J. Phys. Chem. 1988, 92, 6345-6351
6345
Raman Monitoring of Reactive Electrogenerated Species: Kinetics of Halide Addition to o -Quinones Richard T. Packard and Richard L. McCreery* Department of Chemistry, The Ohio State University, Columbus, Ohio 43216 (Received: January 20, 1988; In Final Form: April 4 , 1988)
The suitability of Raman spectroscopy for quantitative monitoring of solution species associated with electrochemical reactions was examined and applied to a previously unstudied reaction. With suitable instrumentation, resonance Raman scatterers generated at an electrode may be quantitatively monitored on a submillisecond time scale. However, laser beam absorption causes significant quantitative error which complicates kinetic analysis. Normal Raman spectroscopy is free of the absorption problem and is applicable to a wider range of chemical systems, albeit with slower response time due to smaller signal strength. The reactions of electrogenerated dopamine quinone were monitored with normal Raman spectroelectrochemistry with both multichannel and single-channel spectrometers. By observing spectral changes and single-wavelength transients during the oxidation of dopamine, it was possible to monitor two of the products of the addition of chloride and bromide to the electrogenerated quinone. Analysis of the concentration vs time profiles for these products permitted the rates and mechanisms of the reactions to be determined.
Introduction Raman spectroscopy has proven to be a useful and informative technique for the observation and identification of species associated with electrochemical processes, both on the electrode surface and in the adjacent solution.I4 Unlike most spectroelectrochemical techniques, the bulk of which employ UV-vis absorption to probe electrochemical events,s7 Raman spectroelectrochemistry (SEC)provides both structural information and the resolution necessary to allow monitoring of individual species in the electrochemical diffusion layer.*-14 The ability of the Raman technique to reveal dynamic and detailed molecular information in both aqueous and nonaqueous environments, and its compatibility with commonly used cell materials and optics have made Raman spectroscopy an attractive alternative to infrared absorption SEC methods.15,16 As with other SEC techniques, Raman SEC is not influenced by background electrochemical effects, such as charging current and surface Faradaic processes, which often plague conventional electrochemical measurements. As well as being a powerful analytical tool capable of supplying both thermodynamic and quantitative information, electrochemistry has the ability to generate a wide variety of stable and reactive species. The fate of these products can subsequently be monitored by electrochemical or spectroscopic means. A comparison can be made between this approach and stopped-flow experiments. The electrochemical method, however, provides a continuously tunable potential which can be carefully controlled to provide the proper amount of “reagent” needed to initiate the reaction. Furthermore, this ability to easily vary the potential circumvents the requirement of selecting a different chemical oxidant or reductant for each particular reaction. The difficulty of applying Raman spectroscopy to chemically reactive electrogenerated species is related primarily to the problem of obtaining an acceptable Raman signal intensity. The inherently weak nature of Raman scattering coupled with the dilute solutions encountered in electrochemical experiments demands either an extremely efficient detection system or a Raman enhancement mechanism, such as resonance or surface enhancement.Ig4 These conditions have limited the utility of Raman SEC as a probe of chemically reactive systems in solution to those cases where resonance enhancement O C C U ~ S . ~ ~ , ~ *Unfortunately, *~~ the wavelengths of electronic absorption bands which lead to resonance enhancement for many molecules of interest are not available with conventional laser sources and resonance Raman SEC has been applied mainly to visible absorbers. Therefore, it is desirable to develop a Raman SEC technique with sufficient sensitivity to relieve the requirement of resonance enhancement.
* Author
to whom correspondence should be addressed.
0022-36S4/88/2092-6345$01.50/0
Recently, we demonstrated a technique capable of obtaining transient Raman spectra of electrogenerated normal and resonance Raman scatterers with time resolution less than 100 ms, and capable of observing the time-dependent concentration of normal Raman scatterers by single-wavelength monitoring starting at a few milliseconds.8 In this report, we will show how this technique can be used to obtain structural, mechanistic, and kinetic information about reactive electrochemical systems. The results are logically separated into three general areas: the quantitative effect of laser beam absorption on resonance Raman intensity transients, the acquistion of time-resolved spectra for the reaction of electrogenerated dopamine-0-quinone with halides, and deduction of reaction rates and mechanisms from normal Raman intensity transients.
Experimental Section The details of the electronic and optical configuration used for both spectral acquisition and single-wavelength monitoring as well as the system’s characteristic sensitivity have been previously reported.8 At the heart of the system is a lowf/number (f/4.2), (1) Van Duyne, R. P. In Chemical and Biological Applications of Lasers; Moore, L. B., Ed.; Academic: New York, 1979; Vol. 4, Chapter 4. (2) Cooney, R. P.; Mahoner, M. R.; McQuillan, A. J. In Aduances in Infrared and Raman Spectroscopy; Clark, R. J., Hester, R. E., Eds.; Heyden: London, 1982; Vol. 9, Chapter 4. (3) Fleischmann, M.; Hill, I. R. In Comprehensiue Treatise of Electrochemistry; White, R. E. et al., Eds.; Plenum: New York, 1984; Vol. 8, Chapter 6. (4) Efrima, S. Mod. Aspects Electrochem. 1985, 16, 253. (5) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; Chapter 14. (6) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 1. (7) McCreery, R. L. In Physical Methods in Chemistry; Rossiter, B., Ed.; Wiley: New York, 1986; Vol. 2, p 561. (8) Packard, R. T.; McCreery, R. L. Anal. Chem. 1987, 59, 2631. (9) Schwab, S. D.; McCreery, R. L.; Gamble, F. T. Anal. Chem. 1986,58, 2486. (10) Jeanmaire, D. L.; Van Duyne, R. P. J . Electroanal, Chem. 1975,66, 235. (1 1) Jeanmaire, D. L.; Suchanski, M. R.; Van Duyne, R. P. J . Am. Chem. SOC.1975, 97, 1699. (12) Van Duyne, R. P.; Haushalter, J. P. J . Phys. Chem. 1984, 88, 2446. (13) Clarke, J. S.; Kuhn, A. T.; Orville-Thomas, W. J. J . Electroanal. Chem. 1974, 54, 253. (14) Van Duyne, R. P. J . Phys. (Paris) Colloq. 1977, 5, 239. (15) Pons, B. S., et al. In Electroanalytical Chemistry; Bard, A. J., Ed.: Marcel Dekker: New York, 1986; Vol. 14, p 310. (16) Pons, B. S.; Bewick, A. In Advances in Infrared and Raman Spectroscopy: Clark, R. J., Hester, R. E., Eds.; Heyden: London, 1985; Vol. 12, Chapter 1. (17) Suchanski, M. R.; Van Duyne, R. P. J . A m . Chem. SOC.1976, 98, 250.
0 1988 American Chemical Society
6346 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
Packard and McCreery
high throughput (transmission = 32% at 515 nm) Instruments SA 640 spectrometer. A PARC OMA HI/1421B, 1000-channel intensified diode array was used to record static and transient spectra, while a cooled RCA 31034 photomultiplier tube was used for single-wavelength monitoring. In all experiments, the input beam from a Coherent Innova 70 Ar' laser reflected off of the working electrode at an angle of 86' to the surface normal, with the input light polarization parallel to the electrode surface to avoid ZOkHz suppressing the Raman intensity from polarized bands. For the case of the OMA experiments, the laser spot on the electrode slightly overfilled the detector, while for single-wavelength experiments with the PMT, the effective area of the spectrometer was slightly underfilled. As pointed out p r e v i o ~ s l y ,the ~ * ~relationship of laser spot size to active spectrometer area affects the behavior of the Raman signal. Transient spectral acqusition was accomplished by utilizing the 00 1.0 20 30 40 50 software provided with the PARC OMA IIL8 Potential step initiation was provided by the output trigger of the OMA 111, and tlnm l..Cl the pulse duration was determined by a Tektronix Model PG 507 Figure 1. Transient Raman intensity of the 1126-cm-l band of CPZ'+ pulse generator. The pulse controlled a PAR Model 173 pogenerated by potential pulses from 0.2 to 0.85 V vs Ag/AgCl. The solid tentiostat, which applied a potential pulse to the electrochemical line is the simulated result assuming no input light absorption by CPZ'+. Points are the actual values. CPZ concentration was 5 mM in 1.0 M cell. Spectra of predetermined integration periods were then HCI in 42% (w/w) MeOH/H20. Input laser beam was 20 mW at 458 obtained at set intervals after the initiation of the potential pulse. nm. Ten transients were averaged; spectral resolution was 7 cm-l. Signal averaging was accomplished by repeating this process as many times as desired, with the duty cycle for the applied potential dependent upon a large number of variables. For the case of never exceeding 3%. The PARC 1421B detector was cooled to diffusion-limited generation, the number of scatterers encountered -20 OC and only the middle 600 channels were scanned to relieve by the incident beam can be readily determined by Fick's laws memory constraints and to shorten the minimum integration of diffusion. It has been shown9 that the Raman scattering, IR period. (photons s-I sr-l), for the present case is given by Single-wavelength monitoring was done by using an EG&G Ortec ACE multichannel analyzer (MCA) to examine the Raman 4PoPNb(Dt)'12 intensity as a function of time after a potential step. At the start IR = (1) ??'I2 cos ff of a transient, an output pulse was generated by the MCA to trigger the pulse generator, which then applied a potential pulse where Pois the input beam power, photons SKI,fi is the differential to the cell in the same manner as used for transient spectrum Raman cross section, cm2 molecule-' sf-', Nb is the bulk number acquisition. The output of the RCA photomultiplier tube was density of starting material, molecules ~ m - D~ is , the diffusion passed through a pulse height discriminator and then read directly coefficient of starting material, cm2 s-I, t is the time after the into the MCA. The MCA recorded the number of photons arpotential step, s, and a is the incident beam angle relative to the riving at the detector during a series of variable-width time surface normal. windows, as performed previously by Van DuyneIo and o ~ r s e l v e s . ~ ~ ~ Several assumptions are made when deriving eq 1. First, it is The duty cycle could be adjusted by varying the number of assumed that ideal planar diffusion occurs to and from the channels acquired per pass, with an upper limit of 4096 channels. electrode surface, with the generation of a stable product at a To correct for any changes in broad-band emission, the time profile diffusion-limited potentiai. Second, any anisotropy of the Raman of a wavelength adjacent to a Raman peak was recorded in a scattering from the solution species is ignored. Third, constant separate experiment and subtracted from the Raman transient. laser irradiance both across the beam and through the sample is For cases where Raman bands of two species overlap, a given assumed, implying no absorption of the incident beam. For most transient was corrected for the contribution of an adjacent peak cases where normal Raman scatterers are generated, these asby subtracting a fraction of the transient for the adjacent band. sumptions are completely valid, with the result being ideal behavior The fraction to be subtracted was determined by assuming the as predicted by eq 1. However, when resonance Raman scatterers Raman bands were symmetric and noting the intensity of the are formed, the last assumption can easily be violated due to interfering band on its side farthest away from the overlap region. absorption of the beam by the resonantly enhanced species. Figure This approach proved sufficient when overlap was not severe or 1 shows the effect of absorption on the intensity vs time plot for when the band of interest was the larger, as was the case for the an electrogenerated resonance Raman scatterer. Ideal behavior, systems studied. as predicted by eq I , would be expected to have a t1I2dependence. The working electrode was fashioned from a Bioanalytical However, it is clear that beam attenuation severely affects the Systems (West Lafayette, IN) platinum-disk voltammetry elecobserved intensity, especially at longer times when absorption is trode of 1.5 mm diameter. Polishing of this electrode to a large. high-quality flat surface was accomplished by securing the The extent of this attenuation is determined by the time of electrode in a 2h-diameter stainless steel cylinder and following electrochemical generation, t , the initial concentration of starting a previously reported procedure.I* Chloropromazine hydromaterial, @, the molar absorptivity of the enhanced species, e, chloride (CPZ) and 3-hydroxytyramine hydrochloride (dopamine, and the incident angle of the laser beam, a,and thereby the optical H,DA) were obtained from Sigma Chemical Co. All reagents path length through the diffusion layer. An analytical solution were used as received, but solutions were passed through a O.l-fim to the magnitude of the intensity attenuation is not easily obtained filter (Millipor; Corp.) prior to use to reduce scattering from small due to the coupling of the electrochemical diffusion process to the insoluble particles. absorption effect. However, one can simulate the Raman SEC response by using standard explicit finite difference method^.'^,^^ Results and Discussion
I
I/
Y
The Raman intensity of a scaterer generated by a potential step and monitored by a beam reflected off an electrode surface is (18) Jan, C. C : Lavine, B K.; McCreery, R. L. Anal Chem. 1985, 57, 752
t 19) Feldberg, S. In ElecfrounalyficalChemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1969; Vol. 3, p 199. (20) Bard, A. J.; Faulkner, L. R. Electrochemical Mezhods; Wiley: New York, 1980; pp 675-698. ( 2 1 ) Mayausky, J . S. Ph.D. Thesis, The Ohio State University, 1982.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6347
Kinetics of Halide Addition to o-Quinones
............ .............. . . ....... 7==.19
. . a .
.......... .................................................................................. .
.
.
7= I
f l 2 I
1
0.5
0.0
1.0
1/2
( t ime/t tot 1
Figure 2. Effect of y upon linear t 1 / 2behavior. Solid lines are simulated results; points are experimental. For experimental data, the following values were used to calculate y: c = 3000 M-I cm-',21 cb = 5.0 mM, D = 4.1 X lod cm2 s-1,22 t,,, = 5 s (y = 0.96) or ttot = 0.20 s (y = 0.19), and (Y = 86'. Solution conditions were the same as those in Figure 1.
-2 1
-1 5
- 0'9
-0'3
0'3
+
0.4
0.8
0
10g11~~1
Figure 3. Transient intensity of the 1126-cm-l band of CPz'+ corrected for absorption by using the values of y calculated as in Figure 2. Data are accumulated from three experiments over the ranges of 50 p s to 5 ms (average of 15000 transients), 2 to 200 ms (300 transients), and 50
ms to 5 s (10 transients). Solution conditions were identical with those in Figure 1.
By using this type of simulation, one can estimate the fractional concentration of any given species within a layer of solution at a given distance from the electrode surface. The laser power attenuation in a layer with thickness A x is given by the Beer's law absorption p = pJO.1O-CAx/cOs J (2)
Potential ( V vs Ag/AgCI)
Figure 4. Cyclic voltammetry of 10 mM H3DA in 3.0 M HCI. Scan rate is 1.8 V/min.
in that layer, C j / @ , DMA is the dimensionless diffusion coefficient, and L is the number of simulation iterations, then by utilizing the relationship20
(3) the change in laser power in layer j , AP,, can be obtained from eq 2 as
OL
where P, is the attenuated laser power exiting layer j, PJois the laser power incident upon layer j, and C, is the concentration of absorber in layer j. If P, is the fractional concentration of absorber (22) Mayausky, J . S.;McCreery, R. L. Anal. Chem. 1983, 55, 308.
-2.303Fj~@(Dt)'/~ APj =
(DMA.L)'/~ cos a
(4)
It is clear from eq 4 that the beam attenuation, and therefore the deviation of the Raman intensity from ideal behavior, is governed by the dimensionless parameter y = t@Dt)1/2/cos a. The effect of y on the linearity of the I vs t l / * plot is shown in
6348 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
I
r'
Packard and McCreery
I
MM II
A
I I
II
I
I
I
I
1450
1500
1550
1600
J I
I
1850
I
1700
Raman shift (cm-') Figure 5. Raman spectra of 20 mM HjDA oxidation at 100 ms after a 0.2-0.85 V vs Ag/AgCl potential step. Curve A was obtained in 1.0 M HBr, curve B in 1.0 M HCI. and curve C in 1.0 M HCIOd. A water band at 1636 cm-' obtained from a blank was subtracted from each curve. Laser power was 75 mW, 488 nm. Spectral band-pass was 7.5cm-I.
Figure 2. It is apparent that enhancement gained in Raman intensity from the absorption required for resonance Raman can be detrimental to a well-behaved SEC response. The deviation plot can be corrected if the magnitude from linearity of the I vs of y is known. A correction of this type was applied to the data obtained from a CPZ oxidation experiment, with the results shown in Figure 3. The correction was accomplished by dividing the Raman intensity at a given time by the fractional deviation from linearity predicted by the digital simulation. Thus, the response can be accurately corrected over 5 orders of magnitude in time, extending from the lower end ( 6 0 N) where slow electrochemical response occurs, to the upper end (>5 s) where solution convection eventually limits ideal diffusion. However, for this technique to be useful as a tool for obtaining accurate quantitative information about an electrogenerated resonance Raman scatterer, c and D must be known. This may not always be possible, particularly when dealing with a chemically reactive species. Moreover, even if the magnitudes of c and D are known, absorption complicates the Raman intensity transient for a reactive species and may prevent accurate extraction of kinetic results. The problem is much simpler if laser absorption does not occur and a correction is unnecessary. In fact, there is a wider range of electrogenerated species which do not absorb light than those that do. However, the absence of resonance enhancement requires greater sensitivity to compensate for the lower scattering cross section. As shown later, such sensitivity is available for species with greater than millisecond lifetimes.
I I500
1600
500 Hz
1700
Raman shift (ern-')
Figure 6. Transient Raman spectra of H,DA oxidation in a solution initially containing 20 mM H3DA in 1 M HBr. The potential pulse is 0.&0.85 V vs Ag/AgCl with a duration of 0.5 s. Spectral conditions are identical with those in Figure 5. Time after potential step: (a) 0 ms; (b) 0-50 ms;(c) 100-150 ms; (d) 200-250 ms; (e) 400-450 ms. Origin of Y axis was displaced upward for successive spectra. Exposure width was 50 ms.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6349
Kinetics of Halide Addition to o-Quinones
The ability of normal Raman SEC to obtain spectra of intermediates and products associated with a reactive system was demonstrated previouslyEfor the case of dopamine oxidation. The electrochemical behavior of catecholamines and their derivatives has been the focus of a large amount of research over the past 20 years.23 It is well-known that the initial product of dopamine (H3DA) oxidation, the o-quinone (HDOQ), is a reactive species which can readily undergo nucleophilic a t t a ~ k . * ~While - ~ ~ the most common nucleophilic addition is that of dopamine's own side-chain amine, the ring is also susceptible to intermolecular nucleophilic attack. Based on a previously reported mechanism for the addition of HC1 to electrogenerated catechol-~-quinone,~~J~ the addition of HX (X = C1, Br) to H3DA would be predicted to occur via eq 5-8, where H2DAX corresponds to 6-halodopamine
+ 2e- + 2H+
H3DA s HDOQ HX
+ HDOQ
k2
H2DAX s DOQX H3DA
+ DOQX
(5)
H2DAX
(6)
+ 2e- + 2H+
(7)
4
K
HDOQ
+ H2DAX
k = 4 s-'
(8)
and DOQX to the substituted o-quinone. Raman SEC is particularly suited to studying the dopamine oxidation and halide addition mechanism because of the selectivity one can obtain by monitoring Raman bands associated with individual species involved in the mechanism. A cyclic voltammogram of 2.0 mM dopamine in 3 M HCI is shown in Figure 4. The absence of a second set of redox peaks and the qualitative similarity between this voltammogram and one obtained in 3 M HC104 indicate a very small difference in the redox potentials of H3DA and H,DACl. The application of standard electrochemical methods to this mechanism is limited due to the overlap of the oxidation potentials. Hawley et al.27 were able to obtain values for k2 by using the chronoamperometric technique of Alberts and Shain2*in the case of 4-methylcatechol; however, differences in K are difficult to distinguish electrochemically since K is related directly to the relative redox potentials of the two couples. Similarly, UV-vis SEC is not expected to be a useful probe for this mechanism because of overlapping electronic absorption bands of the unsubstituted and halogenated quinones. By taking advantage of the inherent selectivity of Raman SEC, on the other hand, information concerning both k2 and K is available. Figure 5 shows the results of a time-resolved experiment in which a potential 200 mV past the oxidation peak potential for dopamine was applied to a platinum electrode in a solution of 20 mM dopamine. The spectra shown in Figure 5 were all taken at times less than 150 ms after the positive potential step. Clearly, in the range of 1400-1800 cm-I, there are peaks from more than one species present: the C=C and C=O stretching frequencies of the o-quinone at 1574 and 1672 cm-I, of the chloroquinone at 1548 and 1693 cm-I, and of the bromoquinone at 1540 and 1695 cm-l, respectively. In 1 M HClO.,, the Raman peaks of the o-quinone at 1572 and 1672 cm-l grow with a t l l z dependence, indicating a stable species on this time scale. As shown in Figure 6, however, in 1 M HBr, the 1540-cm-' peak increases rapidly with time at the expense of the 1572-cm-' peak. At short times (
