1403
J. Phys. Chem. 1986,90, 1403-1407 simplest form is the difference between kinetic and static nonequilibrium entropies of the system. A is a physically relevant quantity, the negative variational derivative of which (being a covariant vector) should replace the gradient of entropy in Onsager's equations if one wants to apply these equations to complex irreversible systems in which entropy depends on the rate of the process as well as on the state of the system.
Acknowledgment. Helpful discussion with J. Nulton is gratefully acknowledged. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society. S.S. acknowledges the kind hospitality of the Department of Mathematical Sciences, San Diego State University. We thank the reviewers for constructive comments which have resulted in an improved manuscript.
Monomer-Dimer Equilibrium of Arylnitroso Spin Traps: An Electrochemical Investigation Marcel Culcasi, Paul Tordo, Laboratoire d'Etude de la Structure et de la Riactivitd des Especes Paramagnetiques, CNRS LA 126, Universiti d'Aix- Marseille I , 13397 Marseille Cidex 13, France
and GBrard Gronchi*' Laboratoire de I'Ecole Supirieure de Chimie de Marseille, Universiti III, 13397 Marseille Cidex 13, France (Received: July 30, 1985)
Pulse voltammetry was used to investigate the monomer-dimer equilibrium for six arylnitroso compounds. Because of the pulse duration, 30 ms, equilibrium drifts during measurements were negligible. The dissociation constant of the nitrosodurene dimer, a commonly used spin trap, is 0.84X lo4 in acetonitrile at 293 K. Under the same conditions, the new spin trap pentamethoxynitrosobenzene exhibits a higher monomer concentration (k = 1.12 X lo-"). The same technique was used to simultaneously investigate the equilibrium and the kinetics of the dissociation of 2,4,6-trimethylnitrosobenzeneat various temperatures, giving the following thermodynamic parameters: AHo = 55.9 kJ mol-'; M0= 151.1 J K-'mol-'; AH* = 85.0 kJ mol-'; AS* = -11.8 J K-' mol-'.
Introduction
Since the preliminary work of Bard in 1974: the spin trapping technique3 has been used to investigate electrochemical processes by different workers."8 For a such applications, the reduction and oxidation potentials of the spin traps are very important parameters, because the traps used need to be electrochemically inactive at the applied potential during the electrochemical process under investigation. On the other hand, even in chemical processes, electron transfer can occur between the different implied species, and the knowledge of redox properties constitutes an aid in understanding the results observed in a spin trapping experiment. Two classes of compounds, nitrones and nitroso compounds, are largely used as spin traps. Their electrochemical behavior has been investigated in order to determine their "potential window^".^,^,^ Although nitrones generally have a larger potential (1) To whom correspondence should be directed. (2) Bard, A. J.; Gilbert, J. C.; Goodin, R. D. J . Am. Chem. SOC.1974,96, 620. (3) Perkins, M. J. In Aduances in Physical Organic Chemistry, Vol. 178, Gold, V., Bethell, D., Eds.; Academic Press: New York, 1980; p 1 . (4) McIntire, G. L.; Blount, H. N.; Stronks, H. J.; Shetty, R. V.; Janzen, E. G. J . Phys. Chem. 1980,84, 916. (5) Belevsky, V. N.; Iarkov, S . P. Dokl. Akud. Nuuk. 1980, 254, 1417. (6) Walter, T. E.; Bancroft, E. E.; McIntire, G. L., Davis, E. R.; Gierasch, L. M.; Blount, H. N.; Stronks, H. J.; Janzen, E. G. Cun. J . Chem. 1982,60, 1621. (7) Sossonkin, I. M.; Belevsky, V. N.; Strogof, G. N.; Damariev, A. N.; Iarkov, S . P. Zh. Org. Khim. 1982, 18, 1504. It must be pointed out that, in this study, the first reduction wave of arylnitroso compounds was wrongly attributed to the dimer. (8) Gronchi, G.; Courbis, P.; Tordo, P.; Mousset, G.; Simonet, J. J. Phys. Chem. 1983.87, 1343.
window, the nitroso compounds present better trapping activityg*I0 and may yield more information, since, in the adduct, the trapped radical is nearer the radical center R*
+
X-N=O
0 R*
+
X-CH=N-Y
t
- p' - i'i 0' R-N-X
R--N-Y
H
However, a disadvantage of nitroso compounds lies in the existence of a monomefdimer equilibrium. Most nitroso compounds are isolated as crystalline dimers which dissociate in solution to give the colored monomers; only the monomeric form can behave as a free radical scavenger. Accordingly, knowledge of the equilibrium in the media used in organic electrochemistry is very important and is mandatory for kinetic studies of the spin trapping proces~.~J~ The monomer-dimer equilibrium of arylnitroso compounds has been primarily investigated by spectrophotometry1&I3and by NMR.'"I6 If the visible absorption band of the monomer is used, (9) Maeda, Y.; Ingold, K. U. J . Am. Chem. SOC.1979, 101,4975. (10) (a) Doba, T.; Ichikawa, T.; Yoshida, H. Bull. Chem. SOC.Jpn. 1977, 50, 3158. (b) Doba, T.; Noda, S.; Yoshida, H. Bull. Chem. SOC.Jpn. 1979, 52, 21. (1 1) Holmes, R. R. J . Org. Chem. 1964, 29, 3076. In this study, Holmes pointed out the existence of a dimer reduction wave in aqueous-alcoholic media but did not use this information. (12) von Keussler, V.; Luttke, W. Z . Elektrochem. 1959, 63, 614. (13) Nakamoto, K.; Rundle, R. E. J. Am. Chem. SOC.1956, 78, 1113.
0022-3654/86/2090-1403$01.50/00 1986 American Chemical Society
1404
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
Culcasi et al. TABLE I: Reduction Potentials' (vs. SCE) of the Monomeric and Dimeric Forms of Substituted Nitrosobenzenes monomer
dimer El/4
compd
Ell2
2,6-dichloro 2,4,6-trichloro 2,4,6-trimethyl 2,3,5,6-tetramethyl pentamethyl pentameth-
-720 (-760) -640 (-640)
-
E1/4
E112
E3/4
Acetonitrile 60 -1030 (-1560) 55 -950 (-1375)
-
E3/4
70 70
-I 150
60
-1625
90
-1 160
55
-1665
100
-1215
60
-1730
95
-1065
60
-1680
110
OXY
- 0.6
- 0.8 E (v
- 1.0 vs
s.c.e.)
Figure 1. Pulse voltammograms of a 3.32 m M solution of 2.6-dichloronitrosobenzene in acetonitrile 0.1 M in tetra-n-butylammonium hexafluorophosphate; working electrode: mercury-coated platinum disk 2 mm in diameter, rotated at 200 rpm; temperature, 293 K. (a) Direct pulse, starting potential A. (b) Direct pulse, starting potential B. (b') Reverse pulse. starting potential B. (c) Reverse pulse, starting potential C.
its low extinction coefficient often leads to poor sensitivity. The UV absorption of the dimer" is not often used since the equilibrium could be disturbed by photoassisted dissociation.16 Proton N M R seems to be a .good method when well-separated signals can be unambiguously assigned to the monomer and the dimer. This is the case for methyl-substituted n i t r o ~ o b e n z e n e s . ' ~ ~ ' ~ During our investigation on the redox behavior of a series of arylnitroso spin traps, we decided that electrochemical techniques should be applied to investigate the monomer-dimer equilibrium in routinely used electrochemical solvents. In acetonitrile or dimethylformamide, the monomer and dimer show two wellseparated reduction waves on platinum or mercury-coated platinum electrcdes.*~" However, the use of voltamperometric currents to obtain the dissociation constant is subject to a problem arising from the dissociation of the dimer during the experiment. At potentials corresponding to the first wave plateau, at which occurs the monoelectronic reduction of the monomer, the concentration of the monomer falls to zero near the electrode, upsetting the equilibrium. A kinetic contribution, a function of the rate of dissociation of the dimer, can then change the measured currents. Results presented here will show that this complication is avoided by using pulse voltammetry on a rotating electrode.I8 With this fast technique, the current readings give a true picture of the composition of the solution, since the pulse duration is sufficiently small (30 ms) to make the equilibrium shift negligible. Because the half-lives of the dimers are longer than 400 s, the variation of t h e dimer concentration during the pulse can be calculated to be less than 0.01%. Results
The general behavior of the nitroso compounds studied is illustrated by the electroreduction of 2,6-dichloronitrosobenzene (14) Stowell, J. C. J . Org. Chem. 1971, 36, 3055. (15) Azoulay, M.; Wettermark, G. Tetrahedron 1978, 34, 2591. Azoulay, M.; Wettermark, G.; Drakenberg, T. J . Chem. Sor., Perkin Trans. 2 1979, 199. (16) Azoulay, M . ; Fischer, E. J. Chent. Soc., Perkin Trans. 2 1982, 637. (17) Sergeev, G. B.; Leenson, I . A. Russ. J . Phys. Chem. 1978, 52, 312 (translation). (18) Myers, D. J.; Osteryoung, R. A,; Osteryoung, J. Anal. Chem. 1974, 46, 2089.
Dimethylformamide 2,6-dichloro -670 (-650) 60 -1060 2,4,6-tri-560 (-570) 60 -955 chloro 2,4,6-tri-1110 (-1150) 60 -1595 methyl 2.3,5,6-tetra- -1090 (-1100) 70 -1680 methyl
(-1260) (-1240)
80 80
(-1900)
80
(-1850)
90
Working electrodes: mercury coated platinum electrode. The values found at a platinum working electrode are given in brackets.
(C = 3.32 mM) in acetonitrile at a mercury electrode (Figure 1).
The direct normal pulse voltamm~gram'~ (curve a) starting at point A shows the monomer wave ( E l j z= -720 mV/SCE; ZL = 118 PA), the dimer wave ( E i l 2= -1030 mV/SCE; fL = 95 PA) and a third wave not shown (E1/2= -1915 mV/SCE; IL= 188 FA). This pattern may be accounted for by the following series of steps? step 1 E = -720 mV
ArNO
+e
step 2 E = -1030 mV
(ArNO),
ArNO-'
-
+ 2e
step 3 E = -1915 mV
ArNO-'
+e
2ArNO-'
(ArNO),-
If the initial potential is set at point B, the potential at which the monomer is consumed near the electrode, the direct scan (curve b) shows a lower dimer reduction wave, and the reverse scan (curve b') shows an anion radical oxidation wave higher than the monomer reduction wave of curve a. These observations clearly demonstrate the contribution of the equilibrium displacement to the currents and confirm the necessity of using the direct pulse technique to obtain reliable information on the dissociation constants. Finally, a reverse scan pulse voltammogram (curve c) initiated at point C, where the monomeric and dimeric forms are reduced near the electrode, shows an increase in the anion radical oxidation wave, which proves the dissociative reduction of the dimer (step 2). The reduction potentials of the compounds used in this study are listed in Table I. (19) In this technique, the working electrode is maintained at the starting potential (chosen so that no electrochemical reaction can occur), except during pulses, the height of which varies linearly with time, ensuring the potential scan in the cathodic or anodic direction. The sum of the pulse durations is only 3% of the total experiment duration; this minimizes the problems arising from electrode pollution by reaction products; owing to the small pulse duration (30 ms) the occurrence of side chemical reactions with slow kinetics (e.g. the studied dissociation) may be neglected. For direct pulse voltammetry, see ref 18. For reverse pulse, see: Osteryoung, J.; Kirowa-Eisner, E. Anal. Chem. 1980, 52, 62.
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1405
Arylnitroso Spin Traps
I
TABLE 11: Dissociation Constants of Substituted Nitrosobenzenes compd 104~ ab AGO' Acetonitrile" 27.4 30.8 3.43 2,6-dichloro 2,4,6- trichloro 107 51.8 2.64 2,4,6-trimethyl 104 51.6 2.66 2,3,5,6-tetramethyl 0.84 6.27 5.47 pentamethyl 1.446 8.13 5.15 11.2 21.0 3.96 pentamethoxy
Dimethylformamide" 2,6-dichloro 31.6 32.6 2,4,6-trichloro 103 50.5 2,4,6-trimethyl 61.6 42.2 2,3,5,6-tetramethyl 4.33 13.7
I
I
I
T
20,uA
I
3.35 2.67 2.96 4.51
" Using 0.1 M tetra-n-butylammonium hexafluorophosphateas supporting electrolyte; temperature, 293 K. Dissociation coefficient for a 0.01 M solution; a is the percent ratio of monomer concentrationto the total concentration expressed in mole of monomer. CFreeenergy of dissociation, kcal/mol. Preliminary result. Investigations on equilibria for these aromatic nitroso compounds were made by measuring the monomer and dimer limiting currents for solutions at different concentrations. Calculations were made with the following equation:
c = c,
+ 2cd = A,I, + AdId
- 0.5
- 1.0
- 1.5
-2.0
E (V v s s.c.e.)
where C is the total concentration expressed as the monomer; C, (cd) is the monomer (dimer) concentration at equilibrium; I , (Id) is the monomer (dimer) limiting current at equilibrium; and A, (Ad) is the proportionality coefficient between current and concentration. Results were treated by a least-squares method which gave values for A,,, and Ad. For each recorded curve, a value of the dissociation constant was obtained from
Figure 2. Direct pulse voltammograms of a 1.96 maw solution of 2,4,6trimethylnitrosobenzenein dimethylformamide0.1 M in tetra-n-butylammonium hexafluorophosphate; mercury-coated platinum disk rotated at 500 rpm; temperature, 293 K; time after solubilization, measured at point A: (a) 160 s; (b) 370 s; (c) 610 s; (d) 890 s; (e) 1190 s; (f) 5450
s (equilibrium is reached).
B
The given value of K is the mean value weighed by the total concentration, since the most precise measurements are obtained at high concentrations. The results obtained in this way at 293 K are listed in Table 11. A second approach is possible when the nitroso compound is highly soluble in the medium; this is the case for 2,4,6-trimethylnitrosobenzene in acetonitrile, which was investigated at various temperatures. Weighed dimer crystals were rapidly dissolved, and the monomer and dimer limiting currents were monitored at different times during the course of the dissociation, until equilibrium was established (Figure 2). The analysis of these results allowed the simultaneous determination of the equilibrium constant and the rate constant of dissociation. This was made by using the following equation, in which I , and I d are now functions of time t :
c = A,I,
+ AdId
(0 5 t 5
m)
A
I
0
1
= 2[Am2/Adl
[zm2/1dl
(t
-
m,
The following assumptions were made in order to calculate the rate constant k of the dissociation: (a) opposite reactions; (b) first-order kinetics for dimer dissociation; and (c) second-order kinetics for the dimerization process. According to these assumptions, the dissociation rate can then be expressed by the equation
3
4
t x10-3
(s)
Figure 3. Variation of the cathodic limiting current of the monomer of 2,4,6-trimethylnitrosobenzeneduring the dissociation process; experi-
mental values (points)are plotted over the calculated curves (solid lines); conditions: (A) temperature, 283 K; concentration, 2.07 mM; (B) temperature, 293 K; concentration, 2.17 mM; (C) temperature, 303 K; concentration, 2.14 mM; (D) temperature, 313 K, concentration, 1.93
mM. where C is the total concentration expressed as the monomer (mole per liter): X i s the monomer concentration at time t . When the equilibrium conditions are introduced, integration of the preceding equation yields
This equation may be used graphically to determine A, and Ad. The constant, K , was then obtained from the equation
K
2
X,2
-
X.(B - x)
where B = [ C X , ] / ( X ,- C ) ; X , is the momomer concentration at equilibrium ( t m). On the basis of this equation, we used an iterative least-squares method to obtain the best fit between experimental values and calculated monomer limiting currents as a function of time. Examples of such fits are shown in Figure 3. The values of the equilibrium constant K and the rate constant obtained at different temperatures for 2,4,6-trimethylnitrosobenzene in acetonitrile containing tetra-n-butylammonium hexafluorophosphate (0.1 M) are listed in Table 111.
1406 The Journal of Physical Chemistry, VoI. 90, No. 7, 1986
Culcasi et al.
TABLE 111: Equilibrium and Rate Constants for the Dissociation of the 2,4,6-TrimethylnitrosobenzeneDimer
7
concn range," mM 2.04/3.22
10
1.82/2.72
5 4
1.96/3.06 1.93/3.46
no.
temp, K 283 293
303 313
of trials
103~6
3.95 (f1.15) 8.26 (f0.80)C 19.72 (h1.93) 37.20 (f7.96)
104/;,bS-1 2.90 (f0.45) 10.7 (f0.50) 35.8 (f3.9) 102.0 (f15.5)
'Total concentration expressed in millimoles of monomer per liter. Solvent: acetonitrile with 0.1 M tetra-n-butylammonium hexafluorophosphate. Limited concentration ranges were used to obtain the best precision from the voltammetric technique. Mean value; 95% confidence limits are indicated in the brackets. cThis value of K at 293 K must be considered as a better estimate than the one which has been noticed in Table 11, since the calculation of the proportionality coefficients A,,, and A,, was supported by a larger number of individual measurements.
-3
Y
Y
C 1
-4
3.2
3.3
3.4
3.5
1 0 ~ 1 ~ Figure 5. Dependence of the rate constant of dissociation of the 2,4,6-
trimethylnitrosobenzenedimer on the temperature. Discussion
-5
The dimeric form of nitroso compounds has the azodioxide structure. Investigations by X-ray diffraction have shown that, in the case of dimeric aryl nitroso compounds (I), the aromatic
/ /
Figure 4. Monomerdimer equilibrium of 2,4,6-trimethylnitroobenzene; variation of the logarithm of the equilibrium constant K vs. 1/T.
Thermodynamic and activation parameters for the dissociation reaction can then be readily calculated over the temperature range investigated. On the basis of the Vant'Hoff equation, a least-squares linear regression leads to the following expression: In K = -6722.78(1/T)
+ 18.19
(A)
with a correlation coefficient r = 0.998. The corresponding enthalpy and entropy values are AHo = 55.9 kJ mol-'
ASo = 151.1 J K-' mol-'
(13.4 kcal mol-I) (36.2 cal K-I mol-])
In a similar manner, application of Eyring's activated complex theory to the kinetic results gives In [ ( h i ) / ( k T ) ]= -10227.8(1/T)
- 1.418
(B)
(where i is the rate constant; T i s the absolute temperature; h is Planck's constant; k is Boltzmann's constant) with a correlation coefficient r = 0.999. This expression yields the activation parameters: AH* = 85.0 kJ mol-l
AS* = -1 1.8 J K-'mol-I
I
rings do not lie in the plane defined by the 0-N=N-O group.20,21 This geometry explains the stability of the dimeric form, since a planar geometry, allowing greater electron delocalization, would lower the N=N double bond character and distabilize the dimer. Indeed, crystallographic studies concerning Z or E dimers of nitroso compounds shown that the N=N bond length is in the range of 1.30 to 1.32 A, which is midway between single and double bond lengths.*O Thus, we may conclude that nonplanar conformers of the monomeric aryl nitroso compounds, in which delocalization is decreased, are needed to reach the transition state of the dimerization process. Accordingly, dimerization should be easier for a monomer exhibiting a low energy barrier for the out-of-plane rotation of the nitroso group, which normally lies in the plane of the aromatic rings due to the conjugation effect. The presence of two ortho substituents having moderate steric requirements (Le. methyl, chlorine) decreases the rotation barrier and leads to the existence of an appreciable concentration of dimer in solution. However, it must be pointed out that a very high steric effect of ortho substituents completely hinders the dimerization, as was observed in the case of 2,4,6-tri-tert-butylnitrosobenzene, which
(20.3 kcal mol-') (-2.8 cal K-'mol-')
Equations A and B and the corresponding experimental points are plotted in Figure 4 and Figure 5, respectively.
(20) Dieterich, D. A.; Paul, I. C.; Curtin, D. Y. J . Am. Chem. SOC.1974, 96, 6372, and references therein. (21) (a) Armand, J.; Armand, Y.; Boulares, L.; Philoche-Levisalles,M.; Pinson, J. Can.J . Chem. 1981, 59, 171 1. (b) Pierot, M.; Vila, F.; Tordo, P., to be submitted for publication.
Arylnitroso Spin Traps
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1407
is monomeric even in the solid state, as evidenced by the green color of the crystals which is characteristic of the monomeric form.13 Conversely, factors increasing the rotation barrier of the nitroso groupZZshift the equilibrium toward the monomer. This is the case for a-donating substituent in the para (or even ortho) position, which increases the double bond character between the ipso carbon and the nitrogen. For example, the value of the C N bond length in 4-iodonitrosobenzene is 1.28 %.,as compared to 1.47 A for nitrosobenzene;z3 4-iodonitrosobenzene and 4-(N,N-dimethylamin0)nitrosobenzene can be isolated as green monomeric crystals. This effect seems to be present to a lesser extent with other para-donor substituents, such as alkyl groups. Our results are in good accord with these hypotheses: the trend of the dissociation coefficients (relative to 0.01 M solutions) for 2,6-dichloronitrosobenzene( a = 30.8%) and 2,4,6-trichloronitrosobenzene ( a = 51 3%) illustrates the influence of the para a-donating chlorine atom in the latter. Similarly, the trend of the dissociation coefficients for the methyl-substituted compounds may be reasonably explained by an increase in the steric hindrance (buttressing effect) in going from 2,4,6-trimethylnitrosobenzene (a = 51.6%) to pentamethylnitrosobenzene (a = 8.13%), and by the lack of the para donor substituent in the case of 2,3,5,6tetramethylnitrosobenzene (a = 6.27%). This approach may be applied to the results obtained in acetonitrile by Azoulay et al., who studied compounds such as 4methylnitrosobenzeneI6 (K(293) = 59.2; a = 99.97%)z4and 2methylnitros~benzene~~ (K(293) = 2.3; a = 99.15%)z4 by 'H N M R at low temperature, and 2,6-dimethylnitrosobenzene ( K (293) = 1.29 X a = 22.4%)z4by visible spectr~photometry.~~ There is a satisfactory agreement between our results and those previously reported. Doba et al. found K(293) = 0.79 X lo4 for for pentamethylnitrosodureneZ4and K(293) = 1.87 X nitrosobenzenez4 in benzene,loa although Maeda and Ingold9 measured K(293) = 4.3 X for nitrosodurene under the same conditions. This is indicative of the scatter of photometric results. Earlier investigations1'-I3 on chloro derivatives and on 2,4,6-trimethylnitrosobenzene in benzene or chloroform gave values of K which are two to four times higher than ours at the same temperature. It is of note that pentamethoxynitrosobenzene has a fairly high dissociation coefficient value (a = 21%) compared to nitrosodurene. This point must be included among the advantages which make this new compound a particularly useful radical scavenger?6 It must be pointed out that the solvent exerts only a small influence on the dissociation constants; results are of the same order of magnitude in acetonitrile and in dimethylformamide, and moreover, this is also true for the values obtained in low polarity Thus, we solvents, such as benzene or chloroform.9-~2~zs~27-29 believe that the values actually available can serve as a general guide in planning spin trapping experiments in any solvent when quantitative measurements are not required. Concerning the investigation on the dissociation of 2,4,6-trimethylnitrosobenzene at various temperatures, the thermodynamic and activation parameters determined by pulse voltammetry are consistent with previously reported data'z,15-z5 obtained by other methods on different nitroso compounds. The value of the rate (22) Calder, I. C.: Garrat, P. J. Tetrahedron 1969, 25, 4023. (23) Hanyu, Y . : Boggs, J. E. J . Chem. Phys. 1965, 43, 3454. (24) The given values of K are calculated at 293 K from the thermodynamic data available in the literature cited. The dissociation coefficient a is related to a total concentration expressed in monomer of 0.01 M. ( 2 5 ) Azoulay, M.; Lippman, R.; Wettermark, G. J . Chem. Soc., Perkin Trans. 2 1981, 256. (20) Vila, F.; Boyer, M.; Gronchi, G.; Duccini, Y.;Santero, 0.;Tordo, P. Tetrahedron Lett. 1984, 25, 2215. (27) Mijs, W. J.; Hoekstra, S.E.; Ulmann, R. M.; Havinga, E. Red. Trau. Chim. (Pays-Bas) 1958, 77, 746. (28) Ingold, C . K.; Piggott, H. A. J . Chem. SOC.1924, 125, 68. (29) Schmid, P.; Ingold, K. U. J . Am. Chem. SOC.1977, 99, 6434.
constant-for the dissociation of 2,4,6-trimethylnitrosobenzeneat 293 K (k = 1.07 X s-l) is comparable to the value obtainecj by cyclic voltammetry for 2-methyl-2-nitrosopropane in DMF ( k = 1.51 X s-l; T = 298 K)4 and in acetonitrile ( k = 0.89 X s-'; T = 293 K)? This corresponds to a half-life for the dimer of about 600 s, and it must be kept in mind, when preparing a solution for a spin trapping experiment, that about 1 h is needed to reach the maximum concentration of the active monomeric form at room temperature. In conclusion, we can state that, in the case of nitroso spin traps, two important parameters, the potential window and the dissociation constant, must be known in order to apply spin trapping to electrochemical or chemical processes. The rate of the dissociation of nitroso compounds makes the herein used technique of pulse voltammetry at a rotated disk electrode particularly suitable for the determination of the dissociation constants in electrochemical media. Experimental Section Nitroso compounds were prepared as described in the literat ~ r e ~and~recrystallized , ~ ~ ~ three ~ ~ times * ~from ~ acetonitrile (chloro derivatives) or from ethanol (methyl and methoxy derivatives). Acetonitrile and N,N-dimethylformamide were purified by standard methods32and stored over 4-A molecular sieves. Solutions of the supporting electrolyte, tetra-n-butylammonium hexafluorophosphate twice recrystallized from a mixture of ethyl acetate (90%) and pentane (lo%), were passed through activated alumina immediately before use. All solutions were deoxygenated by bubbling dry argon through the cell. The working electrode was either a rotating platinum disk electrode or a mercury coated rotating platinum disk electrode,33both 2 mm in diameter. The technique used was pulse voltammetry; a low electrode rotation speed was used to avoid rotation contribution to the current.I8 The reference electrode was an aqueous SCE; a bridge was used to avoid water diffusion into the cell. The cell was thermostated throughout the entire series of experiments (fO.l "C). Pulse voltammograms were recorded on a Tacussel PRG 4 apparatus; the working parameters were as follows: pulse duration, 30 ms; cycle duration, 1 s. To achieve good reproducibility, the electrode was cleaned before each recording by an electrochemical pretreatment in the test solution; 30 s at -2.2 V and 90 s at -0.3
v.
Equilibrium Experiments. Nitroso compound solutions of different concentrations (0.5 to 3 mM)34 were prepared and maintained at 293 K for at least 120 min. Then, for each solution, several voltammograms were recorded to determine that equilibrium was established. Kinetic Experiments. Weighed dimer crystals were introduced in the cell and rapidly dissolved (this never exceeded 30 s for 2,4,6-trimethylnitrosobenzene). Pulse voltammograms were recorded at different times with high potential scan rates (20 mV SKI)until no change in the limiting currents was observed. Registry No. 2,6-Dichloronitrosobenzene,1194-66-7; 2,4,6-trichloronitrosobenzene, 1196-1 3-0; 2,4,6-trimethylnitrosobenzene,1196-1 2-9; 2,3,5,6-tetramethylnitrosobenzene,38899-21-7; pentamethylnitrosobenzene, 65594-36-7: pentamethoxynitrosobenzene, 84802-28-8; 2,6-dichloronitrosobenzene dimer, 100045-99-6; 2,4,6-trichloronitrosobenzene dimer, 84802-32-4; 2,4,6-trimethylnitrosobenzenedimer, 100046-00-2; 2,3,5,6-tetramethylnitrosobenzene dimer, 84802-31-3; pentamethylnitrosobenzene dimer, 84802-30-2; pentamethoxynitrosobenzene dimer, 100046-01 -3. (30) Smith, L. I.; Taylor, F. L. J . Am. Chem. SOC.1935, 57, 2460. (31) Holmes, R. R.; Bayer, R. P. J . Am. Chem. SOC.1960, 82, 3454. (32) Mann, C. K. In Electroanalytical Chemistry, Vol. 111, Bard, A. J., Ed.; Marcel Dekker: New York, 1969; p 57. (33) The electrode was prepared by abrasion under mercury according to: Moros, S.A. Anal. Chem. 1962, 34, 1584. (34) The poor solubility of nitrosodurene and pentamethylnitrosobenzene in acetonitrile did not allow us to test concentrations higher than 2 mM.
