Reactions of Cation Radicals of EE Systems (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33)
A. S.N. Murthy and C. N. R. Rao, Appl. Spectrosc. Rev., 2,69 (1968). A. Bondi, ref 1, p 376. G. R. Wiley and S. I. Miller, J. Am. Chem. Soc., 94, 3287 (1972). E. R. Keams, J. Phys. Chem., 65, 314 (1961). K. B. Whetsel and R. E. Kagarise, Spectrochim. Acta, 18, 329 (1962). T. Matsui, L. G. Hepler. and D. V. Fenby, J. Phys. Chem., 77, 2397 (1973). G. S. Denisov, Opt. Spectrosc., 11, 232 (1961). R. H. Ewell and L. M. Welch, J. Am. Chem. Soc., 63, 2475 (1941). T. S.Pang and S.Ng, Spectrochim. Acta, Part A, 29, 207 (1973). J. H. Gibbs and E. A. DiMarzio, J. Chem. Phys., 28, 373 (1958). A. J. Easteal and C. A. Angell, J. Phys. Chem., 74, 3987 (1970). G. C. Pimentel and A. L. McClellan, ref 6, p 265. M. Goldstein, J. Chem. Phys., 39, 3369 (1963).
1011 (34) M. Goldstein. J. Phys. Chem., 77,667 (1973). (35) J. H. Hildebrand and R. L. Scott, “The Solubility of Nonelectrolytes”, 3d ed, Dover Publications, New York, N.Y., 1964, Chapters IV and XI. (36) A. L. McClellan, “Tables of Experimental Dipole Moments”, W. H. Freeman, San Francisco, Calif., 1963. (37) H. Ratajczak, J. Phys. Chem., 76, 3000 (1972). (38) H. Ratajczak, J. Chem. Phys., 58, 911 (1973). (39) J. R. Hulett, J. A. Pegg, and L. E. Sutton, J. Chem. SOC.,3901 (1955). (40) V. C. G. Trew and G. M. C. Watkins, Trans. Faraday Soc., 29, 660 (1933). (41) T. J. V. Findlay and P. J. Kavanagh. J. Chem. Thermodyn., 6, 367 (1974). (42) M. Lecat. ”Tables Azeotropiques, Vol. I, Azeotropes binaires orthobares”, 2nd ed, Uccle. Brussels, 1949.
Reactions of Cation Radicals of EE Systems. IV. The Kinetics and Mechanism of the Homogeneous and the Electrocatalyzed Reaction of the Cation Radical of 9,lO-Diphenylanthracene with Hydrogen Sulfide’ John F. Evans and Henry N. Blount* Brown Chemical Laboratory, The University of Delaware, Newar!, Delaware 1971 1 (Received October 17, 1975)
Potential step spectroelectrochemical studies of the reaction of H2S with electrogenerated DPA-+ a t PtOTE’s on which PtS has been anodically formed prior to the pulsed experiment shows unusually fast consumption of cation radical during the initial stages of the puls?. Similar experiments using electrodes on which PtS has not been formed as well as those on which PtS has been formed and then cathodically discharged do not give rise to this behavior. On the basis of these experiments, it is concluded that HzS is adsorbed onto PtS where it reacts with electrogenerated DPA.+ and further, that the adsorption process is kinetically slow. Open circuit relaxation spectroelectrochemical experiments of varying pulse duration show that the consumption of DPA.+ is initially a zero-order process involving H2S adsorbed onto the PtS surface and, as this adsorbed material is depleted, the consumption of DPA.+ becomes a second-order process. In concert with the potential step experiments, these pulse time studies show the parallel nature of the heterogeneous and homogeneous reaction pathways. The zero-order rate constant for the heterogeneous process was evaluated from pulse relaxation experiments (by extrapolation to zero pulse duration) to be 1.51(f0.05) X M s-l. The bimolecular rate constant for the homogeneous process was evaluated from potential step and pulsed relaxation spectroelectrochemical measurements and stopped-flow kinetic spectrophotometry to be 6.6(&1.2) M-l s-l.
Introduction Reactions of the cation radical derived from 9,lO-diphenylanthracene (DPA), a representative E E system,2 have been addressed in several recent studies in t h e ~ e ~and -~ other7-12 laboratories. These investigations have shown that reactions of this cation radical (DPA.+) with various nucleophiles afford both addition and electron transfer products. In the course of examining the reactivity of various such “hard” and “soft” nucleophiles22with DPA-+, the reaction of this cation radical with hydrogen sulfide was investigated. It has recently been found that the oxidation of H2S at platinum electrodes in acetonitrile solutions gives rise to the formation of platinum sulfide surfaces on these elect r o d e ~In . ~view ~ of this, the mechanistic role of these anodically modified surfaces in reactions of electrogenerated DPA-+ with H2S was examined. In order to clearly delineate the extent of participation of the platinum sulfide sur-
face in the reaction of DPA-+ with H2S, parallel experiments utilizing stopped-flow kinetic techniques were conducted. This paper reports the results of these studies which afford the first spectroelectrochemica124observation of an electrocatalyzed reaction.
Experimental Section The sources of and purification procedures for acetonitrile, DPA, tetraethylammonium perchlorate (TEAP), and hydrogen sulfide have already been reported.z3 The test electrodes used in this work were vapor deposited platinum optically transparent electrodes (OTE) prepared after the manner of von Benken and Kuwanaz5 and platinum sulfide OTE’s prepared by anodic oxidation of platinum OTE’s as previously reported.z3 These electrodes were incorporated into an electrochemical cell of a type which has been previously described.26The potentiostat used in this work was similar in design to that reported by Pilla.27 The spectroelectrochemical apparatus has also been described.26 The Journal of Physical Chemistry, Vol. 80, No. 9. 1976
1012
John F. Evans and Henry N. Blount
Stopped-flow experiments employed a Durrum Model D-11OB stopped-flow kinetic spectrophotometer. In these experiments, DPA.+ was electrogenerated in an electrochemical cell external to the stopped-flow apparatus by the controlled potential ( E = +1.40 V) electrolysis of acetonitrile solutions of 0.10 M TEAP and the requisite concentration of DPA. These partially electrolyzed solutions were transferred via a closed system to one of the drive syringes of the stopped-flow spectrophotometer. The remaining drive syringe was filled with an acetonitrile solution containing 0.10 M TEAP and the desired concentration of hydrogen sulfide.28 Solutions of this latter reagent were prepared and assayed in the manner previously reported.29 A dedicated NOVA 1200 computer system (Data General Corp.) was used to control both spectroelectrochemical and stopped-flow kinetic reactors, as well as to acquire and reduce the data therefrom. The wavelength employed for all kinetic spectrophotometry was the ,A,, of DPA.+, 653.0 nm.30 All kinetic experiments were conducted a t 25.0(f0.2) "C. All electrode potentials are reported relative to the aqueous saturated calomel electrode. The stoichiometry of the reaction of DPA.+ with HZS was determined by measuring the amount of DPA produced relative to the amount of DPA-+ consumed. These measurements were effected by monitoring the absorbance of DPA at 392.5 nm and indicated that 97(&3)% of the DPA-+ consumed was reduced to DPA by the action of H2S.31Following macroscale reaction of electrogenerated DPA-+ with either gaseous H2S or H2S-saturated acetonitrile, the solvent was removed under vacuum and the residue extracted first with cold benzene and then with water to remove DPA and TEAP, respectively. The remaining residue was determined by mass spectral measurements to be oligomeric elemental sulfur containing up to eight atoms. Results The stoichiometry observed in this study reaffirms the electron transfer character of this reaction as originally reported by Sioda.20 That such an electron transfer process occurs following the selective oxidation of DPA in the presence of H2S is a manifestation of the relative magnitudes of the thermodynamically reversible electrode potentials ( E O ) of the two couples vis-a-vis the relative magnitudes of the observed discharge potentials of these redox couples. The potential a t which H2S undergoes oxidation is shifted anodically upon oxidative modification of the electrode surface (PtS formation)23whereas that of DPA is not as is shown in Figure 1. Cathodic discharge of this modified surface results in the return of the initial voltammetric response observed a t pristeen platinum.23 S i n g l e P o t e n t i a l S t e p Spectroelectrochemistry. Utilizing this technique, the kinetics of the reaction of DPA.+ with H2S were monitored by the in situ measurement of the time-dependent absorbance of the cation radical as the potential of the optically transparent electrode was stepped from a value (+0.60 V) at which the oxidation of DPA (eq 1)does not occur to a value (+1.40 V) at which this process DPA DPA.+ e(1)
-
+
+
2DPA.+ + H2S 2DPA S + 2H+ (2) proceeds at a diffusion-controlled rate.32 At pristeen platinum surfaces (free of PtS) the reaction between DPA-+ and H2S was observed to be first order in each reactant over a wide range of concentrations as shown in Figure 2. The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
+
+
I
100)~A
L
l
I
I
12
16
I
I
I
I
l
08 04 POTENTIAL, V
l
00
Flgure 1. Anodic voltammetric behavior of acetonitrile solution containing 1.0 mM DPA, 0.50 mM H2S, and 0.10 M TEAP at (A) pristeen platinum and (B) platinum sulfide electrodes. Sweep rate = 0.10 VIS, electrode area = 0.36 cm2.
I
0.0
1
-30 -2.0
I
1
I
I
I
-1.0 0.0 1.0 LOG(2,kt,Cz)
I
2.0
I
3.0
Figure 2. Dynamic single potential step spectroelectrochemical response for the DPA-HzS system at pristeen platinum electrodes. Solid line represents theoretical response predicted for lotally re, generative system.32 Symbols mark experimental data: . [DPA] = 28.0 ([DPA] = 1.05 mM); 0 , [H2S]/ DPA] = 62.1 = 1.13 mM); A, [H2S]/[DPA] = 93.7 ([DPA\ = 1.13 mM). TEAP present at 0.20 M in all cases. Bimolecular rate constant, k, is that given in eq 8. Cz is the molar concentration of HzS.
gfi;
It has been shown32 that for totally regenerative systems such as that represented by eq 1 and 2, the absorbance of the product of the electrode reaction (DPA-+) achieves a steady-state value at sufficiently long times of measurement, namely lim ADpA.+(A,t) = A D P A . + ( A )=~ ~ t-=
which is valid if the bulk concentration of HBS,COH~S,exceeds that of DPA, CODPA,by an amount sufficient to ensure pseudo-first-order reaction conditions. Time independent absorbances predicted by eq 3 were manifest in this present study at both pristeen platinum and platinum sulfide surfaces. Representative behavior is shown in Figure 3. When single potential step spectroelectrochemical experiments were carried out on anodically formed PtS surfaces, transient absorbance-time behavior was observed which was extremely dissimilar to that seen when pristeen platinum surfaces were used. Immediately upon application of the potential step perturbation to the platinum sulfide sur-
Reactions of Cation Radicals of EE Systems
1013
TABLE I: Kinetic Data for the Reaction of the DPA Cation Radical with Hydrogen Sulfide Determined by Single Potential Step SDectroelectrochemistry Electrode Method of 103[DPA], 103[HzS], surface evaluation M M
1 i tI
9
A
2.0 0.01
i
.J,
20 ~. 30
10
0
~
50
40
~
TIME, sec
Figure 3. Steady-state single potential step spectroelectrochemical response observed for DPA-H2S systems: (curve A) [DPA] = 1.13 mM, [H2S] = 106. mM, pristeen platinum electrode: (curve E) [DPA] = 1.05 mM, [H2S] = 29.5 mM, platinum sulfide electrode: (curve C) [DPA] = 1.05 mM, [H2S] = 15.5 mM, platinum sulfide electrode. TEAP present at 0.20M in all cases. I
0,O -30
I
I
I
I
5.27(&0.78) 1.05 29.5 Pt SPSU 70.2 7.62(&0.81) SPS 1.13 Pt 106 6.85(*0.68) SPS 1.13 Pt 29.5 6.35(&0.24) SPSb 1.05 PtS 70.2 6.92(&0.77) SPS 1.13 PtS 106 6.84(h0.86) SPS 1.13 PtS 15.5 4.42(&0.38) 1.05 Pt SPS/SSc 4.58(&0.35) 1.05 29.5 SPSISS Pt 70.2 6.35(&0.38) 1.13 Pt SPSISS 106 6.36(&0.31) 1.13 SPSISS Pt 15.5 4.11(&0.69) SPSISS 1.05 PtS 29.5 4.56(&0.67) SPSISS 1.05 PtS 70.2 6.56(&0.62) 1.13 PtS SPSISS 106 6.21(&0.39) 1.13 PtS SPSISS a Single potential step (SPS) dynamic response. Single potential step dynamic response following decay of surface reaction (see Figure 4). Single potential step steady state response according to eq 3. Bimolecular rate constant (eq 8).
I
I
0.0 10 LOG( 2,k'tCz)
-20 -1.0
k,d M-l s-l
2.0
3.0
Figure 4. Dynamic single potential step spectroelectrochemlcalresponse for the DPA-H2S system at platinum sulfide electrodes showing initial rapid consumption of DPA.+. Solid line represents theoretical response predicted for totally regenerative system.32 Symbols mark ex erimental data: I, [H2S]/[DPA] = 28.0 ([DPA] = 1.05 mM); A, fH2S]/[DPA] = 93.7([DPA] = 1.13 mM). TEAP present at 0.20 M in all cases. Bimolecular rate constant, k, is that glven in eq 8. CZ is the molar concentration of HnS. face, the absorbance of DPA.+ was abnormally low but grew with time to coalesce with the dynamic response observed at pristeen platinum. This behavior, shown in Figure 4, indicates a rapid consumption of DPA.+ immediately following potential step perturbation due to a local excess of reducing agent (H2S) in intimate proximity to the electrode. The coincidence of the longer time behavior reflects a surface-dependent phenomenon which decreases in significance with time and is not recurrent during the potential step. If, however, the solution is stirred or sufficient time allowed between potential step experiments to permit reequilibration of the P t S surface, then reproducible shorttime, rapid consumption of DPA.+ upon application of the potential step is again observed. The necessity of the anodically modified surface to the short-time behavior was demonstrated by reductively discharging the PtS at -0.70 V and then repeating the potential step oxidation DPA. The time-dependent normalized absorbance noted in such cases was identical with that observed at nonmodified platinum (Figure 2).
OD
-
0.0
0.1 0.2 TIME, sec
0.3
Figure 5. Open circuit relaxation spectroelectrochemical behavior of DPA-H2S system following pulse of short duration at platinum 1.05 mM, [HzS] = 29.5 sulfide electrode: 7 = 0.100 s, [DPA] mM, [TEAP] = 0.20M, coefficient of correlation = -0.9726. Kinetic parameters obtained from single potential step measurements at pristeen platinum, steady-state measurements a t both pristeen platinum and platinum sulfide, and single potential step measurements a t platinum sulfide surfaces following the decay of the surface phenomenon are summarized in Table I. In all cases, the rate of reaction was seen to be independent of DPA concentration. Open Circuit Relaxation Spectroelectrochemistry. The utility of this technique of examining the reactions of electrogenerated species has already been d e m o n ~ t r a t e d . l ~ * ~ ~ - ~ ~ This experimental method utilizes a potential step perturbation of controlled duration followed by disconnection of the working electrode thereby allowing the system to relax without e l e c t r ~ l y s i s In . ~ ~the present study this technique afforded a unique way of probing the nature of the surfaceassociated reaction occurring a t the onset of the potential step oxidation of DPA at platinum sulfide surfaces. Choice of short pulse times permits examination of the surface reaction under relaxation conditions while the relaxation beThe Journal of Physical Chemistry, Vol. 80, No. 9, 1976
1014
John F. Evans and Henry N. Blount TABLE 11: Kinetic Data for the Reaction of the DPA Cation Radical with Hydrogen Sulfide Determined by Open Circuit Relaxation at Platinum Sulfide Electrodesa 103[DPA], M
a
103[HzS],M
1.05 29.5 8.56(k0.56) 1.13 70.2 5.99(f0.19) 1.13 106 6.67(f0.52) 7 > 2 s. Bimolecular rate constant (eq 8).
TABLE 111: Kinetic Data for the Reaction of the DPA Cation Radical with Hydrogen Sulfide Determined by Stopped-Flow Kinetic Soectroohotometrv
-4.90
I I
0.0
J I
n
2.0
4.0 TIME.=
8.0
6.0
Figure 6. Open circuit relaxation’ spectroelectrochemical behavior of DPA-HpS system followin pulse of long duration at platinum SUIfide electrode: 7 = 10.0 s, IDPA] = 1.05 mM, [H&] = 29.5 mM, [TEAP] = 0.20 M, coefficient of correlation = -0.9868.
104[DPA],aM
103[HzS],M
4.96 4.96 4.96 1.40 0.50 0.50 [DPA.+]o N 5 X constant (eq 8).
k , b M-l s-1
140 5.62(k0.70) 82.5 7.38(&0.33) 69.6 8.55(&0.25) 88.0 6.42(&0.25) 69.6 7.71(&0.46) 27.3 5.60(f0.99) M in all experiments. Bimolecular rate
*
num sulfide surfaces and single potential step experiments a t pristeen platinum electrodes reflect the reaction of DPA.+ with H2S wherein the electrode per se is not a direct participant, the homogeneous reaction of DPA.+ with H2S was examined via stopped-flow kinetic spectrophotometry. Over a tenfold range of DPA-+/H2S concentration ratios, the reaction was observed to be the first order in both cation radical and hydrogen sulfide, and independent of DPA. The results of these experiments are shown in Table 111.
Discussion 40
60
80
100
7 ,sec
Figure 7. Dependence of the observed bimolecular rate constant for the reaction of DPA.+ with H2S(eq 8) on pulse duration for open circuit relaxation spectroelectrochemistry at platinum sulfide electrodes: [DPA] = 1.05 mM, [H2S] = 29.5 rnM, [TEAP] = 0.20 M.
havior following pulses of longer duration chiefly reflects the “bulk” reaction. Under open circuit relaxation conditions following short duration pulses a t platinum sulfide surfaces, the decay of the absorbance of DPA.+ was essentially linear with time indicating a predominantly zero order consumption of DPA.+ as shown in Figure 5. When pulses of considerably longer duration were employed, a first-order consumption of DPA.+ was in evidence during open circuit relaxation. Representative data are shown in Figure 6. As the pulse time, T, was varied from 0.100 to 10.0 s, the apparent first-order rate constant for the consumption of DPA-+ decreased with increasing pulse duration, reaching an invariant value at r > 1s as shown in Figure 7 . The attendant uncertainty in the fit of the data to firstorder behavior also decreased with increased pulse duration, corresponding to increased predominance of the firstorder process.36 Kinetic parameters derived from open circuit relaxation behavior following long duration pulses (7 > 2 s) a t platinum sulfide electrodes are shown in Table 11. Stopped-Flow Kinetic Spectrophotometry. In order to determine the degree to which the open circuit relaxation kinetic behavior following pulses of long duration a t platiThe Journal of Physical Chemistry, Vol. 80, No. 9, 1976
T h e Bulk Reaction. The coincidence with theory32 of both the transient and the steady-state absorbance behavior of DPA.+, electrogenerated in the presence of HZS, is indicative of a totally regenerative process in which the initial encounter of DPA.+ with reducing agent is rate determining. Since the conversion of H2S to elemental sulfur requires the release of two electrons per molecule of H2S oxidized, participation in the reaction sequence by a second DPA.+ is implied.37 Incorporation of this implication into a mechanism consistent with the observed kinetics and reaction products suggests the following reaction sequence DPA
-
DPA.+
DPA-+ + H2S DPA(SH2).+
+B
+
DPA(SH)* DPA-’ DPA(SH)+
+B
+ e-
(1)
k +
+
-
DPA(SH2)*+
+
DPA(SH)* BH+
+
DPA(SH)+
DPA
+ DPA
+ S + BH+
(4) (5)
(6) (7)
where eq 4 is rate determining and B represents a general base38 present in the acetonitrile s o l ~ t i o n . ~ Solution ~ , ~ O of the kinetic equations describing the processes embodied in eq 4-7 affords the rate law, d[DPA.+]/dt = -2k [DPA.+][HzS]
(8)
Alternatively, bond formation may not occur in the ratedetermining step (eq 4) and a mechanism of the following form may be tenable:
1015
Reactions of Cation Radicals of EE Systems
where eq 9 represents the rate-determining step. The overall reaction stoichiometry is maintained in this scheme, and with the stipulation that HS-BH+ is more reactive than H2S toward DPA.+, this reaction sequence also gives rise to the rate law expressed in eq 8. The applicability of either of these mechanistic pathways and the attendant rate law is demonstrated by the results derived from the three kinetic techniques shown in Tables I, 11, and 111. That single potential step spectroelectrochemical measurements at pristeen platinum electrodes as well as long time open circuit relaxation and potential step measurements a t platinum sulfide electrodes reflect the “bulk” process is shown by the agreement of the kinetic parameters arising from these electrochemical techniques (Tables I and 11) with those determined via a purely homogeneous method (stopped flow; Table 111).For 143 observations utilizing the three kinetic methodologies, the composite second-order rate constant (eq 8) was evaluated to be 6.6(&1.2) M-l s-l. The Surface Reaction. In considering the anomalously fast consumption of DPA-+ generated at the modified surface, it is necessary to characterize the nature of this ancillary reaction. There is a high local excess of nucleophile at the electrode surface which reacts with the DPA-+ produced in the initial stages of the pulse perturbation. Clearly this is not simply H2S adsorbed onto platinum since the anomalous kinetic effect is observed only after anodic modification of the electrode. The possibility of the P t S per se being the highly active reducing agent must also be considered. If this were the case, the PtS would be expended in reaction with DPA.+ which would reduce the PtS, thereby eliminating this heterogeneous reaction pathway. That this is not the case is shown both (a) by the presence of the cathodic wave for the reduction of the PtS surface (-0.70 V) following a potential step experiment yielding data of the type shown in Figure 4, and (b) by the reversible nature of the catalytic phenomenon. That is, when the solution is either stirred or allowed to stand for some time following the potential step experiment (ca. 60-100 s) to restore the initial conditions of the system (i.e., homogeneous distribution of the precursor; CDPA(x,~) = CODPA),a subsequent potential step spectroelectrochemical experiment again gives rise to this fast initial consumption of DPA-+. The continual repeatability of this sequence is indicative of adsorption of H2S onto the PtS surface and reaction therefrom, rather than reaction of the PtS layer. Hydrogen sulfide is well known to be strongly adsorbed on platinum s ~ r f a c e s . ~Yet l-~~ on pristeen platinum, it is not as reactive toward DPA.+ as it is on the PtS surface. Even though the PtS is not consumed in the reaction of the adsorbed H2S with electrogenerated DPA-+, it nonetheless participates in the reaction as a specifically active surface for the process in question. This kinetic demeanor may well be an effect of a preferred molecular orientation of H2S in the adsorbed state on PtS that renders it more reactive than the corresponding orientation in the adsorbed state on platinum. In this work the bulk concentration of H2S exceeded that of DPA by as much as 100-fold. It seems reasonable, then,
to ask why H2S, present in this much greater concentration, is not readsorbed from solution as the originally adsorbed material reacts. The most probable answer to this is that the adsorption of H2S onto PtS is a kinetically slow process. Such slow adsorption processes are known44,45and this phenomenon is substantiated in this case by the time required between potential step experiments to reestablish the conditions for initial fast reaction. We are now addressing two reaction pathways which can be represented as DPA.+
+ &Sads
-
---L
DPA-+ + H2Ssolv
products
(12)
products
(13)
where H2Sads and H2Ssolvrepresent the adsorbed and solvated H2S molecules, respectively. The surface reaction (eq 12) is of decreasing importance to the overall consumption of DPA-+ as the adsorbed H2S is reacted. If one assumes a limiting sequential model for these two pathways, namely, that the adsorbed H2S reacts before the solvated H2S as is suggested by the behavior shown in Figure 4, then the time required to generate sufficient DPA-+ to strip the surface of adsorbed H2S and consequently, the time a t which the reaction pathway described by eq 1 2 becomes inoperative, can be calculated from elementary considerations. Taking the distance between centers of adsorbed H2S molecules41 to be 4.5 A, one calculates the surface excess, rHzS, for monolayer coverage to be 8.2 X mol/cm2. From the working electrode area of 0.36 cm2, one further calculates that for 0 = 1.0, 3.0 X mol of H2S is in the adsorbed state. On the basis of the stoichiometry given in eq 2, twice this amount of DPA.: or 6.0 X mol of the cation radical must be generated for complete reaction with the adsorbed hydrogen sulfide. In a 1.0 mM solution of DPA, the time required for generation of the requisite amount of cation radical is calculated from an integrated form of the Cottrell equation and a knowledge of the diffusion coefficient of DPA in this medium11 to be ca. 175 ms. In order to evaluate the extent to which this limiting model of “adsorbed H2S reacting first” holds, open circuit relaxation spectroelectrochemical experiments were carried out on composite DPA-H2S systems. At pulse times, T , short compared to the calculated time, tcalcd, necessary for the production of the amount of DPA-+ required for complete reaction of the adsorbed H2S, the observed relaxation kinetics should primarily reflect the heterogeneous reaction. However, when T >> t&d, the observed relaxation kinetics should mirror the homogeneous process. While the first-order rate constant for the homogeneous reaction of DPA.+ achieves a constant value when the bulk reaction prevails (Figure 7), the corresponding zero-order treatment of the observed kinetic data should not give rise to invariant values of the zero-order rate constant (ho) a t short pulse duration. Rather the zero-order rate constant embodies the activity of the adsorbed hydrogen sulfide and this parameter changes with surface coverage. Assuming dominance of the heterogeneous reaction, the adsorbed layer is being removed with a t dependence in reaction with DPA-’ which, in turn, is being generated with a t1J2dependence. This means that the observed zero-order rate constant should be approximately linear in d I 2 , and extrapolation to T = 0 affords a measure of ko at 0 = 1.0. This T ~ de/ ~ pendence and attendant extrapolation are shown in Figure 8 and afford a ho value at T = 0 of 1.51(&0.05) X loa2 M s-1. The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
John F. Evans and Henry N. Blount
1016
h c \* 1 I
Conclusions
I
From these studies, it is concluded that hydrogen sulfide is adsorbed onto platinum sulfide and that this adsorbed material is more reactive than hydrogen sulfide adsorbed onto platinum toward the electrogenerated cation radical of DPA. Moreover, this adsorption of H2S onto PtS is a kinetically slow process. It is further concluded that the reaction of DPA.', electrogenerated at a platinum sulfide surface onto which hydrogen sulfide is adsorbed, proceeds via parallel heterogeneous (adsorbed H2S) and homogeneous (solvated H2S) pathways.
15'0 14.0
13.0
m-
!? 5.
r
12.0
11.0
'B
A
tt
10.0 Y
9.0
.\.\@
t
References and Notes
\-
0
(1) Presented at the 149th Meeting of the Electrochemical Society, Wash-
ington, D.C., May 1976. (2) "EE" systems are those species capable of undergoing two monoelec-
-
tronic oxidation (or reduction) processes, i.e.
A\
A
A*+ f e-
(€1)
A,+ -+ ~ 2 ++ e-
(€2)
>
6.0 L 0.0
I
0.5
I
1.0
1.5
Pulse duration dependence of zero-order rate constant derived from open circuit relaxation spectroelectrochemical experiments for the reaction of DPA-' with adsorbed H2S at platinum SUIfide electrodes: I, [H2S]/[OPA] = 28.0 ([DPA] = 1.05 mM); 0 , [H*S]/[DPA] = 62.1 ([DPA] = 1.13 mM); A, [H2S]/[DPA] = 93.7 ([DPA] = 1.13 mM). TEAP present at 0.20 M in all cases. Figure 8.
From Figure 7 , the pulse time corresponding to realization of a clean first-order decay of cation radical (second order overall) for relaxation experiments is approximately 1 s. Correspondingly, the time at which the expected second-order kinetic behavior (first order in radical ion) manifests itself during potential step experiments can be seen from Figure 4 to be approximately 2 s. The prolonged period required for the surface process to become an insignificant contributor to the observed kinetic behavior of this system provides clear indication of the parallel nature of the heterogeneous and homogeneous processes, rather than a stepwise "adsorbed first, solvated next" reaction sequence. The parallel nature of these processes is further supported by the data ensuing from the short pulse duration, open circuit relaxation experiments. Close examination of Figure 5 reveals some curvature present in this zeroorder plot, and this is taken as indication of a higher order contribution to the observed kinetics even for cases of short pulse duration. Upon cursory examination, it would seem that the short time behavior reflected in Figure 4 could be simulated in a straightforward fashion.34 Such is not the case because of the necessity of knowing the adsorption isotherm for HzS on PtS which dictates the functional dependence of K O on 8. Assuming a h e a r isotherm, a [I - (.th~&-J)"'] consumption of &Sads, and parallel nature of the homogeneous and heterogeneous processes, an approximate fit to the behavior shown in Figure 4 has been achieved. Although kinetic parameters in agreement with those shown in Table I for the homogeneous portion of the process can be extracted, the interrelation of the zero-order and second-order rate constants in such a simulation is open to question. The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
where €2 €1. (3) (a) H. N. Blount, J. Electroanal. Chem., 42, 271 (1973); (b) D. T. Shang and H. N. Blount, /bid., 54, 305 (1974). (4) J. F. Evans and H. N. Blount, presented at the 168th Natlonal Meeting of the American Chemical Society, Atlantic City, N.J., 1974, Abstracts of Papers, ANAL-7. (5) J. F. Evans and H. N. Blount, presented at the 169th National Meeting of the American Chemical Society, Philadelphia, Pa. 1975, Abstracts of Papers, ANAL-106. (6) J F. Evans and H. N. Blount, J. Org. Chem., 41, 516 (1976). (7) G. Manning, V. D. Parker, and R . N. Adams, J. Am. Chem. Soc., 91, 4564 - - 119691 --(8) V. D. Parke; and L. Eberson, Tetrahedron Lett., 2839 (1969). (9) V. D. Parker and L. Eberson, Acta Chem. Scand., 24, 3542 (1970). (IO) P. T. Kissinger and C. N. Reilley, Anal. Chem., 42, 12 (1970). (11) G. C. Grant and T. Kuwana. J. flectroanal. Chem., 24, 11 (1970). (12) H. N. Blount and T. Kuwana, J. €lectroanal. Chem., 27, 464 (1970). (13) R. Dietz and B. E. Larcombe, J. Chem. SOC.8,1369 (1970). (14) L. Marcoux, J. Am. Chem. Soc., 93, 537 (1971). (15) V. D. Parker, J. flectroanal. Chem., 36, App. 8 (1972). (16) 0. Hammerich and V. D. Parker, J. Electroanal. Chern., 38, ADD. .. 9 \
(1972).
C. J. Ludman, E. M. McCarron, and R. F. O'Malley, J. Electrochem. SOC.,119, 674 (1972). R. P. Van Duyne and C. N.Reilley, Anal. Chem., 44, 158 (1972). U. Svenholm and V. D. Parker, Acta Chem. Scand., 27, 1454 (1973). R. E. Sioda, J. Phys. Chem., 72, 2322 (1968). L. Papauchado, R. N. Adams, and S. W. Feldberg, J. Electroanal. Chem., 21, 408 (1969). R. G. Pearson, Ed., "Hard and Soft Acids and Bases," Dowden, Hutchinson, and Ross, Inc., Stroudsburg, Pa., 1973. J. F. Evans, H. N. Blount, and C. R. Ginnard, J. Electroanal. Chem., 59,
169 11975). (24) For a comprehensive discussion of spectroelectrochemistry, see T. Kuwana and N. Winograd in "Electroanalytical Chemistry", Vol. 7, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1974. (25) W. von Benken and T. Kuwana, Anal. Chem., 42, 1 1 14 (1970). (26) J. W. Strojek and T. Kuwana, J. Electroanal. Chem., 16, 471 (1968). (27) A. A. Pilla, J. Electrochem. Soc., 118, 702 (1971). (28) Supporting electrolyte must also be present in this reactant solutlon to prevent thermal effects and changes in refractive index upon mixing. (29) J. F. Evans and H. N. Blount, Anal. Lett., 7, 445 (1974). (30) At this wavelength, the value of eopA.+&pA"* em loyed for spectroelectrochemlcal measurements was 30.8 M-l which is In agreement with previously reported values (ref 1 1 ) . (31) The DPA absorbance at 392.5 nm is free from interferences by all other specles present. See ref 20 for details. (32) N Winograd, H. N. Blount, and T. Kuwana, J. Phys. Chem., 73, 3456
s-*I
(1969). (33) H. N. Blount, N. Winograd, and T. Kuwana, J. Phys. Chem., 74, 3231 (1970). (34) T. Kuwana, Ber. Bunsenges. Phys. Chem., 77, 858 (1973). (35) R. F. Broman. W. R. Heineman, and T. Kuwana, Discuss. Faraday SOC., 56, 16 (1973). (36) At pulse times greater than 1 s, the coefficient of correlation, R, for first-order klnetic fits was greater than 0.985 in all cases. (37) That the cation radical rather than the electrode functions as the ultimate electron receptor is further supported by the regenerative stoichiometry observed under purely homogeneous conditions. (38) B Is most likely water. In the most carefully purified acetonitrile, water is present in at least millimolar concentration. Although water is re-
1017
Fluorinated Benzene Derivatives in a High-Frequency Discharge ported to be a fiftyfold stronger base than hydrogen sulfide in acetonitrile [I. M. Kolthoff, Anal. Chem., 46, 1992 (1974)], at the higher concentrations of hydrogen sulfide used in this work, HzS may compete favorably with H20 as the deprotonatingagent, B. The identity of the base involved, however, is inconsequential to the proposed mechanism and the attendant kinetic assumptions. (39) Without question, the processes reflected in eq 5 and 6 involve some measure of reversibllity. This does not, however, compromise the ensuing klnetic treatment so long as these processes are fast relative to the ratedetermlning step. (40) The order in which the carbon-sulfur bond cleavage and deprotonation
steps occur in eq 7 is not clear. The process reflected in this equation may indeed be far more complex than depicted here, e.g., the formation of various polysulfides may well complicate this step. The only necessary stipulation for this process is the rapidity of the kinetics of regeneration of DPA relative to the ratedetermining consumption of DPA.+, eq 4. (41) H. P. Bonze1and R. Ku, J. Chem. Phys.. 58,4617 (1973). (42) J. M. Saleh, Trans. FaradaySoc., 67, 1830 (1971). (43) J. Wnard, Catal. Rev., 3, 93 (1969). (44) T. LouCka, J. €/ectroanal. Chem., 36, 369 (1972). (45) J. D. E. Mclntyre, private communication.
Emission Spectra and Chemical Reactions of Fluorinated Benzene Derivatives in a High-Frequency Discharge R. Gilbert and A. TheorBt* Physicaland Organic Chemistry Group, Materials Science Department, Hydro-Quebec institute of Research (iR€Q), Varennes, Quebec, Canada (Received November 11, 1975) Publication costs assisted by Hydro-Quebec Institute of Research
The emission spectra of fluorinated benzene derivatives excited by radiofrequency discharges were measured and compared with those obtained using other modes of excitation. In the spectra of C6H5F and 0-,m-, and p-Cs&Fz, the emissions were assigned to the fluorescence spectra of these molecules. Molecular emission spectra were never detected for C6HFb and C6F6 although emission bands due mainly to CFz radicals were observed. The reaction products were analyzed by infrared spectroscopy, gel permeation chromatography, gas chromatography, and mass spectrometry, and the results show that they are formed mainly of medium and high molecular weight compounds. The degree of polymerization is higher as the monomer passes from C&,F to C6F6 and, also, as the power applied to the discharge is increased. On the basis of the product analysis and electrical emission study, a general reaction mechanism was proposed involving interactions between excited molecules and fragmentation species. Finally an interesting relationship was found to exist between both the emission features and the degree of polymerization of these molecules in electrical discharges and fluorescence and triplet yield measurements reported by other authors.
Introduction
Many papers have already been published on the photochemistry of fluorinated benzene derivatives in the gas phase.1-8 The quantum yield and lifetime of the first singlet (SI) and triplet states (TI) of these compounds have been measured as functions of the exciting wavelength and the pressure. Analysis of the energy relaxation processes emphasizes the following interesting point: the value of the quantum deficit 1- (+sl +TJ, where +sland 4~~represent the luminescence quantum yield of the first singlet and triplet states, respectively, tends to increase with both the energy of the photons and the number of fluorine atoms on the benzene ring (Table I). The value of the quantum deficit gives an indication of the importance of photochemical processes over other modes of energy relaxation; in other words, photochemical reactions tend to become more important a t higher vibronic levels of the first electronic excited state and with higher degrees of fluorine substitution on the benzene ring. Thus it appears from these earlier studies that a large fraction of these compounds in their excited states use photochemical processes to release their excess energy by dissociation, isomerization, or polymerization.
+
Monofluorobenzene (CsH5F) has also been studied by controlled electron impact in the gas phase. Ogawa et a1.,9 in experiments carried out with an electron energy of 60 and 300 eV, have observed the fluorescence of C&=,F as well as the emission of species resulting from molecular fragmentation. Shukla et and Singh and Singh,ll for their part, have investigated the high-resolution emission spectra of monoand difluorobenzenes as excited by uncondensed transformer discharges and have made an exhaustive interpretation of the fine structure. Although most of the authors referred to above make occasional mention of the formation of isomers and polymers, very few investigations have been carried out to identify the specific reaction products resulting from the interaction of fluorinated benzene derivatives with particles of high energy. The purpose of the study presented in this paper, therefore, was to obtain such additional information. The paper deals particularly with the reactions of CeH5F, the isomers of difluorobenzene (0-,m-, and P - C ~ H ~ Fpentafluorobenzene ~), (C&F5), and hexafluorobenzene (C6F6) in a capacitively coupled radiofrequency (rf) plasma discharge.12 First, the emission spectra of these compounds excited by electrical discharges are compared with those obtained with The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
