Ϫ
Table 1 Observed rate constants kobs and diffusion corrected rates k for the DDQ/DDQ couple in different solvents at T = 293 K. η denotes the
et
2
solvent viscosity and τL the longitudinal solvent relaxation time. γ = (1/n Ϫ 1/ε ) is the solvent parameter according to the Marcus theory (Pekar
s
factor)
8
Ϫ1 Ϫ1
8
Ϫ1 Ϫ1
8
Ϫ1 Ϫ1 a
b
b
b
Solvent
k /10 M
s
kobs/10 M
s
kdiff/10 M
s
η/cP (293 K)
τL/ps
γ (293 K)
et
CH CN
Acetone
Benzonitrile
29.2 ± 0.3
15.5 ± 0.6
1.1 ± 0.05
48.5 ± 0.2
33.7 ± 1.5
25.3 ± 0.3
14.4 ± 0.6
8.9 ± 0.05
36.7 ± 0.2
26.0 ± 1.5
190
206
52.4
151
114
0.341
0.316
1.24
0.43
0.57
0.20
0.3
6.9
0.33
2.9
0.530
0.494
0.388
0.380
0.270
3
CH Cl2
2
CHCl3
a
b
From eqn. (10). For detailed references on solvents, see ref. (4).
2
3
4
5
6
7
8
9
G. Grampp and W. Jaenicke, Ber. Bunsenges. Phys. Chem., 1984, 88,
25.
G. Grampp and W. Jaenicke, J. Chem. Soc., Faraday Trans. 2, 1985,
1, 1035.
G. Grampp and W. Jaenicke, Ber. Bunsenges. Phys. Chem., 1991, 95,
04.
T. T.-T. Li and C. H. Brubaker Jr., J. Organomet. Chem., 1981, 216,
23.
T. T.-T. Li, M. J. Weaver and C. H. Brubaker Jr., J. Am. Chem. Soc.,
982, 104, 2381.
D. Jürgen, S. U. Pedersen, J. A. Pedersen and H. Lund, Acta Chem.
Scand., 1997, 51, 767.
R. Fuhlendorff, T. Lund, H. Lund and J. A. Pedersen, Tetrahedron
Lett., 1987, 28, 5335.
3
8
9
2
1
H. Larsen, S. U. Pedersen, J. A. Pedersen and H. Lund,
J. Electroanal. Chem., 1992, 331, 971.
1
0 M. J. Weaver and G. E. McManis III, Acc. Chem. Res., 1990, 23,
94.
2
Ϫ1/2
11 M. J. Weaver, Chem. Rev., 1992, 92, 436.
Fig. 3 Solvent dependence of ln ket (right side) and ln(k τ γ
)
et
L
2
12 H. Larsen, Y. A. Khan and G. Grampp, J. Chem. Soc., Perkin Trans.
(
left side) versus the solvent parameter γ = (1/n Ϫ 1/ε ).
s
2
, 1997, 2555.
1
3 R. A. Marcus and N. Sutin, Biochem. Biophys. Acta, 1985, 811, 265.
found for the homogeneous electron self-exchange reactions
14 R. A. Marcus, J. Chem. Phys., 1956, 24, 966.
15 R. A. Marcus, J. Chem. Phys., 1957, 26, 876.
Ϫ
Ϫ
ϩ
of the TCNE/TCNE
,
TCNQ/TCNQ and TTF/TTF
1
6 N. Sutin, Prog. Inorg. Chem., 1983, 30, 441.
17 S. J. Formosinho, L. G. Arnant and R. Fausto, Prog. React. Kinet.,
998, 23, 1.
8 S. F. Nelsen, Y. Kim and S. C. Blackstock, J. Am. Chem. Soc., 1989,
couples (TCNE = tetracyanoethylene, TCNQ = tetracyano-
4,34–36
quinodimethane, TTF = tetrathiafulvalene).
Also several
1
heterogeneous electrochemical electron transfer reactions are
1
37–41
reported showing a solvent dynamical effect.
Fig. 3 also shows the results for ln k and ln (k τ
et L
1
11, 2045.
Ϫ1/2
) versus
19 S. F. Nelsen, R. F. Ismagilov and D. A. Trieber II, Science, 1997, 278,
et
γ. A straight line confirms the influence of τ on the solvent
846.
L
2
0 G. Rauhut and T. Clark, J. Chem. Soc., Faraday Trans., 1994, 90,
dynamical effect. A rough calculation of the outer sphere
1
783.
reorganization energies using the continuum sphere model
2
1 G. Rauhut and T. Clark, J. Am. Chem. Soc., 1993, 115, 9127.
R. M. Nelsen, J. T. Hupp and D. I. Yoon, J. Am. Chem. Soc., 1995,
Ϫ1
approximation of eqn. (2), gave λ = 81.9 kJ mol
for
O
Ϫ1
acetonitrile and 41.7 kJ mol for chloroform, respectively.
1
17, 9085.
Ϫ1
Theoretical calculations of λ result in 42.1 kJ mol using the
22 S. F. Nelsen, in Advances in Electron Transfer Chemistry, ed. P. S.
Mariano, JAI Press Inc., London, 1991, vol. 3, pp. 167–189.
2
2
i
Ϫ1
42
PM3 procedure and 45.3 kJ mol from the AM1-method.
3 G. Grampp, Rev. Sci. Instrum., 1985, 56, 2050.
Temperature dependent measurements are in progress to get
more detailed information about the activation parameters of
the reaction.
4 M. Oyama, F. Marken, R. D. Webster, I. A. Cooper, R. G.
Compton and S. Okazki, J. Electroanal. Chem., 1998, 451, 193.
5 C. Corvaja, L. Pasimeni and M. Brustalon, Chem. Phys., 1976, 14,
2
1
77.
2
2
2
2
3
3
3
3
3
6 D. Gordon and M. J. Hove, J. Chem. Phys., 1973, 59, 3419.
Conclusions
7 B. Kirste, Anal. Chim. Acta, 1992, 265, 191.
Ϫ
8 G. Grampp and G. Stiegler, J. Magn. Reson., 1986, 70, 1.
9 G. Grampp, Spectrochim. Acta, Part A, 1998, 54, 2349.
0 R. W. Fawcett, Chem. Phys. Lett., 1992, 199, 153.
1 M. Bixon and I. Jortner, Chem. Phys., 1993, 176, 467.
2 H. Heitele, Angew. Chem., Int. Ed. Engl., 1993, 32, 359.
3 H. Sumi and R. A. Marcus, J. Chem. Phys., 1986, 84, 4894.
4 G. Grampp, W. Harrer and W. Jaenicke, J. Chem. Soc., Faraday
Trans. 1, 1987, 83, 161.
The DDQ/DDQ electron self-exchange couple clearly shows
a solvent dynamical effect in its solvent behaviour. This indi-
cates an adiabatic reaction behaviour.
Acknowledgements
K. R. would like to thank the Danish Government for a
scholarship. The financial support of the VW-Foundation,
Deutsche Forschungsgemeinschaft and the Austrian Academic
Exchange Service is gratefully acknowledged. The authors
would like to thank S. F. Nelsen for helpful discussions during
his guest professorship at Technical University Graz and for
providing theoretical calculations.
3
3
3
5 W. Harrer, G. Grampp and W. Jaenicke, Chem. Phys. Lett., 1984,
1
12, 263.
6 G. Grampp, W. Harrer and G. Hetz, Ber. Bunsenges. Phys. Chem.,
990, 94, 1343.
1
7 A. Kapturkiewicz and B. Behr, J. Electroanal. Chem., 1984, 163, 189.
38 M. Opallo and A. Kapturkiewicz, Electrochim. Acta, 1985, 30,
1301.
39 W. R. Fawcett, Langmuir, 1989, 5, 661.
40 W. R. Fawcett and M. Opallo, J. Phys. Chem., 1992, 96, 2920.
References
41 G. Grampp, A. Kapturkiewicz and W. Jaenicke, Ber. Bunsenges.
Phys. Chem., 1990, 94, 439.
1
G. Grampp, in Electron Paramagnetic Resonance, Eds. B. C. Gilbert,
N. M. Atherton and M. J. Davies, The Royal Society of Chemistry,
Cambridge, 1999, vol. 16, pp. 234–267.
4
2 S. F. Nelsen, personal communication.
Paper 9/03394G
J. Chem. Soc., Perkin Trans. 2, 1999, 1897–1899 1899