883
Anal. Chem. 1991, 63,883-885
Electron Paramagnetic Resonance Imaging of Electrochemically Generated p-Benzosemiquinone Radicals Minoru Sueki,Sandra S. Eaton, and Gareth R. Eaton* Departments of Engineering and Chemistry, University of Denver, Denver, Colorado 80208
Spectral-spatial EPR Imaging was used to determine the spatial distribution and spatlally varying linewidths for p benzosemiqulnone radicals that were generated electrochemlcaily In an EPR cavlty. When p-benzosemlquinone was produced in DMF by reduction of p-benzoqulnone, the line width of the EPR signal on the side of the electrode toward the bulk solution was increased by rapld electron transfer between the radlcal and the hlgher concentration of parent p-benzoqulnone on this slde of the electrode. Generation of p -benzosemlquinone from hydroquinone was demonstrated to occur in a two-step mechanism with reduction of p benzoquinone detected at the auxlllary electrode when the working electrode was set at 1.43 V vs an Ag-wire reference electrode.
-
INTRODUCTION Simultaneous EPR/electrochemistry techniques permit observation of radicals produced in electrochemical processes (1-4). The EPR spectrum that is observed is the “average” spectrum for all regions of the cavity. Recently, it has been shown that spectral-spatial EPR imaging permits detection of the spatial variation of the intensity and line shape of the EPR signal (5, 6), which can provide greatly increased information about radical generating processes (7). The images reported in this paper have one spatial dimension that is parallel to the external magnetic field. EPR spectra are recorded a t a series of magnetic field gradients that correspond to projections a t varying angles in the spectral-spatial plane (Figure 1). a = 0’ is defined as zero gradient, and N = f90’ corresponds to infinitely large gradient. The magnetic field sweep a t each angle, a,is given by sweep width = f i ~ ~ / c oas
(1)
The image is generated from the projections by convoluted back-projection (8). The length of the spatial axis, L, is related to the length of the spectral axis (AH), the maximum angle at which data are collected (a,), and the maximum gradient (Gmax) by L = (tan a m m ) A H / G m a x (2) For a given maximum gradient, the resolution of the image is improved by using a set of projections that does not span a full 180’ and by using an iterative “missing angle” image reconstruction algorithm to generate the image (8). The EPR spectra of semiquinone radicals have been studied extensively (9). Detailed studies of the quinone-semiquinone-hydroquinone system, especially the sensitivity of the processes to the concentration of hydrogen ions, have revealed exquisite complexity of the redox processes of these biologically important species (10-12). In aprotic solvents there is a well-behaved one-electron reduction that corresponds to Q e- Q(3) Although p-benzosemiquinone is readily prepared by air oxidation of alkaline solutions of hydroquinone, the generation
+
-
of p-benzosemiquinone in aprotic solvents is thought to occur by a two-step mechanism:
QH,
-
Q + 2H+ + 2e-
Q + e-
-
(4)
Q’
We are not aware of simultaneous EPR and electrochemical studies of the oxidation of hydroquinone. Furthermore, the distinction between direct oxidation to the p-benzosemiquinone and indirect formation of p-benzosemiquinone via p-benzoquinone can be made readily by EPR imaging.
EXPERIMENTAL SECTION Apparatus. The electrochemical cell that was used for these experiments in shown in Figure 2. The internal dimensions are 3.5 X 1.0 X 0.04 cm. It was oriented in the cavity with the 1.0-cm dimension along the direction of the magnetic field, which is also the direction along which the magnetic field gradient was generated. The working and auxiliary electrodes were 0.25 mm diameter Pt wires. The reference electrode was 0.1 mm diameter Ag wire. Electrodes were repositioned in the cell for each measurement, which caused some variability in the distances between the electrodes. The electrochemistry was controlled with a Bioanalytical Systems CV-27 instrument. The oxidation potential for ferrocene in DMF in this cell was +0.96V. The cell was purged with nitrogen gas to avoid oxygen broadening of the EPR spectra and to increase the lifetimes of the radicals. EPR spectra were recorded on a Varian E9 spectrometer. The experimental conditions for data collections were 7-mW microwave power, 0.1-G magnetic field modulation at 100 kHz, and a time constant of 0.016 s. The magnetic field gradients were generated with a Hewlett Packard HP601OA power supply and anti-Helmholtz coils mounted on the magnet pole faces as described previously (13). A current of 2 A generated a gradient of 200 G/cm. Gradient control and magnetic field sweeps were performed with a VAXstation I1 and Stanford Research SR245 computer interface. Reagents. Dimethylformamide (DMF) was pursed by vacuum distillation and stored over 4A molecular sieves. p-Benzoquinone (Eastman) and hydroquinone (Mdinckrodt) were recrystallized from 90% ethanol/water until the melting points were constant. Tetrabutylammonium perchlorate (Eastman) was used as received. Solutions were 0.2 M in supporting electrolyte (tetrabutylammonium perchlorate) and 10 mM in p-benzoquinone or hydroquinone. The average equilibrium concentration of radicals was less than lW3 M, as judged by the intensity of the EPR signals, although, as illustrated below, there was a concentration gradient in the cell. Freshly prepared solutions were used for all measurements. Procedure. p-Benzoquinone was reduced at a potential of -0.72 V vs the Ag-wire reference electrode. Hydroquinone was oxidized at a potential of +1.43 V vs the Ag-wire reference electrode. The potential was applied for about 15 min prior to the start of data collection to achieve a steady-state distribution of p-benzosemiquinone radicals in the cell. The time for each scan was 13 s. To permit time for heat removal from the gradient coils, which are heat-sinked to the poles of the magnet, there is a variable waiting time between scans. The longest wait occurs after the highest gradient scans. The waiting time added about 50% to the total experiment time. Two types of data sets were collected with either (a) 128 points/projection, 62 experimental projections, and 2 “missing” projections or (b) 256 points/projections, 124 experimental projections,and 4 “missing”projections. Images were reconstructed on the VAXstation I1 with 5 cycles
0003-2700/91/0363-0883$02.50/00 1991 American Chemical Society
884
ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991
spatial
I I
9c
A
spectral
Flgure 1. Sketch of the pseudoobject with dimensions AH X L that is examined in the spectral-spatial imaging experiment and definition of the angle a.
0.2 cm H
Flgwe 3. (A) First derivative of row 17 from the image shown in Figure 2. (B) First derivative of row 36 from the image shown in Figure 2. (C) Integral of columns 48-75 for each row of the image shown in Figure 2, which corresponds to the spatial distribution of the EPR signal.
- 1 4 G Figure 2. Spectral-spatial EPR image of p-benzosemiquinoneradicals
generated electrochemically from p-benzoquinone. The image was reconstructed iteratively from 62 experimental and 2 “missing” projections on a 128 X 128 grid. The maximum gradient was 200 G/cm. A sketch of the electrochemical cell is included. A, W, and R denote the auxiliary, working, and reference electrodes, respectively. The spacing between adjacent pairs of electrodes was 0.3-0.4 cm. of the iterative algorithm (8)and a Hamming filter parameter set to 0.8.
RESULTS AND DISCUSSION A spectralspatial image of the p-benzosemiquinone radicals generated by reduction of p-benzoquinone is shown in Figure 2. To facilitate image reconstruction, the spectral dimension is presented as the absorption spectrum rather than the first-derivative display that is traditionally used in EPR. The characteristic five-line p-benzosemiquinone spectrum is observed in the vicinity of the working electrode. The ridges that run top-to-bottom through the image are artifacts that arise from changes in the EPR signal intensity during the time required to collect a complete set of data. Slices through the image permit closer examination. The first derivatives of rows 17 and 36 from the image in Figure 2 are shown in Figure 3A,B, respectively. The peak-to-peak line widths of the p-benzosemiquinone signals are 0.63 G for row 17 and 0.93 G for row 36. These line widths are typical of values observed for the two sides of the electrode. Row 17 corresponds to a location in the limited volume between the working electrode and the end of the cell. Row 36 is on the
side of the working electrode toward the bulk of the solution. Electron transfer occurs between the p-benzosemiquinone radical and the parent p-benzoquinone. At sufficiently high concentrations of p-benzoquinone, electron transfer can be fast enough to cause broadening of the p-benzosemiquinone EPR spect,rum. The difference in line widths for the signals on the two sides of the electrode indicates that the p-benzoquinone concentration is different in the two regions. The narrow line width in row 17 is presumably due to partial depletion of the p-benzoquinone in the restricted volume between the working electrode and the nearby end of the cell. Due to the spatial variations in line widths of the EPR signals, a single column from the image does not reflect accurately the spatial distribution of signal intensity. Columns 48-75 encompass the central line of the spectrum. Integration of these columns as a function of location in the sample is displayed in Figure 3C. The concentration of radicals is comparable on the two sides of the electrode and falls off rapidly at greater distances from the electrode. The separation between the two maxima in the concentration profile is greater than the diameter of the electrode. Small deviations of the electrode orientation from precisely vertical in the cavity contribute to this discrepancy. Also, it has been observed that the presence of a metallic electrode in the cavity perturbs the 100-kHz magnetic field modulation, which decreases the intensity of the observed signal adjacent to the electrode along the direction of the external field (14). This effect would also increase the size of the region, along the spatial axis of the image, in which the decreased EPR signal is observed. When hydroquinone was oxidized in DMF a t a potential of 1.43 V vs the Ag-wire electrode, the spectral-spatial image (Figure 4) showed negligible signal intensity in the vicinity
ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991
W R
885
A
a1 4 G
-
Figure 4. Spectral-spatial EPR image of p-benzosemiquinone radicals generated electrochemically from hydroquinone. The image was reconstructed iteratively from 124 experimental projections and 4 “missing” projections on a 256 X 256 grid. The maximum field gradient was 200 G/cm.
of the working electrode. Instead, the intensity was localized in the vicinity of the auxiliary electrode. Due to the high impedance of the solution, when the working electrode was at +1.43 V, the auxiliary electrode was at about -0.9 V, which is the potential needed to reduce p-benzoquinone to pbenzosemiquinone. Thus the p-benzosemiquinone is produced from the p-benzoquinone, and not directly from the hydroquinone, which is consistent with the two-step mechanism in eqs 4 and 5. An alternate explanation might be that the EPR signal was due to an impurity of p-benzoquinone in the hydroquinone solution. However, a cyclic voltammogram of the hydroquinone solution lacked the reduction wave expected for p-benzoquinone. The two-step mechanism also is supported by the observation that immediately after the potential was applied, the EPR signal was weak but increased within about 5-6 min to an approximately constant value. This observation is consistent with the need for elapsed time to permit diffusion/ convection of the p-benzoquinone from the working to the auxiliary electrode. By contrast, when the p-benzosemiquinone was produced directly from p-benzoquinone, the strongest signal was observed immediately after the potential was applied. To confirm the impact of the large potential drop between the auxiliary and working electrodes, an image was obtained for a p-benzoquinone solution with the working electrode potential set for oxidation at 1.43 V. Again the p-benzosemiquinone was produced at the auxiliary electrode. The first derivative of a spectral slice through the image in Figure 4 is shown in Figure 5A. The narrow lines indicate little impact of electron transfer between p-benzoquinone and p-benzosemiquinone, which is consistent with the two-step mechanism and low p-benzoquinone concentrations. A spatial slice through the image is shown in Figure 5B and shows the
Figure 5. (A) Spectral slice through the image shown in Figure 4. (B) Spatial slice through the image shown in Figure 4 at the magnetic field that corresponds to the peak of the central line of the p-benzosemiquinone spectrum. This slice represents the spatial distribution of the EPR signal.
localization of the p-benzosemiquinone radical at the auxiliary electrode.
CONCLUSION Spectral-spatial EPR imaging permits detection of spatial variation in intensity and line widths of electrochemically generated radicals. This technique provides a new dimension in simultaneous EPR/electrochemistry measurements. Heat removal from the gradient coils in our current setup increases the length of time required for data collection. Work is underway to speed up the data collection and improve heat removal from the gradient coils and to minimize various artifacts in the images.
LITERATURE CITED Maki, A. H.; Geske, D. H. J. Chem. Phys. 1959, 30, 1356-1357. Gale, R. J., Compton, R. G.; Waller, A. M. I n Spectroekc~ochemis~; Ed.; Plenum Press: New York, 1988; Chapter 7. Goldberg, I. B.; Bard, A. J. I n Magnetic Resonance in Chemistry and Biology; Herak, J. N., Adamic, K. J., Eds.; Marcel Dekker: New York, 1975; Chapter 10. McKinney, T. M. I n Electroanalytical Chemistry: A Series of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10, p 97-278. Eaton, S. S.; Eaton, G. R. I n Modern Pulsed and Continuous-Wave Electron Spin Resonance; Kevan, L., Bowman, M. K., Eds.; Wiley: New York, 1990; Chapter 9. Maltempo, M. M.; Eaton, G. R.; Eaton, S. S. I n EPR Imaging and in vivo EPR; Eaton, G. R., Eaton, S. S., Ohno, K., Eds.; CRC Press: Boca Raton, FL, 1991; Chapter 13. Sueki, M.; Quine, R. W.; Eaton, S. S.;Eaton, G. R. J. Chem. Soc., Faraday Trans. 1990, 86, 3181-3184. Maltempo, M. M.; Eaton, S. S.; Eaton, G. R. J. Magn. Reson. 1988, 77, 75-83. Pedersen, J. A. Mndbook of EPR Spectra from Quinones and Quinols; CRC Press: Boca Raton, FL, 1985. Chambers, J. Q. I n The Chemistry of the QuinonoH Compounds;Patai, s., Ed.; John Wiley: New York, 1974; Part 2, Chapter 14. Eggins. B. R.; Chambers, J. Q. Chem. Commun. 1969, 232-233. Eggins, B. R.; Chambers, J. Q. J. Eiectrochem. SOC. 1970, 117, 186-192. Maltempo, M. M.; Eaton, S. S.; Eaton, G. R. J. W g n . Reson. 1987, 72, 449-455. Compton. R. G.; Waller, A. W. J. Ekctroanal. Chem. Interfackil Electrochem . 1985, 195, 289-297.
RECEIVED for review November 5,1990. Accepted January 23, 1991.
