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Kinetics of electron-transfer and in tetrahydrofuran
Volume 9, number 4
KINETICS AND
15 May 1971
CHEMICAL PHYSICS LETTERS
OF
B’ , Na+
ELECTRON-TRANSFER
+ n + B + D; , Na+
M. FISHER, Go RAMME, Institute
B:
+ IT -
B + v-
IN TETRAHYDROFURAN
S. CLAESSON and M. SZWARC
of Physical Chemihy, Uppsala Unium-sity, P. 0.23, 532, 75121 UppsaLa 1, Su~cdcm Received 8 March 1971
Photoejection of clcctrons from pyrenideLx7. in the presence of excess of bghcnyt yields biphenylide, B’, nnd the rate constants of the renctions B’ + 7i -B + Z7 and B’, Nn+ + li - 51’) Nnt’ + B were determined.
Obviously,
1. INTRODUCTION
electron
photoejection
takes place on flash and the ejected Photoejection of electrons from solutions of radical-ions was first observed by Hoytink et al. [l-3] in rigid media and by Eloranta and
Linschitz [4] and more recently by Giling et al. [5] in liquid phase. We have studied now a liquid system in which an electron is photoejected from a radical-ion Al: in the presence of a large excess of a suit,able acceptor A2 having a substantially lower electron affinity than Al. The electron capture process is indiscriminate and therefore the electron is initially captured by A2 forming the AZ_’ radical-ion. However, if the equi1ibri.m A2’ + AI = A2 + Ai7 favors the right side, A2 should rapidly transfer the electron back to Al. The system sodium pyrenide, 6, mixed with a 30 to 300-fold excess of biphenyl, B, fulfils the required conditions. The electron affinity of pyrene is higher than that of biphenyl [6] by about 0.5 eV, and hence such a solution contains no measurable quantities of biphenylide. Flash photolysis of such a mixture in THF by light of a wavelength longer than 420 nm leads to a perfectly reversible reaction. Transient spectrum of biphenylide B’ appears after each flash while that of the pyrenide is bleached. Nevertheless, the original spectrum is restored after a few hundred microseconds. These changes are depicted in fig. 1. Even after 25 consecutive flashes, each causing initially about ‘IO% of r7 bleaching (determined from the decrease in the absorption at 493 nm), the original spectrum of the solution eventually reappears *. * Exclusion reversibility
306
of UV light is imperative
to ascertain
and roproducibiiity of the process.
the
from
d
electron,
, in
whatever form, is captured by the excess of biphenyl in a time much shorter than our resolu-
tion time (-10 I.rsec). The kinetics of the subsequent electron-transfer reaction may be followed by analyzing the changes in light transmittance at 493 nm (X,;Lx of 6) or at 400 nm and 630 nm @max ‘s of 3 ), see, e.g., fig. 2. 1.0 0.0.
n
.a -
Fig. 1. Visible spectrum showing the formation of transient nbsorption due to BY following rho flash photolysis of n 3 x 10-T M solution of n” in THF: tbc spectrum before flashing or at d > 1000 psec: .-.- the spectrum 50 psec after flaeh; --- the spectrum 100 ,usec nfter flash.
Volume 9, number 4
CHEMICAL PHYSICS LETTERS
15 hlny 1971
began 50 wee after the flash, the association equilibrium was established at that time. Hence, in these experiments the ratio [BTJ/[By, Na+] was 3-4, and therefore the observed eIectron return involved mainly the free B7 ions, the contribution of B‘, Naf pairs to the process being small. Similar calculations prove that in the second series of experiments the electron return involved more titan 90% of B:, Nat ion-pairs, because the concentration af the free Na+ ions, formed by the dissociation of Na+, BPhi, was
Pig.
2. Oscilloscope
Lrnccs
obtained
by monitoring
the
light tronsmittnncc nt 493 nm nnd 400 runfollowing II flash.
Tim&ax
Two distinct system,
200
processes
/Lxx
per division.
may occur in this
viz.,
k2 + B - B+d,Na+.
(2)
Reaction of free ions may be observed in sufficiently dilute solutions, whereas that of the ionpairs should predominate in more concentrated solutions, especially in the presence of readily dissociated sodium salts, e.g., Na+, BPhi. Consequently, two series of experiments were performed, one in a 60 cm long quartz optical cell containing a solution of 2.4-3.9 X 10-7 M of d, Na+, 2.1-2.9 x 10-7 M of 51with about 300 fold excess of B; the other in a 10 cm cell filled with about 2 x 10-6 M solution of ?-, Na+ mixed with an approximately equivalent amount of 71, ~30 fold excess of B and a 10 fold excess of Na+, BPh& The dissociation constants of n:, Naf and By, Na+ in THF are 7 x 10m6 M and 1 x 10e6 M, respectively [7]. Hence, at least 95% of s, Na+ is dissociated into free ions when its concentration is within 2.4-3.9 x 10-7 M range. Under these conditions the photoejection takes place from the free d ions. The relaxation time of the association equilibrium, Bs + Na+ =: B’, Na+, was calculated to be less than 30 psec if the rate constant of the ion association is lOI ~-1 set-1 (certainly its lower limit). Since of the kinetics of the reaction,
the observation
(see e.g. fig. l), the relevant
extinction
coeffi-
known r6] (c49&Y_) = 4.9 X 104, E40&) = 4.0 x 104). The kinetic results conform with the bimol$_cular law involving T;and BT (in 60 cm cell) or B’ , Naf (in 10 cm cell in the presence of a large excess of Na’, BPh$ as the respective substrates. The bimolecuIar rate constants derived from the first series of experiments were: 2.0 K 1010 M-l set-1 from 10 runs at 493 nm, 2.1 ,: lll10 ~-1 set-1 from 5 runs at 400 nm, and 2.4 x 1010 M-l set-1 from 7 runs at 630 nm. The scatter of the results did not exceed 20%, the determination of the initial concentration of I; being the largest source of the experimental error. Thus, &I is at least 2.0 x 1019 M-1 set-L and the correction accounting for the presence of I;~, Naf pairs slightly raises this value but not above 2.7 x 1010 ~-1 see-l. The results of the second series of experiments led to rate constants of 0.86 x 1010 ~-1 see-1 for 4 runs performed at 493 nrn, 0.82 x 1010 M-1 set-1 for 3 runs at 400 nm, and 0.86 x lOlo ~-1 set-1 for 2 runs at 630 nm. Hence, kz = 0.7 X lOLo M-l see-l (takirg into account the contribution of 5 - 10% of the free ions). cients
and B:,Na+
about 1.6 x 10-5 M. The conversion of d into B’, or vice versa, is stoichiometric. This was established by comparing A(o.d. 493), A(o.d. 400) and A(o.d. 630) being
2. DISCUSSION The kinetics of electron transfer, B: + 6, in isopropanol, was investigated by Arai et al. [8] who utilized pulse-radio1 sis techni ue. For that medium a vaiue of 5 X LO% M-l set- 9 was reported for RI. The rate of electron-transfer should depend on the medium, and indeed, the relation between the appropriate rate constants and Marcus’ A parameters has been demonstrated 307
Volume 9, number 4
CHEMICAL PHYSICS LETTERS
[9,10]. On this basis, one expects to find a higher value of R1 in THF than in isopropanol, and that reported by us, viz. 2-3 x 1019 ~-1 see-1, seems to be plausible. The exothermic reaction g + D 4 B + d is expected to be faster than a similar tIzermoneutrat reaction, e.g., the exchange of naphthalenide ions with naphthalene, if both are performed in the same solvent (THF). This is indeed the case, since the data of Chang and Johnson [ll] indicate that the latter reaction is about 8 times slower than (1). Electron-transfers, studied by ESR technique [9,11,12] demonstrate that, depending on the nature of the solvent. the rate of exchange between free naphthalenide ions (NT) and naphthalene is faster, sometimes by two orders of magnitude, than the exch;inge involving the corresponding ion-pairs, NT, Na+ + N - exchange. We found, however, that the electron transfer BT + ITinvolving free ions (1) is faster, but only by a factor of 3-4 than the corresp:;nding reaction of ion-pairs (2). Since we investigated an exothermic reaction (BT + B 4 B + IT;, AIf x -12 kcal/mole) while the exchanges studied by ESR are thermoneutral, the smaller difference between the rates of free ions and ion-pairs observed by us may be fully justified,
ACKNOWLEDGEMENT : The financial support of this investigation by the Swedish National Science Research Council and the National Science Foundation (U.S.A.) is gratefully acknowledged. REFERENCES [l] G. J. Hoytink and P. J. Zondstra. Mol. Phya. 3 (1960) 371. [Z] J. D. W-van Voorst and G. J.Hoytink, J. Chom. Phys. 42 (1965) 3995. _ .~J. Chern. .131 * J.D. W-van Voorst and G. J. Hovtink. Phys. 45 (196G) 3918. I41 - _ J. Elorants and II. Lonschitz. J. Chom. Phys. 38 (1963) 2214. [5] L. J. Gfling. J. G. Kloosterboer. R. I?. H. Rottschnick and J.D..W..van Voorst. Chem. Phys.Letters 8
(1971) 457. [6] J. Jagur-Grodzinski,
[7] [S] [9] [lo] [ll] [12]
308
15 May 1971
M. Fcld, S. L. Yang and 111. Sxwnrc, J. Phys. Chem. 69 (1965) 628. P. Chang, R.V. Slates and M. Szwarc, J.Phys. Chem. TO (1966) 3180. S.hrai, D.A.Drev and L. M. Dorfman. J. Chem. Phys. 46 (1967) 2572. K.Hafclmann, J. Jagur-Grodzinski and M. Szwarc J.Am. Chem. Sot. 91 (1969) 4645. J. R. Brandon and L.M. Dorfman, J. Chcm. Phys. 53 (1970) 3849. R. Chang and C. S. Johnson, J.Am. Chem. Sot. 88 (1966) 2338. P. J. Znndstrn and S. I. Weissmnn, J.Am. Chem. Sot. 84 (1962) 4408.
KINETICS AND
15 May 1971
CHEMICAL PHYSICS LETTERS
OF
B’ , Na+
ELECTRON-TRANSFER
+ n + B + D; , Na+
M. FISHER, Go RAMME, Institute
B:
+ IT -
B + v-
IN TETRAHYDROFURAN
S. CLAESSON and M. SZWARC
of Physical Chemihy, Uppsala Unium-sity, P. 0.23, 532, 75121 UppsaLa 1, Su~cdcm Received 8 March 1971
Photoejection of clcctrons from pyrenideLx7. in the presence of excess of bghcnyt yields biphenylide, B’, nnd the rate constants of the renctions B’ + 7i -B + Z7 and B’, Nn+ + li - 51’) Nnt’ + B were determined.
Obviously,
1. INTRODUCTION
electron
photoejection
takes place on flash and the ejected Photoejection of electrons from solutions of radical-ions was first observed by Hoytink et al. [l-3] in rigid media and by Eloranta and
Linschitz [4] and more recently by Giling et al. [5] in liquid phase. We have studied now a liquid system in which an electron is photoejected from a radical-ion Al: in the presence of a large excess of a suit,able acceptor A2 having a substantially lower electron affinity than Al. The electron capture process is indiscriminate and therefore the electron is initially captured by A2 forming the AZ_’ radical-ion. However, if the equi1ibri.m A2’ + AI = A2 + Ai7 favors the right side, A2 should rapidly transfer the electron back to Al. The system sodium pyrenide, 6, mixed with a 30 to 300-fold excess of biphenyl, B, fulfils the required conditions. The electron affinity of pyrene is higher than that of biphenyl [6] by about 0.5 eV, and hence such a solution contains no measurable quantities of biphenylide. Flash photolysis of such a mixture in THF by light of a wavelength longer than 420 nm leads to a perfectly reversible reaction. Transient spectrum of biphenylide B’ appears after each flash while that of the pyrenide is bleached. Nevertheless, the original spectrum is restored after a few hundred microseconds. These changes are depicted in fig. 1. Even after 25 consecutive flashes, each causing initially about ‘IO% of r7 bleaching (determined from the decrease in the absorption at 493 nm), the original spectrum of the solution eventually reappears *. * Exclusion reversibility
306
of UV light is imperative
to ascertain
and roproducibiiity of the process.
the
from
d
electron,
, in
whatever form, is captured by the excess of biphenyl in a time much shorter than our resolu-
tion time (-10 I.rsec). The kinetics of the subsequent electron-transfer reaction may be followed by analyzing the changes in light transmittance at 493 nm (X,;Lx of 6) or at 400 nm and 630 nm @max ‘s of 3 ), see, e.g., fig. 2. 1.0 0.0.
n
.a -
Fig. 1. Visible spectrum showing the formation of transient nbsorption due to BY following rho flash photolysis of n 3 x 10-T M solution of n” in THF: tbc spectrum before flashing or at d > 1000 psec: .-.- the spectrum 50 psec after flaeh; --- the spectrum 100 ,usec nfter flash.
Volume 9, number 4
CHEMICAL PHYSICS LETTERS
15 hlny 1971
began 50 wee after the flash, the association equilibrium was established at that time. Hence, in these experiments the ratio [BTJ/[By, Na+] was 3-4, and therefore the observed eIectron return involved mainly the free B7 ions, the contribution of B‘, Naf pairs to the process being small. Similar calculations prove that in the second series of experiments the electron return involved more titan 90% of B:, Nat ion-pairs, because the concentration af the free Na+ ions, formed by the dissociation of Na+, BPhi, was
Pig.
2. Oscilloscope
Lrnccs
obtained
by monitoring
the
light tronsmittnncc nt 493 nm nnd 400 runfollowing II flash.
Tim&ax
Two distinct system,
200
processes
/Lxx
per division.
may occur in this
viz.,
k2 + B - B+d,Na+.
(2)
Reaction of free ions may be observed in sufficiently dilute solutions, whereas that of the ionpairs should predominate in more concentrated solutions, especially in the presence of readily dissociated sodium salts, e.g., Na+, BPhi. Consequently, two series of experiments were performed, one in a 60 cm long quartz optical cell containing a solution of 2.4-3.9 X 10-7 M of d, Na+, 2.1-2.9 x 10-7 M of 51with about 300 fold excess of B; the other in a 10 cm cell filled with about 2 x 10-6 M solution of ?-, Na+ mixed with an approximately equivalent amount of 71, ~30 fold excess of B and a 10 fold excess of Na+, BPh& The dissociation constants of n:, Naf and By, Na+ in THF are 7 x 10m6 M and 1 x 10e6 M, respectively [7]. Hence, at least 95% of s, Na+ is dissociated into free ions when its concentration is within 2.4-3.9 x 10-7 M range. Under these conditions the photoejection takes place from the free d ions. The relaxation time of the association equilibrium, Bs + Na+ =: B’, Na+, was calculated to be less than 30 psec if the rate constant of the ion association is lOI ~-1 set-1 (certainly its lower limit). Since of the kinetics of the reaction,
the observation
(see e.g. fig. l), the relevant
extinction
coeffi-
known r6] (c49&Y_) = 4.9 X 104, E40&) = 4.0 x 104). The kinetic results conform with the bimol$_cular law involving T;and BT (in 60 cm cell) or B’ , Naf (in 10 cm cell in the presence of a large excess of Na’, BPh$ as the respective substrates. The bimolecuIar rate constants derived from the first series of experiments were: 2.0 K 1010 M-l set-1 from 10 runs at 493 nm, 2.1 ,: lll10 ~-1 set-1 from 5 runs at 400 nm, and 2.4 x 1010 M-l set-1 from 7 runs at 630 nm. The scatter of the results did not exceed 20%, the determination of the initial concentration of I; being the largest source of the experimental error. Thus, &I is at least 2.0 x 1019 M-1 set-L and the correction accounting for the presence of I;~, Naf pairs slightly raises this value but not above 2.7 x 1010 ~-1 see-l. The results of the second series of experiments led to rate constants of 0.86 x 1010 ~-1 see-1 for 4 runs performed at 493 nrn, 0.82 x 1010 M-1 set-1 for 3 runs at 400 nm, and 0.86 x lOlo ~-1 set-1 for 2 runs at 630 nm. Hence, kz = 0.7 X lOLo M-l see-l (takirg into account the contribution of 5 - 10% of the free ions). cients
and B:,Na+
about 1.6 x 10-5 M. The conversion of d into B’, or vice versa, is stoichiometric. This was established by comparing A(o.d. 493), A(o.d. 400) and A(o.d. 630) being
2. DISCUSSION The kinetics of electron transfer, B: + 6, in isopropanol, was investigated by Arai et al. [8] who utilized pulse-radio1 sis techni ue. For that medium a vaiue of 5 X LO% M-l set- 9 was reported for RI. The rate of electron-transfer should depend on the medium, and indeed, the relation between the appropriate rate constants and Marcus’ A parameters has been demonstrated 307
Volume 9, number 4
CHEMICAL PHYSICS LETTERS
[9,10]. On this basis, one expects to find a higher value of R1 in THF than in isopropanol, and that reported by us, viz. 2-3 x 1019 ~-1 see-1, seems to be plausible. The exothermic reaction g + D 4 B + d is expected to be faster than a similar tIzermoneutrat reaction, e.g., the exchange of naphthalenide ions with naphthalene, if both are performed in the same solvent (THF). This is indeed the case, since the data of Chang and Johnson [ll] indicate that the latter reaction is about 8 times slower than (1). Electron-transfers, studied by ESR technique [9,11,12] demonstrate that, depending on the nature of the solvent. the rate of exchange between free naphthalenide ions (NT) and naphthalene is faster, sometimes by two orders of magnitude, than the exch;inge involving the corresponding ion-pairs, NT, Na+ + N - exchange. We found, however, that the electron transfer BT + ITinvolving free ions (1) is faster, but only by a factor of 3-4 than the corresp:;nding reaction of ion-pairs (2). Since we investigated an exothermic reaction (BT + B 4 B + IT;, AIf x -12 kcal/mole) while the exchanges studied by ESR are thermoneutral, the smaller difference between the rates of free ions and ion-pairs observed by us may be fully justified,
ACKNOWLEDGEMENT : The financial support of this investigation by the Swedish National Science Research Council and the National Science Foundation (U.S.A.) is gratefully acknowledged. REFERENCES [l] G. J. Hoytink and P. J. Zondstra. Mol. Phya. 3 (1960) 371. [Z] J. D. W-van Voorst and G. J.Hoytink, J. Chom. Phys. 42 (1965) 3995. _ .~J. Chern. .131 * J.D. W-van Voorst and G. J. Hovtink. Phys. 45 (196G) 3918. I41 - _ J. Elorants and II. Lonschitz. J. Chom. Phys. 38 (1963) 2214. [5] L. J. Gfling. J. G. Kloosterboer. R. I?. H. Rottschnick and J.D..W..van Voorst. Chem. Phys.Letters 8
(1971) 457. [6] J. Jagur-Grodzinski,
[7] [S] [9] [lo] [ll] [12]
308
15 May 1971
M. Fcld, S. L. Yang and 111. Sxwnrc, J. Phys. Chem. 69 (1965) 628. P. Chang, R.V. Slates and M. Szwarc, J.Phys. Chem. TO (1966) 3180. S.hrai, D.A.Drev and L. M. Dorfman. J. Chem. Phys. 46 (1967) 2572. K.Hafclmann, J. Jagur-Grodzinski and M. Szwarc J.Am. Chem. Sot. 91 (1969) 4645. J. R. Brandon and L.M. Dorfman, J. Chcm. Phys. 53 (1970) 3849. R. Chang and C. S. Johnson, J.Am. Chem. Sot. 88 (1966) 2338. P. J. Znndstrn and S. I. Weissmnn, J.Am. Chem. Sot. 84 (1962) 4408.