Aromatic Disulfide Radical Anions
A R T I C L E S
substituted phenylthiyl radicals 3 (eq 6) were determined by digital
simulation of the experimental cyclic voltammetric curves for oxidation
of the corresponding thiophenoxide ions 4. The anions were generated
by electroreduction of the parent disulfide and also chemically, by
reaction of the pertinent thiophenol with tetrabutylammonium hydrox-
ide. The results obtained by the two methods agreed with each other.
The rate constants for the homogeneous reaction between series of
aromatic radical anion donors and 1a-d,f-h were measured by
different electrochemical techniques. Whereas cyclic voltammetry (CV)
and linear scan voltammetry (LSV) were the electrochemical techniques
of choice for the reactions having rate constants (khom) larger than 1
M-1 s-1, we employed a potentiostatic technique for the study of the
reaction mechanism. We found that the energies and character
of the lowest unoccupied molecular orbitals (LUMOs) and of
the SOMOs change significantly along the series. Electron-
donating and mildly electron-withdrawing substituents make
both the LUMO and the SOMO display a pronounced σ*
character. On the other hand, with more powerful electron-
withdrawing substituents, while the LUMO is a π* orbital, it
becomes a σ*-type SOMO because of the large S-S bond
elongation. Eventually, with the nitro substituent, the energy
of the π* LUMO is such that the SOMO also is a π* orbital.
Nevertheless, upon S-S bond elongation, a new SOMO,
displaying the same features observed with the other substitu-
ents, was characterized. Therefore, independently of the disul-
fide, the actual S-S bond fragmentation involves a σ*-type
radical anion and occurs through a thermally activated ender-
gonic reaction.
reactions having khom ) 10-3-10 M-1 s-1 17
. To attain the situation in
which the reaction kinetics was controlled solely by the forward ET
from the radical anion to the disulfide (eq 4, in which “e” symbolizes
the reductant), the experiments were usually carried out at low mediator
concentrations (1-2 mM). For the radical anion of 1h, however, the
cleavage in eq 5 was so slow that mediator concentrations as low as
0.2-0.5 mM had to be employed, leading to a higher uncertainty in
the determination of khom. Moreover, when the potentiostatic method
was used in the study of the slow reactions, methyl p-toluenesulfonate
was added to the solution in order to trap in a fast SN2 reaction the
thiophenoxides formed in steps 5 and 6. Thereby, any influence on the
reaction kinetics from the reverse reaction of eq 5 and accordingly from
the fast and usually diffusion-controlled backward ET involving 2 and
the mediator could be neglected. In this respect, the rate measurements
carried out by the CV and LSV techniques are less influenced because
the reaction between 2 and the mediator is less exergonic and thus
generally slower.
Computational Details. The calculations were performed at the MO
ab initio level with the Gaussian 98 series of programs18 run on a Silicon
Graphics 4CPU MIPS R10000, SGI-ORIGIN 2000/16, and CRAY T3D
MCA 128-8 supercomputer. The molecules 1 and anions 4 were studied
at the HF/6-311G*//HF/6-311G* level, while the corresponding unre-
stricted UHF scheme was employed for the open-shell species 2 and
3. Stationary points were located through full geometry relaxation. The
single-point frozen core (f.c.) MP2 energies also were obtained. For
the open-shell systems, the spin projection operator19 was applied to
remove contamination from higher spin states. For several of the radicals
examined, however, the value of s2 was greater than 0.75. The solvent
effect (having ꢀ ) 37) on the total molecular energy was estimated by
using the Self-Consistent Reaction Field (SCRF) facility employing
the polarized continuum model (PCM) performed with the Onsager
reaction field theory and implemented in the Gaussian 98 package. The
energy profiles were constructed as Morse-like potential energy
functions of the S-S bond distance by employing the second derivative
of the molecular energy corresponding to the relaxed molecular
structure, with respect to the S-S bond length coordinate, following a
procedure reported previously.20 While the energy profiles were obtained
by employing the energy of the critical points at the MP2/6-311G*//
HF/6-311G* level, the exponential â factors were calculated at the
HF/6-311G*//HF/6-311G* level (the unrestricted UHF scheme was
used for the radical anions).
Experimental Section
Chemicals. N,N-Dimethylformamide (Acros Organics, 99%) was
treated for some days with anhydrous Na2CO3, under stirring, and then
distilled at reduced pressure (17 mmHg) under a nitrogen atmosphere.
Tetrabutylammonium perchlorate (Fluka, 99%) was recrystallized from
a 2:1 ethanol-water solution. Tetrabutylammonium tetrafluoroborate
was prepared by reacting sodium tetrafluoroborate with tetrabutylam-
monium hydrogensulfate in water. It was recrystallized from dichlo-
romethane. The syntheses of the disulfides were carried out according
to the procedure outlined in ref 14. The purity was confirmed by means
of 1H NMR and GC-MS. The mediators were commercially available
or prepared as described elsewhere.15
Electrochemical Apparatus. The electrochemical measurements
were conducted in all glass cells. For direct electrochemistry (cyclic
voltammetry and convolution analysis), an EG&G-PARC 173/179
potentiostat-digital coulometer, an EG&G-PARC 175 universal pro-
grammer, and a Nicolet 3091 12-bit resolution digital oscilloscope were
used. To minimize the electrical noise, the experiments were carried
out inside a double-wall copper Faraday cage, using the various
precautions described in detail previously.16 For the redox catalysis
measurements, the experimental setup and procedures were as previ-
ously described.17 For both types of experiment, the feedback correction
was applied to minimize the ohmic drop between the working and the
reference electrodes.
The electrochemical experiments were carried out in DMF containing
0.1 M Bu4NClO4 (TBAP), direct reduction, or 0.1 M Bu4NBF4 (TBAT),
mediated reduction, using a glassy-carbon working electrode and a
platinum plate as the counter electrode. A platinum electrode was
employed for the oxidation studies. The reference electrode (either Ag/
AgCl or Ag/AgI) was calibrated after each experiment against the
ferrocenium/ferrocene couple. All potentials were then converted to
the KCl saturated calomel electrode (SCE). The reference and the
counter electrodes were separated from the catholyte by glass frits and
Tylose-TBAP-saturated bridges.
Electrochemical Procedures. For the determination of the hetero-
geneous parameters, the digitalized, background-subtracted curves were
analyzed by using a homemade voltammetry-convolution software and
compared with the corresponding digital simulations. The DigiSim 3.03
package was used for the simulations, using a step size of 1 mV and
an exponential expansion factor of 0.5. The standard potentials of the
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M.; Farkas, C. O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P.; Cui, Y. Q.; Morokuma, K. D.; Malick, K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, L. R.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(14) (a) Bogert, M. T.; Stull, A. Organic Synthesis; New York, 1932; Vol. I.
(b) Gundermann, K.-D.; Hu¨mke, K. Houben-Weyl; Klamann, D., Ed.; Georg
Thieme Verlag: Stuttgart, 1985; p 129.
(15) Occhialini, D.; Kristensen, J. S.; Daasbjerg, K.; Lund, H. Acta Chem. Scand.
1991, 46, 474.
(16) Antonello, S.; Musumeci, M.; Wayner, D. D. M.; Maran, F. J. Am. Chem.
Soc. 1997, 119, 9541.
(19) (a) Schlegel, H. B. J. Chem. Phys. 1986, 84, 4530. (b) Schlegel, H. B. J.
Phys. Chem. 1988, 92, 3075. (c) Sosa, C.; Schlegel, H. B. Int. J. Quantum
Chem. 1986, 30, 55.
(17) Daasbjerg, K.; Pedersen, S. U.; Lund, H. Acta Chem. Scand. 1991, 45,
424.
(20) Benassi, R.; Taddei, F. J. Phys. Chem. A 1998, 102, 6173.
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