Kinetics of the decay reactions of the N,N-dimethyl-p-toluidine cation radical in acetonitrile. Acid-base interaction to promote the CH2-CH2 bonding

Article abstract of DOI:10.1021/jp026073h

The decay reaction of N,N-dimethyl-p-toluidine (DMT) cation radical (DMT?⁺) in acetonitrile (AN) was analyzed using an electron-transfer stopped-flow (ETSF) method. In the ETSF method, DMT?⁺ is generated by mixing AN solutions of DMT and tris(p-bromophenyl) amine cation radical (TBPA?⁺). When DMT?⁺ was generated quantitatively without DMT via 1:1 mixing of DMT and TBPA?⁺, it was found that DMT?⁺ was fairly stable in AN. On the other hand, when DMT remained with DMT?⁺ under the control of the mixing ratio of DMT/TBPA?⁺ (> 1), the neutral DMT was found to promote the decay reaction of DMT?⁺. From the determined rate law, -d[DMT?⁺]/dt = k [DMT?⁺] [DMT] (k = 6.5 × 10² M^-1 s^-1), the initial acid-base reaction between DMT?⁺ and DMT was clarified to be the rate determining step. The acid-base interaction was also confirmed by observing the decay reaction of DMT?⁺ in the presence of pyridine derivatives. The identical rate law, which indicates the rate-determining acid-base interaction, was obtained for eight pyridine derivatives examined, though the final product was different from the case of DMT. The ETSF method has permitted the straightforward analysis and given a definite kinetic conclusion concerning the acid-base reaction between DMT?⁺ and DMT.

Full text of DOI:10.1021/jp026073h

J. Phys. Chem. A 2002, 106, 8103-8108
8103
Kinetics of the Decay Reactions of the N,N-Dimethyl-p-Toluidine Cation Radical in
Acetonitrile. Acid-Base Interaction to Promote the CH2-CH2 Bonding
Masashi Goto, Hyun Park,† and Koji Otsuka
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Sakyo-ku, Kyoto 606-8501, Japan
Munetaka Oyama*
DiVision of Research InitiatiVes, International InnoVation Center, Kyoto UniVersity,
Sakyo-ku, Kyoto 606-8501, Japan
ReceiVed: May 7, 2002; In Final Form: June 26, 2002
The decay reaction of N,N-dimethyl-p-toluidine (DMT) cation radical (DMT•+) in acetonitrile (AN) was
analyzed using an electron-transfer stopped-flow (ETSF) method. In the ETSF method, DMT•+ is generated
by mixing AN solutions of DMT and tris(p-bromophenyl)amine cation radical (TBPA•+). When DMT•+ was
generated quantitatively without DMT via 1:1 mixing of DMT and TBPA•+, it was found that DMT•+ was
fairly stable in AN. On the other hand, when DMT remained with DMT•+ under the control of the mixing
ratio of DMT/TBPA•+ (> 1), the neutral DMT was found to promote the decay reaction of DMT•+. From
the determined rate law, -d[DMT•+]/dt ) k [DMT•+] [DMT] (k ) 6.5 × 102 M-1 s-1), the initial acid-base
reaction between DMT•+ and DMT was clarified to be the rate determining step. The acid-base interaction
was also confirmed by observing the decay reaction of DMT•+ in the presence of pyridine derivatives. The
identical rate law, which indicates the rate-determining acid-base interaction, was obtained for eight pyridine
derivatives examined, though the final product was different from the case of DMT. The ETSF method has
permitted the straightforward analysis and given a definite kinetic conclusion concerning the acid-base reaction
between DMT•+ and DMT.
Introduction
Numerous studies have been devoted to the kinetic analysis
of aromatic cation radicals in aprotic solvents,1 and work still
continues in this area.2-9 In addition to cation radical-
nucleophile combination reactions, proton-transfer reactions
have been analyzed because of the acidic property of cation
radicals to release H+.2 By analyzing the reaction of cation
radicals with bases, such as pyridine derivatives, the recent
advances have shown the detailed mechanism of the proton-
transfer reactions.4,5
When we consider the acidic property of aromatic cation
radicals, it may be necessary to elucidate the property as a base
of the neutral molecules, in particular, in the cases of aromatic
amine cation radicals as discussed previously.8,9 In a previous
communication,10 we have analyzed the reaction of the N,N-
dimethyl-p-toluidine (DMT, Figure 1) cation radical (DMT•+)
in acetonitrile (AN) using an electron-transfer stopped-flow
(ETSF) method, in which DMT•+ was generated via the
electron-transfer reaction between DMT and tris(p-bromophen-
yl)amine (TBPA) cation radical (TBPA•+). In the ETSF method,
the amount of neutral molecules coexisting with the cation
radicals can be easily controlled.10-15 Utilizing this technique,
the effect of DMT on the decay reaction of DMT•+ was
analyzed to form N,N,N′,N′-tetramethyl-R,R′-bi-p-toluidine
(TMBT, Figure 1),10 while the electrochemical results on the
oxidation of DMT in AN to form TMBT had been reported
previously.16,17
Figure 1. Structural formulas of N,N-dimethyl-p-toluidine (DMT) and
N,N,N′,N′-tetramethyl-R,R′-bi-p-toluidine (TMBT).
In the present full paper, at first, the details of the reaction
of DMT•+ controlled by the neutral DMT are described with
the ETSF analysis. In the electrochemical methods, it had been
very difficult to analyze the effects of the neutral molecules
explicitly because the neutral molecules surely coexist when
the cation radicals are generated on the electrodes. Compared
with the electrochemical results, a definite conclusion on the
reaction kinetics between DMT•+ and DMT is given by the
results obtained using the ETSF method.
Next, the reactions of DMT•+ in the presence of pyridine
derivatives (X-Py s), instead of the neutral DMT, are analyzed.
By mixing the solution of 0.10 mM TBPA•+ with the solution
containing both 0.10 mM DMT and x mM X-Py, the reaction
between 0.050 mM DMT•+ and x/2 mM X-Py can be observed
in the absence of DMT in the ETSF method. The reaction rates
* Corresponding author. Tel: +81-75-753-9152, Fax: +81-75-753-9145,
E-mail: oyama@iic.kyoto-u.ac.jp.
† Present address: POSCO, Republic of Korea.
10.1021/jp026073h CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/06/2002

8104 J. Phys. Chem. A, Vol. 106, No. 35, 2002
Goto et al.
should reflect the basic property of X-Py s as in the previous
work on hexamethylbenzene cation radicals.18 In the present
work, the relationship between the basic property and the
reaction rates of DMT•+ is compared by determining the decay
rates of DMT•+ in the presence of eight X-Py s; i.e., pyridine
(Py), 2-methylpyridine (2Me-Py), 4-methylpyridine (4Me-Py),
2,4-dimethylpyridine (2,4Me-Py), 2,6-dimethylpyridine (2,6Me-
Py), 2,4,6-trimethylpyridine (2,4,6Me-Py), 4-cyanopyridine
(4CN-Py), and 4-methoxypyridine (4MeO-Py).
In addition, the basic property of N,N-dimethylformamide
(DMF) is evaluated by observing the decay reaction of DMT•+
in the presence of DMF in AN. While DMF has been used as
the solvent in the recent studies with the fast scan cyclic
voltammetric analysis,8,9 the basic properties of DMF and DMT
are compared through the decay reaction of DMT•+ in the
present work.
Figure 2. Cyclic voltammograms of (A) DMT and (B) TBPA in AN.
Concentration of the substrates: 1.0 mM. Supporting electrolyte:
tetrabutylammonium hexafluorophosphate 0.10 M. Working elec-
trode: platinum disk electrode (diameter: 1.6 mm). Scan rate: 100
mV s-1
.
Experimental Section
The concept and actual measurements of the ETSF method
were described previously.11,12 In this method, unstable cation
radicals (N•+) are formed via the electron transfer with another
long-lived cation radical (M•+) whose formal potential is
positive to that of N•+, as expressed by eq 1.
N + M•+ f N•+ + M
(1)
Figure 3. Cyclic voltammograms of DMT in AN. [DMT]: (A) 2.0
mM, (B) 1.0 mM, and (C) 0.50 mM. Supporting electrolyte: tetrabu-
tylammonium hexafluorophosphate 0.10 M. Working electrode: plati-
In the present work, DMT•+ is produced via the electron-transfer
num disk electrode (diameter: 1.6 mm). Scan rate: 100 mV s-1
.
reaction of eq 2.
dehydrated (Wako chemicals, H2O < 50 ppm) was used as
received. As the supporting electrolytes for the electrochemical
measurement, tetrabutylammonium hexafluorophosphate (Fluka,
puriss. electrochemical grade > 99.0%) was used.
DMT + TBPA•+ f DMT•+ + TBPA
(2)
All the measurements were carried out in AN as a solvent.
Because TBPA•+ is stable enough,19 the AN solution was
prepared by a batchwise electrolysis and used after diluting it
to an appropriate concentration by AN. The concentration was
determined by the absorption measurement using the reported
molar absorptivity of TBPA•+ at 705 nm, ꢀ ) 3.2 × 104 M-1
Results and Discussion
Cyclic Voltammograms of DMT. Figure 2 shows the cyclic
voltammograms of TBPA and DMT in AN. While the redox
potential of TBPA•+/TBPA is 0.69 V, the oxidation peak
potential of DMT is 0.32 V. Because the oxidation potential of
TBPA is 0.4 V positive to that of DMT, eq 2 should proceed
quantitatively when the solutions of TBPA•+ and DMT are
mixed. In addition, some irreversibility can be recognized for
the oxidation process of DMT (Figure 2A). This implies the
presence of the consecutive chemical reactions of DMT•+ as
reported previously.16,17
Figure 3 shows the changes in cyclic voltammograms
depending on the concentration of DMT. With increasing the
concentration of DMT, the ratio of the oxidation current versus
the reduction current was increased as shown in this figure,
implying the presence of consecutive chemical reactions. As
well as the DMT concentration, the scan rate is a factor to
influence the irreversibility. When the scan rate was relatively
slower, the irreversibility was increased. Actually, almost
reversible response was obtained for the low concentration, 0.20
mM, DMT at the higher scan rate of 500 mV s-1 (data is not
shown here).
cm-1 19
.
For the stopped-flow measurements, a rapid-scan stopped-
flow spectroscopic system, RSP-601 (Unisoku Co. Ltd., Hiraka-
ta, Japan) was used. In this apparatus, dynamic transformation
of absorption spectra can be observed with the minimized time
interval of 1.0 ms after mixing two solutions. The obtained decay
curve at a wavelength was composed of 512 points. The
simulation analysis was carried out using Microsoft Excel 2000.
Cyclic voltammograms were obtained using a computer-
controlled PAR 263 potentiostat. The working electrode used
was a 1.6 mm diameter platinum disk electrode (BAS Co. Ltd),
and the reference electrode used was a Pt | (I3-, I-) electrode
in AN.
Product analysis was carried out for the solution obtained
after mixing the solutions of TBPA•+ and 5-fold excess of DMT
in the case of the reaction between DMT•+ and DMT. It was
evaporated to ca. 1/10 amount and extracted with ether after
adding water. The extract was evaporated and separated using
a silica gel column with hexane + ethyl acetate as an eluent.
1
The isolated product was identified using a H NMR. For the
Decay Reaction of DMT•+ Influenced by the Presence of
DMT. Generally, in electrochemical oxidation processes, the
neutral molecules are necessarily present in the vicinity of the
electrode to be oxidized. Thus, the complex reactions between
the cation radicals and the neutral molecules might bring about
some difficulties in analyzing the electrochemical responses,
though the cation radical-substrate coupling is one of the typical
oxidative dimerization reactions.20 For the clear-cut observation
of the reaction processes involving cation radicals and the neutral
case of the reaction between DMT•+ and X-Py, the products
were analyzed using the same procedure after mixing the
solution of TBPA•+ with the solution containing both DMT
and pyridine. After obtaining the authentic products identified
1
using the H NMR, the products in the reactions with X-Py s
were confirmed by a thin-layer chromatography.
The reagents, DMT, TBPA, and X-Py s, were obtained from
Aldrich, and used as received. For a solvent, acetonitrile

DMT•+ Decay Reaction Kinetics
J. Phys. Chem. A, Vol. 106, No. 35, 2002 8105
Figure 5. (A) Time changes in absorbance at 470 nm of DMT•+
observed after mixing the AN solution of 0.10 mM TBPA•+ with the
AN solution of (a) 0.10, (b) 1.0, (c) 2.0, (d) 5.0, or (e)10.0 mM DMT.
After mixing the concentration of DMT coexisting with 0.05 mM
DMT•+ is (a) 0, (b) 0.45, (c) 0.95, (d) 2.45, (e) 4.95 mM, respectively.
(B) Simulated results obtained assuming the rate law of -d[DMT•+]/
dt ) k[DMT•+] [DMT] and using the identical k value of 6.5 × 102
Figure 4. Changes in absorption spectra observed after mixing of the
AN solution of 0.10 mM TBPA•+ with the AN solution of DMT.
[DMT]: (A) 0.10 mM and (B) 1.0 mM. Time interval of each
spectrum: 1.0 s. Composed of 10 spectra. While DMT•+ is almost
stable without DMT as in A, monotonic decrease of DMT•+ was
observed in the reaction of 0.05 mM DMT•+ with 0.45 mM DMT as
shown in B.
M-1 s-1
.
of DMT•+, the decay curves of DMT•+ were recorded in the
presence of several concentrations of DMT. Figure 5A shows
the changes in absorbance at 470 nm, which is the absorption
maximum of DMT•+ (Figure 4), depending on the concentration
of DMT coexisted in the reaction solutions. As shown in Figure
5A, the decay rate of DMT•+ was accelerated with the increase
of the concentration of DMT.
molecules, we are proposing the ETSF method,10-15 in which
kinetic observations can be carried out in complete homogeneous
solution.
Figure 4 shows the remarkable features of the reactions of
DMT•+ revealed explicitly using the ETSF method. In this
method, when the solutions of 0.10 mM TBPA•+ and 0.10 mM
DMT were mixed, the solution of 0.05 mM DMT•+ is formed
in the optical cell in the stopped-flow apparatus, considering
the dilution due to the equivolume mixing. That is, the solution
containing only DMT•+ without DMT can be produced easily
in homogeneous solution using the ETSF method, while DMT
is necessarily present in the conventional electrochemical
generation of DMT•+. The absorption spectra of Figure 4A that
can be reasonably assigned to that of DMT•+ judging from the
complete disappearance of TBPA•+ after the mixing indicate
that the decrease of DMT•+ was very little during 10 s. While
the stability of the cation radicals in AN is susceptible to water
content in many cases, DMT•+ was stable enough, even in the
presence of H2O. The absorption spectra of DMT•+ unaltered
by 50 mM H2O were observed using the ETSF method. In
contrast, as shown in Figure 4B, the remarkable decrease of
DMT•+ was observed in the reaction of 0.05 mM DMT•+ with
0.45 mM DMT, recorded after mixing the solutions of 0.10 mM
TBPA•+ with 1.0 mM DMT. Figure 4B indicates clearly that
the neutral DMT promotes the decay reaction of DMT•+.
Thus, two conclusions are summarized here from the results
of Figure 4. First, when DMT•+ is formed quantitatively without
DMT, DMT•+ is fairly stable (Figure 4A), though this situation
is very difficult to prepare in electrochemical measurements.
While the phenyl C-C coupling proceeds at the para position
through the cation radical-cation radical coupling in the
reactions of methyldiphenylamine and diphenylamine cation
radicals,11,12 the sufficient blocking effects of the p-methyl
substituent of DMT•+ are verified to prevent the phenyl C-C
coupling judging from the stability of 10 s. Second, as apparently
from Figure 4B, the neutral DMT promotes the decrease of
DMT•+. That is, the stability of DMT•+ is very susceptible to
the neutral DMT.
To analyze the effect of DMT quantitatively, simulation
analysis was carried out for the decay curves b-e in Figure
5A. By taking into account the consumption of both DMT•+
and DMT during the reaction, and by using an identical value
of k, 6.5 × 102 M-1 s-1, numerical calculation was performed
using computer software. Consequently, working curves show-
ing the fairly good fit could be obtained as shown in Figure 5B
by changing only the concentration of DMT on the basis of the
rate law (eq 3).
-d[DMT•+]/dt ) k[DMT•+] [DMT]
(3)
This rate law means that the initial interaction between DMT•+
and DMT is the rate-determining step (rds). Considering the
previous results of the electrochemical oxidation of DMT to
form TMBT,16,17 the proton extraction from DMT•+ by DMT
can be regarded as the rds. That is, the acid-base interaction
between DMT•+ and DMT is significant in the decay reaction
of DMT•+ as in eq 4.
DMT•+ + DMT f (DMT-H)• + DMT(H+)
2 (DMT-H)• f TMBT
(4)
(5)
If the acid-base reaction of eq 4 was in equilibrium, the
protonated from of DMT (DMT(H+)), which increases with the
progress of the reaction, should affect the decay rate of DMT•+
by participating in the rate law. Thus, the excellent fit with eq
3 in Figure 5B means that the acid-base reaction of DMT•+
and DMT is the rds. The redox reaction between DMT•+ and
(DMT-H)• might be imaginable in the proposed scheme after
eq 4. However, the consumption of one DMT•+ per one DMT
was taken into account in the simulation of Figure 5B. Thus,
after the rds, the dimerization reaction of two (DMT-H)• is
considered to proceed rapidly.
Kinetic Analysis of the Reaction between DMT•+ and
DMT. To make clear the effects of DMT on the decay reaction

8106 J. Phys. Chem. A, Vol. 106, No. 35, 2002
Goto et al.
Figure 7. Cyclic voltammograms of 1.0 mM DMT in AN. (A) With
5.0 mM Py, (B) without Py. Supporting electrolyte: tetrabutylammo-
nium hexafluorophosphate 0.10 M. Working electrode: platinum disk
electrode (diameter: 1.6 mm). Scan rate: 100 mV s-1
.
Figure 6. (A) Time changes in absorbance at 470 nm of DMT•+
observed after mixing of the AN solution of 0.10 mM TBPA•+ with
the AN solution containing both 0.10 mM DMT and (a) 5.0, (b) 10.0,
(c) 20.0, or (d) 50.0 mM Py. After the mixing, the concentration of Py
coexisting with 0.05 mM DMT•+ is (a) 2.5, (b) 5.0, (c) 10.0, and (d)
25.0 mM, respectively. (B) Simulated results obtained assuming the
rate law of -d[DMT•+]/dt ) k[DMT•+] [Py] and using the identical k
value of 2.3 × 102 M-1 s-1
.
In the above mechanism, by working as a base, DMT can
promote the formation of TMBT. Such character of DMT as a
base is reasonably expected because it is an amine compound.
Thus, in the case of DMT, it was found that one electron brought
about the conversion of the acidic and basic character between
DMT•+ and DMT (eq 6).
-
+
DMT
(base)
- e T
(6)
DMT•
(acid)
Figure 8. (A) Dependence of the reaction rates of DMT•+ with X-Py
s on the base strengths (pKaB, ref 18), together with (B) the previous
result on the deprotonation rates of the hexamethylbenzene cation
radical.18
Effect of Pyridine Derivatives on the Reaction of DMT•+.
To clarify the Brønsted acid-base interaction to promote the
decay reaction of DMT•+, next we carried out the kinetic
analysis of the reaction of DMT•+ in the presence of X-Py s,
because Py is known as a typical base in AN. In the ETSF
method, this reaction is easily observable with no hindrances
of DMT by mixing the AN solution of 0.10 mM TBPA•+ with
the AN solution containing both 0.10 mM DMT and varied
concentration of X-Py. Because the oxidation potential of Py
is much higher than that of TBPA,21 the reaction between
DMT•+ and Py can be observed after the electron transfer of
eq 2.
2,6Me-Py, 5.8 × 102 M-1 s-1; 2,4,6Me-Py, 1.7 × 103 M-1 s-1
;
4CN-Py, 2.4 × 10-1 M-1 s-1; 4MeO-Py, 1.45 × 103 M-1
s-1
.
The changes in the reaction rates involving pyridine deriva-
tives have been investigated on the cation radical-nucleophile
reaction21 and the proton extraction reaction from the cation
radicals.18 The present results showed the relationship similar
to the latter case,18 in which the rates were plotted versus pKa
B
(KaB ) [X-Py][H+]/[X-PyH+]). Figure 8 shows the relationship
B
between log k and pKa in the present case, together with the
previous result for the hexamethylbenzene cation radical.18 This
similarity supports a view that the decay reaction of DMT•+ is
promoted through the proton extraction by the coexisting
Brønsted base molecules.
As a consequence, the acceleration of the decay reaction of
DMT•+ was observed with the increase of the concentration of
Py as shown in Figure 6A, which was very similar to the result
of Figure 5A. From the results of the simulation analysis, it
was found that the rate law was expressed as eq 7, and k was
Reaction Products and Mechanisms. In the electrochemical
method, it was established that the dimer compound, TMBT
(Figure 1), was produced in the oxidation of DMT in AN.16,17
In the present work, to investigate the products in the ETSF
procedures, product analysis was carried out for the solution
after mixing TBPA•+ and DMT. As a result, the formation of
TMBT as a main product is also confirmed. Thus, the reaction
mechanisms can be summarized as shown in Figure 9. After
the rate determining proton extraction by DMT, the coupling
reaction of two radical species, (DMT-H)•, proceeds to form
TMBT.
The product analysis was also performed for the products
obtained in the reactions with X-Py s. In the presence of Py,
the result of NMR showed the isolated main product was N,
N′-dimethyl-N, N′-di-p-tolylethylenediamine (DMDTE). That is,
the proton extraction was found to occur at the N-methyl
position. As well as the remarkable difference in the NMR
determined to be 2.3 × 102 M-1 s-1
.
-d[DMT•+]/dt ) k[DMT•+] [Py]
(7)
Thus, the acid-base interaction to promote the decay reaction
of DMT•+ was verified using the typical base, Py, instead of
DMT. The results of cyclic voltammetry also showed the
qualitative promotion of the consecutive reactions as shown in
Figure 7.
Substituent Effect of Pyridine Derivatives on the Decay
Reaction Rates of DMT•+. The effects of X-Py s on the
reactions of DMT•+ were systematically investigated using the
ETSF method. Consequently, the rate law of eq 7 was obtained
for all other seven derivatives examined, and the reaction rates,
k, were determined to be: 2Me-Py, 5.0 × 102 M-1 s-1
;
;
4Me-Py, 7.5 × 102 M-1 s-1; 2,4Me-Py, 1.55 × 103 M-1 s-1

DMT•+ Decay Reaction Kinetics
J. Phys. Chem. A, Vol. 106, No. 35, 2002 8107
Figure 11. Time changes in absorbance at 470 nm of DMT•+ observed
after mixing of the AN solution of 0.10 mM TBPA•+ with the AN
solution containing both 0.10 mM DMT and (A) 2.5, (B) 5.0, or (C)
7.5 M DMF. After mixing, the concentration of DMF coexisting with
0.05 mM DMT•+ is (a) 1.25, (b) 2.50, and (c) 3.75 M, respectively.
the N-methyl group.22 Thus, the formation of DMDTE does not
seem to be exceptional. Rather than DMDTE, it might be better
to consider a specific interaction for the formation of TMBT,
e.g., the π-π stacking interaction between DMT•+ and DMT to
promote the H+ extraction from the p-methyl group of DMT•+.
However, this is a merely possible speculation from the reaction
products.
Decay Reaction of DMT•+ in the Presence of DMF.
Amatore and co-workers revealed the reaction mechanisms of
aromatic amine derivative cation radicals using the fast scan
cyclic voltammetry, in which DMF was used as a solvent.8,9
While the basic property of the neutral substrate was considered
in the case of p-anisidine,8 the interaction with DMF as a base
was significant in the cases of p-halogenoanilines.9
Figure 9. Mechanism of the dimerization reaction of DMT•+ with
the presence of DMT.
To evaluate the basic property of the present case of DMT,
we observed the decay reaction of DMT•+ in the presence of
DMF in the AN solutions. Figure 11 shows the decay curves
of DMT•+ observed after mixing the solution of DMT•+ in AN
with the solution containing 2.5, 5.0, or 7.5 M DMF in AN.
Compared with the absorbance change of only DMT•+ in AN
(Figure 5Aa), remarkable decrease was found in the cases with
a large excess of DMF, though the change in Figure 11 is quite
different from Figures 5A and 6A (i.e., do not follow a certain
rate law). However, if the solvent was totally replaced to DMF,
it is expected that DMT•+ diminished totally even just after
the mixing in the optical cell. Thus, the basic property of DMF
(if it is used as the solvent) is considered to be stronger than
that of the neutral DMT (used as the reactant).
The present ETSF method can be utilized for such comparison
and the evaluation of basic properties. In the present work,
despite the relative lower basic property of DMT compared with
p-anisidine,8 which is suspected via DMF solvent, the significant
contribution of the neutral DMT to the decay reaction of DMT•+
was clearly revealed in AN.
Figure 10. Mechanism of the dimerization reaction of DMT•+ with
the presence of Py.
results, TMBT and DMDTE can be distinguished in a thin-
layer chromatography. Thus, the products in the reactions with
the other seven X-Py s were examined using the thin-layer
chromatography. Consequently, the main product in the presence
of X-Py s was identified to be DMDTE in all the cases. Thus,
the reaction mechanism with Py can be summarized as Figure
10, and this is true for the other X-Py s.
Conclusions
In the present work, first, the stability of DMT•+ in the
absence of DMT in AN has been revealed using the ETSF
method, though the previous electrochemical work reported the
instability of DMT•+; e.g., the spectroscopic detection of
DMT•+ in AN had been very difficult.16 This can be attributed
to the difference between the ETSF and electrochemical
methods. In the electrochemical oxidation, the neutral DMT
molecules surely exist in bulk solution, so that the reaction of
DMT•+ is promoted by the basic property of DMT. The function
of DMT as a base is a significant driving force of the CH2-
CH2 bond formation in the reaction of DMT•+.
Except the final product, both the reaction schemes are
essentially the same; after the rate determining proton extraction
by DMT or Py, the coupling reaction of two radical species
proceeds. The present results of the products analysis showed
that a proton of N-methyl group of DMT•+ is subtracted by
Py, while a proton of p-methyl group is subtracted by the neutral
DMT. The reason for this difference is unclear at present.
However, the formation of DMDTE was reported in the reaction
of DMT and tert-butoxy radical through a proton extraction from
For the electrochemical dimerization reactions of aromatic
cation radicals, mechanistic attention has been devoted to the
terms of “the radical-radical coupling” (RRC) or “the radical-

8108 J. Phys. Chem. A, Vol. 106, No. 35, 2002
Goto et al.
substrate coupling” (RSC) mechanisms.20 In the present case,
the RRC is very slow, if at all, as revealed in the absence of
the substrate DMT (Figure 5Aa). This is quite reasonable
considering the blocking effect of p-methyl group, while the
RRC mechanism was confirmed for methyldiphenylamine and
diphenylamine cation radicals.11,12 Whereas the RSC implies
the direct coupling reaction in many cases,20,23 the present
reaction can be regarded as an interesting example. This is
because little kinetic attention has been paid on the acid-base
interaction of the present type, despite the rate law being
identical to the RSC (when the first encounter step is the rds).
In the elucidation of the reactions of cation radicals, in some
cases, it might be necessary to take into account that such acid-
base interactions with the neutral molecules surely exist in
solution. While the voltammetric measurements were usually
carried out at a certain concentration, ca. 1 mM or so, diverse
changes in the concentration of the neutral molecules versus
the cation radicals, as in the present work, would be necessary
to clarify the such interaction.
Education, Culture, Science, Sports and Technology, Japan, No.
13640602. M.O. thanks the Mitsubishi Foundation for the
financial support. H.P. thanks the JSPS (Japan Society for the
Promotion of Science) Postdoctoral Fellowship for Foreign
Researchers.
References and Notes
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Acknowledgment. This work was supported in part by a
Grant-in-Aid for Scientific Research from the Ministry of