter, G.; Stevens, B., Ed.; Pergamon Press: Oxford, 1961; Vol. 1, pp 161-86. (2) Czapski, G.;Schwarz, H. A. J . Phys. Chem. 1962, 66, 471-4. (3) Gordon, S.;Hart, E. J.; Matheson, M. S.; Rabani, J.; Thomas, J. K. J . Am. Chem. SOC.1963,85, 1375-7. (4) Anbar, M.; Hart, E. J. Adu. Chem. Ser. 1968, No. 81 (Vol. I), 79-94. (5) Buxton, G.V.;Dainton, F. S.; Hammerli, M. Trans. Faraday SOC. 1967,63, 1191-7. (6) Baxendale, J. H.;Beaumond, D.; Rodgers, M. A. J. Trans. Faraday SOC.1970, 66, 1996-2003. (7) Bockrath, B.; Dorfman, L. M. J . Phys. Chem. 1973, 77, 1002-6. (8) Bockrath, B.;Dorfman, L. M. J . Phys. Chem. 1973, 77, 2618-22. (9) Bockrath, B.;Dorfman, L. M. J. A m . Chem. Soc. 1974,96, 5708-15. (10) Salmon, G.A,; Seddon, W. A. Chem. Phys. Lett. 1974, 24, 366-8. (1 1) Salmon, G.A.; Seddon, W. A,; Fletcher, J. W. Can. J . Chem. 1974, 52, 3259-68. (12) Langan, J. R.; Salmon, G. A. J . Chem. SOC.,Faraday Trans. 1 1982, 78. 3645-57.

0022-365418412088-2368$01.50/0

used in these studies are tetrahydroaluminate and tetraphenylborate salts of alkali metals, whose addition to the THF solutions results in ion pairs of the negatively charged species, generated by pulse radiolysis, with the alkali metal cations. The rate constants for several types of reactions have been determined for the free and paired states. The values are smaller for the paired states than for the free states except for those of proton transfer reactions of benzyl anions from water and alcohol^.^ This subject has also been studied by flash photolysis of aromatic radical anions in THF, demonstrating that the reactivities of the solvated electrons and aromatic radical anions are reduced upon ion-pair formation with the alkali metal cations from the added salts.I4-l6 We have recently reported that the lifetime of the biphenyl radical cations formed in dichloromethane by pulse radiolysis is extended by the addition of salts having complex metal halide anions, such as diphenyliodonium, triphenylsulfonium, and tetrabutylammonium hexafl~orophosphates.'~This effect has been attributed to the stabilization of the radical cations through ion-pair formation with the nonnucleophilic anions from the salts. It was also found that the stabilization of the monomer and dimer (13) Langan, J. R.; Salmon, G. A. J . Chem. SOC.,Faraday Trans. I 1983, 79, 589-97. (14) Fisher, M.; Ramme, G.; Claesson, S.; Szwarc, M. Chem. Phys. Left. 1971, 9, 306-8. (15) Fisher, M.; Ramme, G.; Claesson, S.; Szwarc, M. Chem. Phys. Lett. 1971, 9 , 309-12. (16) Ramme, G.; Fisher, M.; Claesson, S.; Szwarc, M. Proc. R. SOC. London, Ser. A 1972, 327, 467-19. (17) Mah, S.;Yamamoto, Y . ; Hayashi, K. J . Phys. Chem. 1983, 87, 297-300.

0 1984 American Chemical Society

The Journal of Physical Chemistry, Vol. 88, No. 1 1 , 1984 2369

Pulse Radiolysis of BP and Py in T H F radical cations of styrene and a-methylstyrene results in the enhancement of the radiation-induced cationic polymerization in the presence of diphenyliodonium and triphenylsulfonium hexafluorophosphates.'8-20 The formation of dimer radical cations is enhanced in the presence of the salts. Similar results have been reported for the formation of dimer dianions of styrene derivatives in THF, which is enhanced in the presence of sodium and lithium

1 , at the end of the pulse

2 , 100 ns a f t e r l h e pulse

d

3 , 500 ns a f t e r the pulse

0

tetra hydro alum in ate^.'^ The lifetimes of the radical ions generated by pulse radiolysis are generally short since they are in highly reactive free-ion states and rapidly decay by neutralization with the counterions. From this point of view, the salt effect in low polar solvents is interesting in connection with the enhancement of reactions other than neutralization. The present pulse radiolysis study is concerned with the effects of various kinds of quaternary ammonium salts on the decay behaviors of the biphenyl (BP) and pyrene (Py) radical anions in THF. The contributions of the quaternary ammonium cations and counteranions of the salts to the reactions of radical anions are described.

Experimental Section THF (Wako Pure Chemical Co.) was distilled over calcium hydride, and the middle fraction was stored under vacuum over calcium hydride. Zone-refined BP (Tokyo Kasei Kogyo) was used without further purification. Py (Wako) was purified by vacuum sublimation. Bu4NPF6, CeMe3NPF6, BzMe3NPF6, and PhMe3NPF6(Bu, butyl; Ce, cetyl; Me, methyl; Bz, benzyl; and Ph, phenyl) were prepared from corresponding bromides (Tokyo Kasei) and KPF6 (Aldrich Chemical Co.) and twice recrystallized from water-methanol mixtures. Bu4NI and Bu4NBF4 (Tokyo Kasei) were also purified by recrystallizations. The samples for pulse radiolysis were prepared in a high vacuum operation and sealed into Suprasil cells of 10-mm optical path length. Suprasil cells of 1-mm optical path length were used for experiments to determine the absolute rate constants of the neutralization reactions. An L-band linear accelerator operating a t 28 MeV was used for the pulse radiolysis. The pulse width was 8 ns unless otherwise noted (the beam diameter, ca. 4 mm). A 450-W xenon pulse lamp (OPG-450, Osram), a monochromator (Nikon G-250),a photomultiplier (Hamamatsu-TV R928), and a programmable digitizer (Tektronix 7912 AD) were used. The pulse radiolysis was carried out at room temperature kept at ca. 22 OC. Results and Discussion Pulse radiolysis of solutions of aromatic compounds in T H F is known to yield radical anions by attachment of solvated electrons, and their decay is due to neutralization with solvent counterions, THF(H+).6,'2q13,21

THF w- THF+. e,-

-

+ Ar

+ e;

Ar-.

+ T H F THF(H+) + THF(-H) Ar-. + THF(H+) Ar(H) + T H F

THF'.

4

(1)

(2)

(3) (4)

The transient absorption spectra obtained in the present study with BP and Py agreed with the spectra of their radical anions in the literature.6,22 The decay behaviors of the transient absorption were significantly affected by the addition of quaternary am(18) Mah, S.; Yamarnoto, y.; Hayashi, K. J. Polym. Sci., Polym. Chem. Ed. 1982. 20. 1709-16. (19) Mah,'S.; Yamamoto, Y.; Hayashi, K. J . Polym. Sci., Polym. Chem. Ed. 1982, 20, 2151-8. (20) Mah, S.; Yamamoto, Y.; Hayashi, K. Macromolecules 1983, 16, 681-5. (21) Shaede, E. A.; Kurihara, H.; Dorfman, L. M. Int. J . Radiat. Phys. Chem. 1974, 6, 47-54. (22) Buschow, K . H. J.; Dieleman, J.; Hoijtink, G. J. J. Chem. Phys. 1965, 42, 1993-9. ~~~

~

Wavelength / nrn

I , at the end of the pulse 2 , 100 ns a f t e r the p u l s e 3 , 500 ns a f t e r the pulse

1

0

0

D

I

600

I

700

I

I

Add i t ive I

A * none

T i m e I ns

Figure 2. Effects of salts on the decay of BP-in 4 X M BP solution. The optical densities a t the end of the pulse are A, 0.44; B, 0.46; and C, 0.36.

monium salts, whereas no particular change in the shape and intensity of the absorption spectra at the end of the pulse was observed. Pulse Radiolysis of BP Solutiom. Figure 1 shows the transient absorption spectra of 3 X lo-* M BP solutions irradiated in the M Bu,NPF,. The absorption absence and presence of 4 X bands at 400 and 630 nm are attributed to BP-., although there is a contribution from a long-lived species having an absorption band at 360 nm which is assigned to the first triplet excited state of BP.23 Comparison of the spectra at different times after the pulse shows that the lifetime of B P . is extended by the addition of the salt. The decay of BP-. was also retarded by the addition of Bu4NBF4, Bu4NI, and CeMe3NPF6. But it was accelerated by the addition of BzMe3NPF6and PhMe3NPF6. This indicates the occurrence of reactions of BP-. with quaternary ammonium cations having aromatic substituents, probably neutralization (23) Porter, G.; Windsor, M. W. Proc. R . SOC.London, Ser. A 1958,245, 238-58.

1

-

I

. ._..,.-.. ...... -.

I

I

I

-

"'

0.81 0

1

I /

I

I

I

I

I

2

4

6

8

]

C S ~ I x~ 103 I / M

Figure 4. Relative values of the slopes of the second-order kinetic plots vs. salt concentration: [BPI, 4 X lo-* M. 2.0

/ 200 400 E00

1.5

600

OO

T i m e / ns

Figure 3. Second-order kinetic plots for the decay of BP. in 3 X M BP solution.

reactions like that of hydrated electrons with B Z M ~ ~ N The +.~~ retardation and acceleration of the decay monitored at 630 nm in the presence of Bu4NPF6and BzMe3NPF6,respectively, are illustrated in Figure 2. Kinetic Analysis of the Decay of B P . Retarded by the Addition of Tetraalkylammonium Salts. The decay of B F - obeys second-order kinetics if it is due exclusively to the neutralization reaction BP-* THF(H+) BP(H) + THF (5)

+

d 0

1.0

0.5

500

600

700

800

Wavelength

500

600

700

800

/ nm

Figure 5. Transient absorption spectra of 6 X alone; B, with 6 X M Bu4NPF6.

M Py solution: A,

+

Figure 3 shows an analysis of the second-order kinetics, based on the assumption [BP.] = [THF(H+)], for the decay of the absorption at 630 nm in the absence and presence of Bu4NPF6. The plot for a solution containing the salt gives a good straight line except for the very early stage after the pulse where a small fast decay is observed. The plot for the salt-free solution deviates from linearity in the later stage as well as in the very early stage. The deviation in the later stage may be due to a small contribution from any long-lived species, which is important in the absence of the salt because of the fast decay of BY-. N o explanation for the fast decay just after the pulse, which was not observed in the case of Py-. as seen later, can be offered at the present time. The slope of the plot for the salt-free solution was determined from the line in the range from 30 to 130 ns after the pulse. By comparing the slopes, the lifetime of B P . in the presence of the salt is estimated to be about 20 times longer than that in the absence of the salt. The slope of the second-order kinetic plot corresponds to k5/sl, where kS is the rate constant of reaction 5 , B is the molecular extinction coefficient of BP., and I is the optical path length of the pulse radiolysis system which is close to the diameter of the electron beam, ca. 4 mm. Reproducible values for the slopes were obtained in the same experimental run during a consecutive operation of the accelerator, keeping the beam diameter constant. The experimental uncertainty in the slopes determined in the same experimental run is less than k5%. The slope was invariant with BP concentration in the range examined from 3 X to 6 X 1W2M as well as the intensity of absorption at the end of the pulse. The electron attachment to BP is complete in this concentration range. When the other tetraalkylammonium salts, Bu4NBF4,Bu4NI, and CeMe3NPF6,were added, the second-order kinetic plots also gave straight lines. The slopes of the plots for solutions containing Bu4NPF6, Bu4NBF4, and CeMe3NPF6were similar at the same salt concentrations. Figure 4 shows the relative values of the slopes ~~

(24) Bobrowski, K.J . Phys. Chem. 1981,85, 382-8.

for the solutions containing Bu4NPF6and Bu4NI plotted against salt concentration. For Bu4NI, the concentration did not exceed 3X M because of its low solubility. The slopes decrease with increasing salt concentration and attain constant values. The constancy of the slope at high salt concentrations suggests that the retardation of the decay of B P . is due to ion-pair formation of BP-. and/or THF(H+) with counterions from the salts. Pulse Radiolysis of Py Solutions. Figure 5 shows the transient absorption spectra of 6 X M Py solutions irradiated in the M Bu4NPF6. The intense, sharp absence and presence of 6 X peak at 495 nm and the weak, broad one at 740 nm are attributed to Py;. They are rather long-lived in the presence of the salt as shown by the spectrum taken 2 ps after the pulse. On the contrary, the fast decay in the absence of the salt is demonstrated by the spectrum taken at 800 ns, where only small absorption peaks remain. Another striking feature observed in the presence of the salt is a slow rise in the absorption in the range from 520 to 600 nm, which attains a maximum at 2 ps and then slowly decays. The formation of species having an absorption band in this wavelength region has already been studied for Py and trans-stilbene in THF solutions irradiated in the presence of LiA1H4.6*12 They have been proposed as the stable products, (Ar.AlH,.THF)-. Although the composition of the species produced in the present system is not clear, the lifetime of Py-. might be correlated with the formation since it was similarly observed with the other salts, Bu4NBF4, Bu4NI, CeMe3NPF6,BzMe3NPF6,and PhMe3NPF6,whose addition extends the lifetime of Py-. as described below. Contrary to the case of BP., the decay of Py-. was retarded by the addition of BzMe3NPF6and PhMe3NPF6as well as by the addition of the tetraalkylammonium salts Bu4NPF6,Bu4NBF4, Bu,NI, and CeMe3NPF6. The decays in the presence of BzMe3NPF6and PhMeJW", were fast compared with those in the presence of the tetraalkylammonium salts, as illustrated in Figure 6, and obeyed neither first- nor second-order kinetics. These results suggest that Py-. reacts with BzMe3N+and PhMe3N+as well as BP--but with much smaller rates than BY. probably because of the higher electron affinity of Py. That is to say, the

The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2371

Pulse Radiolysis of BP and Py in T H F

TABLE I: Effects of BulNPFr on the Relative Yields and Decay Rates of Radical Anions concn/M (OD)/€(rel) [BPI PYl [Bu4NPF61 OD 1 none none 0.46b 6 X loF2 1.1 none 6X 0.49b 6X 0.84 none 1.54c none 6 X lo-' 0.95 6X 6X 1.74' none 0.35 2 x 10-3 none 0.65d 6 X lo-' 0.86 2 x 10-3 6X 1.5gd 6X

(slope)

slope," s-l

M-1

4.9 x 107 2.3 X lo6 1.6 x 107 1.3 x 105 1.5 x 107 1.4 x 105

cm-l

6.1 X 2.9 X 8.0 x 6.5 x 7.5 x 7.0 x

X

6,

s-I

10" 1Olo 10" 109 10" 109

'The slope of the second-order kinetic plot for the decay of BP-- or Py-.. bAt 630 nm at the end of the pulse. CAt495 nm at the end of the pulse. dThe maximum value at 495 nm attained after the pulse.

-

-

C A d d i t i v e l ' 8x10-3H A . none

'. s

1.6-

1.20

0

-

0.8-

0

6

4

2

Time I

ps

Figure 6. Effects of salts on the decay of Py-. in 6 X

1 0

M Py solution.

decay of Py-. is accelerated in the presence of BzMe3N+ and PhMe3N+,although the net decay rate is reduced by the addition of BzMe3NPF6and PhMe3NPF6. Therefore, the retardation of the decay of Py-. by the addition of these salts can be attributed primarily to the anions of the salts, PF;, BFc, and I-, which form ion pairs with THF(H+) resulting in the retardation of the neutralization reaction Py-.

+ THF(H+)

+

Py(H)

+ THF

(6)

Pulse Radiolysis of BP-Py Solutions. The pulse radiolysis of M BP solutions containing 1 X 10-3-6 X M Py was 6X carried out, and the effect of added Bu4NPF6 on the electron transfer reaction from the initially formed BP-. to Py was investigated.

(7) The transient absorption spectra a t the end of the pulse were similar to those for the BP solutions not containing Py, and the formation of Py-- was observed after the pulse. Figure 7 shows the rise of absorption at 495 nm at Py concentrations of 1 X M and in the absence and presence of 6 X and 6 X M Bu4NPF6 for each solution. In the presence of the salt the maximum optical densities attained after the pulse at different Py concentrations are similar. But in the absence of the salt a higher optical density is attained at higher Py concentrations. Furthermore, in the presence of the salt the rate of formation of the 495-nm band was consistent with the decay rate of the 630-nm band, whereas in the absence of the salt the latter was faster than the former. These results indicate that reaction 5 competes with reaction 7 in the absence of the salt but not in the presence of the salt. The retardation of the decay of Py-. by the addition of the salt is also demonstrated in Figure 7. A slow rise in the absorption in the 520-600-nm region was also observed in BP-Py solutions containing Bu4NPF6. The behaviors of the rise and decay of absorption were independent of Py concentration. Kinetic Analysis of the Electron Transfer Reaction from B P . to Py in the Presence of Bu4NPF6. The formation of Py-- in the

I

I

200

1

I 400 Time

1

0

I

I

200

I

I 400

1

/ ns

Figure 7. Effect of Bu,NPF, on the formation and decay of Py-. in BP-Py solutions: [BPI, 6 X loF2M; [Py], (I) 1 X M and (11) 6 X

10-3 M. BP-Py solutions containing Bu4NPF6can be analyzed according to first-order kinetics because reaction 7 is much faster than reactions 5 and 6. The kinetic analysis was carried out with the results obtained by using a 3-ns electron pulse. The first-order kinetic plots for the rise of absorption at 495 nm gave straight M. lines in the Py concentration range from 1 X to 6 X The slopes of the plots, the pseudo-first-order rate constants, were then plotted against the Py concentration. From the linear plot the second-order rate constant, k7,in the presence of the salt was determined to be 9.0 X lo9 M-' s-I. The same rate constant was determined from the decay rate of the absorption at 630 nm. The rate constants of the electron transfer reactions from the free and Na+-paired BP-- to Py in THF have previously been determined by flash photolysis methods.I4J6 The reported values are 4.8 X 1Olo and 6 X lo9 M-] s-' a t ca. 20 OC for the free and Na+-paired BP., respectively. The value obtained in the presence of Bu4NPF6 is close to the reported value for the Na+-paired BP-. rather than to that for the free BP-.. Unfortunately, kinetic analysis of the formation of Py-. in the absence of the salt is impossible because of the competition of reactions 5 and 6. However, as seen in Figure 7, the rise in the absorption in the absence of the salt appears to be faster, or at least not slower, than that in the presence of the salt despite the simultaneous occurrence of reactions 5 and 6. Thus, it is quite likely that the reactivity of BP-. in the presence of Bu4NPF6is lower than that in the absence of the salt. This may be attributed to ion-pair formation with Bu4N+. Consequently, the tetraalkylammonium cations may contribute to the retardation of reaction 5 as well as the anions of the salts. On the other hand, no evidence was obtained for the reduction in the reactivity of Py-. in the presence of tetraalkylammonium cations, and their contribution to the retardation of reaction 6 should be

2372 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 Time / ps

0 I

2 I

200

4 I

I

400

I

0

4

2

0

I

200

I

I

400

Time I n s

Figure 8. Second-order kinetic plots for the decay of Py-. in (I) 6 X lo-* M M Py solution and (11) 6 X M BP solution containing 2 X

PY. negligibly small compared with that of the anions of the salts as described above. This might be explained in terms of the larger dissociation constant of the larger ions according to Bjerrum theory.25 Kinetic Analysis of the Decay of Py-.Retarded by the Addition of Tetraalkylammonium Salts. Figure 8 shows the second-order kinetic plots for the decay of Py-- monitored at 495 nm in the Py and BP-Py solutions and in the absence and presence of Bu4NPF6 for each solution. The slopes of the plots for the BP-Py solutions, which were independent of Py concentration in the range from 1X to 6 X lod3M, are similar to those for Py solutions in both the absence and presence of the salt, although the linearity of the plots for the former solutions is better. Comparison of the slopes shows that the decay rate in the presence of the salt is two orders of magnitude smaller than that in the absence of the salt. The second-order kinetic plots for the Py solutions containing Bu4NBF4,Bu4NI, and CeMe,NPF6 also gave straight lines. The slopes at high salt concentrations above ca. 3 X lom3M were constant and similar to that for solutions containing Bu4NPF6. (25) See, for example, Bockris, J. OM.; Reddy, A. K. N. "Modern Electrochemistry";Plenum Press: New York, 1970; Vol. 1, pp 257-60.

Yamamoto et al. Rate Constant of Neutralization Reactions of B P - and Py-.. The results of the pulse radiolysis of the BP, Py, and BP-Py solutions obtained in the same experimental run are listed in Table I. The optical densities divided by e and the slopes of the second-order kinetic plots multiplied by e are presented as the relative yields and decay rates of the radical anions. The values of q,30(BP--) and ~ ~ ~ ~ ( Pused y - s are ) 1.25 X lo4 and 5.0 X lo4 M-' cm-I, respectively, according to the l i t e r a t ~ r e . * ~The * ~ ~yields of BP-- and Py-- are approximately similar except for the yield of Py-. in the salt-free BP-Py solution, where reactions 5 and 6 compete with reaction 7. The slight increase in the yields by the addition of Bu4NPF6to the BP and Py solutions may be attributed to the retardation of the decay within the pulse duration. The decay rates of BP-. and Py-. in the absence of the salt are similar and are reduced by the addition of the salt by factors of 21 and 120, respectively. The pulse radiolysis of the BP solution containing Bu4NPF6 was examined in irradiation cells of 1- and 2-mm optical path lengths in order to determine the absolute rate constant of reaction 5. The second-order kinetic plots for the decay of B P . monitored at 630 nm gave straight lines. The slopes of the plots which correspond to k5/elwere 9.6 X lo6 and 4.9 X lo6 s-l for experiments with 1- and 2-mm cells, respectively. This demonstrates that the slope is inversely proportional to the optical path length and that the absolute rate constant determined from the slopes is valid. Th,e pulse radiolysis of the salt-free BP solution was also carried out in the 1-mm cell. The values of k5 determined in the absence and presence of Bu4NPF6are (2.6 f 0.4) X 10" and (1.2 f 0.2) X 1O'O M-' s-l, respectively. Consequently, the value of k6 for the salt-free solution is also ca. 3 X 10" M-' s-', which is smaller than the reported value, 1 X 10l2 M-' s-1.21 A kinetic analysis of the decay of charged species will be undertaken for additional systems. Registry No. BP, 92-52-4; Py, 129-00-0; THF, 109-99-9; THF(H+), 27659-93-4; BU4NPF6, 3109-63-5; Bu~NBF~, 429-42-5; Bu~NI,31 1-28-4; BzMe3NPF6,6427-70-9; PhMe3NPF6,2932-48-1; BP-, 34509-93-8; Py-., 34512-41-9; PFC, 16919-18-9; BF4-, 14874-70-5; I-, 20461-54-5; Bu~N", 10549-76-5; BzMe3N+,14800-24-9; PhMe3Nt, 3426-74-2. (26) Gill, D.; Jagur-Grodzinski, J.; Szwarc, M. Trans. Faraday SOC.1964, 60, 1424-31. (27) Jagur-Grodzinski, J.; Feld, M.; Yang, S. L.; Szwarc, M. J . Phys. Chem. 1965, 69, 628-35.