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Cell for combined electrochemistry and ESR measurements at variable temperatures in a varian TE102 microwave cavity
JOURNAL
OFMAGNETIC
RESONANCE
68, 551-555(1986)
Cell for Combined Electrochemistry and ESR Measurements at Variable Temperatures in a Varian TEI02 M icrowave Cavity K.R.FERNANDO,
A.J.McQuILLAN,B.M.PEAKE,*
ANDJ. WELLS
Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
ReceivedDecember12, 1985;revisedMarch 4, 1986
Since the first report in 1960 by Geske and Maki (1) of an electrochemical cell that could be used to prepare paramagnetic speciesin situ for ESR study, there have been a number of reports of successfulcell designsof this type (2-12). However, only a few of these (3, 6, 9-l I) enable ESR measurementsto be m a d e at variable temperatures which is often a necessaryrequirement to obtain optimal resolution of the hyperfme structure in ESR spectra of organic radical ions, O n the other hand these latter cells often have no reference electrode and hence do not allow accurate electrochemical measurementsand cyclic voltammograms (CV) to be recorded on low dielectric solvent systemsused in ESR measurements.Some cells (1, 8-10) also require the use of ESR m icrowave cavities other than the TEloz configuration, which is the only one available, at least on the widely used Varian E-4 and E-104 spectrometers. In this note we describe a cell which allows the recording of a CV of a sample in a low dielectric constant solvent rigorously degassedunder vacuum conditions. ESR spectra can then be observed on this same sample under controlled electrochemical conditions and at variable temperatures in a TELo2cavity. The cell (Fig. 1) consists of three m a in parts, A, B, C, together with a small sample compartment D. In this way, the cell can be easily dismantled for electrode cleaning. The lower portion of C is a normal cylindrical Suprasil quartz ESR sample tube (3 m m i.d.) connected via a graded seal to the remaining Pyrex component of this part. All other glass parts of the cell are constructed from Pyrex. B is the most important and critical part of the cell. Platinum leads (0.25 mm) to the working and secondary electrodes (WE and SE, respectively) are sealed in the wall of this section. The WE lead passesdown through the inside of one of two capillaries (1 m m i.d.) which share a common wall, and emergesat the bottom to form a WE helix of two closely wound turns (seeinset). This WE capillary is sealedat both ends to prevent any electrochemical action occurring other than at the WE. This also has the effect of firmly attaching the electrode assembly to B. The SE consists of a m u ltitum helix wound around the twin capillaries and about 1.5 cm above the WE. A silver wire (0.27 mm) referenceelectrode, which is joined to a platinum wire sealedin the wall of A, slides down inside the openended capillary when A and B are fitted together. W h e n in place the tip of the silver * To whom correspondence shouldbe addressed. 551
0022-2364186 $3.00 Copyright 0 1986 by Academic press, Inc. All rights of reproduction in any form reserved
552
NOTES Vacuum
k P
Ag wire RE
B S E lead 3
WE
(Pt)
C 1 quartz
lead
(Pt)
L
Pyrex graded: seal ./
/’
/’
/’
0’
f
1.5
cm
(a 1
'\ '\
-\ '. '.
FIG. I. Combined electrochemistry/ESR cell for variable temperature measurements with low dielectric constant solvents.
wire is about 1 mm from the end of the capillary which is itself about 1 mm above the WE helix. The presence of a reference electrode as well as the normal working and secondary electrodes allows the WE potential to be measured much more accurately than for a two electrode cell. This is particularly important when low dielectric solvents such as ethers are used because these can give rise to large potential differences between the WE and SE. Although in this cell, the Ag wire acts as a pseudo-reference electrode (13) it simplifies the cell design, minimizes possible contamination that may arise
553
NOTES
from any reference electrode system, and provides adequate stability over a period of several hours (+-lo mV). Uncompensated resistance (14) is minimized by the close proximity of the reference electrode to the working electrode and the geometric arrangement of the working and secondary electrodes gives a uniform current density over the surface of the working electrode. The active area where electrochemical reactions occur is small enough to fit inside the Varian E-257 variable temperature insert. Hence both CVs and ESR spectra can be conveniently measured over a wide range of temperatures. Conversely the secondary electrode is located outside the active area of the microwave cavity and hence any reaction products electrochemically generated in this region will not be detected in the ESR spectrum. Although the inner twin capillary assembly is Pyrex the outer tube in C is quartz and hence any spurious signals from paramagnetic species in this tube are minimized. Oxygen can also be rigorously excluded in this cell design and this can enhance the resolution of radical ion ESR spectra. The procedure to use this cell can be summarized as follows: (1) Pretreat the Pt working electrode by soaking briefly in concentrated nitric acid, rinsing with distilled water, and then drying. It is then oxidized at + 1.O V vs saturated calomel electrode (SCE) in sodium perchlorate (0.1 mol dm-3) for 5 min and followed by reduction at -0.6 V vs SCE in the same solution for 10 min. (2) Assemble the parts in the order B, A, C, and D. Particular care is necessary in guiding the reference electrode wire into the twin capillary assembly. (3) Dry and check for any leaks by attaching to a vacuum line. (4) Insert into D preweighed substrate and supporting electrolyte to give final concentration typically 0.005 and 0.1 mol dme3, respectively. (5) Evacuate and then distil solvent into D from a storage bulb. Alternatively the solvent can be introduced at the time of adding the substrate and supporting electrolyte. It can then be rigorously degassedby freeze-pump-thaw cycles. (6) Remove from the vacuum line, dissolve the substrate and supporting elec-
1
I
I
I
I
0.4
0.6
0.8
1.0
I
E/V FIG. 2. Cyclic voltammogram at -30°C of phenazine (5 X 10e3 mol dmm3) in I ,2dimethoxyethane tetrabutylammonium perchlorate (0. I mol dm-‘). Scan rate 30 mV SK’with Ag reference electrode.
with
554
NOTES
FIG. 3. ESR spectrum of phenazine produced by electrolysis at -30°C under the above conditions. Time constant 0.128 s, scan time 4 min, 100 kHz modulation with 0.1 G amplitude, microwave power 1 mW.
trolyte, and tip sufficient volume from D into the lower section of C to cover the SE but not the top of the capillary tubes. (7) Insert the whole assembly in the variable temperature unit within the TEIo2 spectrometer cavity and connect the three electrodes to the appropriate terminals of a suitable electrochemical control system. We have used this cell successfully in studies of the radical ions derived from a number of heterocyclic aromatics (IS) and organometallic metal carbonyls (16). For example, a typical CV measured in this cell at -30°C for phenazine and TBAP in 1,2-dimethoxyethane is shown in Fig. 2. Allowing for the high uncompensated resistance which must necessarily arise whenever this solvent (dielectric constant of 7.2) is used even in a conventional CV cell, the observed peak separation of 160 mV for this wave is quite acceptable. The ESR spectrum shown in Fig. 3 was recorded at -30°C on this same sample with a Varian E- 104 spectrometer after electrolyzing at a potential of - 1.1 V for 5 min at this temperature. The resolution of the hyperfine structure in this spectrum is comparable to that obtained using traditional alkali metal reduction methods (17). We conclude that this cell provides a number of advantages over previous designs and enables satisfactory electrochemical and ESR measurements to be made in low dielectric solvents at variable temperatures using the Varian TEKU cavity. ACKNOWLEDGMENTS We thank G. P. Speck for assistance in the phenazine measurements. K.R.F. acknowledges the award of a Senior Demonstratorship from the University of Otago. REFERENCES 1. D. H. GESKE AND A. H. MAKI, J. Am. Chem. Sot. 82,267 1 (1960). 2. L. H. PIETTE, P. LUDWIG, AND R. N. ADAMS, J. Am. Chem. Sot. 83, 3909 (196 1); 84, 42 12 (1962); Anal. Chem. 34, 1084 (1963).
NOTES
555
3. J. E. HARRIMAN AND A. H. MAKI, J. Chem. Phys. 39,778 (1963). 4. M. T. JONES, E. A. LALANCETTE, AND R. E. BENSON, J. Chem. Phys. 41,401 (1964). 5. J. K. D~HRMANN AND K. J. VETTER, J. Electroanal. Chem. %I,23 (1969). 6. D. H. LEW AND R. J. MYERS, J. Chem. Phys. 41, 1062 (1964). 7. I. B. GOLDBERG AND A. J. BARD, Z. Phys. Chem. 75, 3281 (1971). 8. R. D. ALLENDOERFER, G. A. MARTINCHEK, AND S. BRUCKENSTEIN, Anal. Chem. 47,890 (1975). 9. F. GERSON, H. OHYA-NISHIGUCHI, AND C. WYDLER, Angew. Chem. Znt. Ed. 15,552 (1976). 10. H. OHYA-NISHIGUCHI, Bull. Chem. Sot. Jpn. 52,2064 (1979). 11. G. BOWMAKER, P. D. W. BOYD, G. K. CAMPBELL, J. M. HOPE, AND R. L. MARTIN, Znorg. Chem. 21, 1152 (1982). 12. C. LAMY AND P. CROVIGNEAU, J. Electroanul. Chem. 150,545 (1983). 13. R. R. GANGE, C. A. KOVAL, AND G. C. LISENSKY, Znorg. Chem. 19,2854 (1980). 14. A. J. BARD AND L. R. FAIJLKNER, “Electrochemical Methods-Fundamentals and Applications,” Wiley, New York (1980). 15. R. K. FERNANDO, Ph.D. thesis, Chemistry Department, University of Otago, Dunedin, New Zealand, 1984. 16. A. D~WNARD, Ph.D. thesis, Chemistry Department, University of O@o, Dunedin, New Zealand, 1984. 17. A. CARRINGTON AND J. D. SANTOS-VEIGA, Mol. Phys. 5,2 1 (1962).
OFMAGNETIC
RESONANCE
68, 551-555(1986)
Cell for Combined Electrochemistry and ESR Measurements at Variable Temperatures in a Varian TEI02 M icrowave Cavity K.R.FERNANDO,
A.J.McQuILLAN,B.M.PEAKE,*
ANDJ. WELLS
Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
ReceivedDecember12, 1985;revisedMarch 4, 1986
Since the first report in 1960 by Geske and Maki (1) of an electrochemical cell that could be used to prepare paramagnetic speciesin situ for ESR study, there have been a number of reports of successfulcell designsof this type (2-12). However, only a few of these (3, 6, 9-l I) enable ESR measurementsto be m a d e at variable temperatures which is often a necessaryrequirement to obtain optimal resolution of the hyperfme structure in ESR spectra of organic radical ions, O n the other hand these latter cells often have no reference electrode and hence do not allow accurate electrochemical measurementsand cyclic voltammograms (CV) to be recorded on low dielectric solvent systemsused in ESR measurements.Some cells (1, 8-10) also require the use of ESR m icrowave cavities other than the TEloz configuration, which is the only one available, at least on the widely used Varian E-4 and E-104 spectrometers. In this note we describe a cell which allows the recording of a CV of a sample in a low dielectric constant solvent rigorously degassedunder vacuum conditions. ESR spectra can then be observed on this same sample under controlled electrochemical conditions and at variable temperatures in a TELo2cavity. The cell (Fig. 1) consists of three m a in parts, A, B, C, together with a small sample compartment D. In this way, the cell can be easily dismantled for electrode cleaning. The lower portion of C is a normal cylindrical Suprasil quartz ESR sample tube (3 m m i.d.) connected via a graded seal to the remaining Pyrex component of this part. All other glass parts of the cell are constructed from Pyrex. B is the most important and critical part of the cell. Platinum leads (0.25 mm) to the working and secondary electrodes (WE and SE, respectively) are sealed in the wall of this section. The WE lead passesdown through the inside of one of two capillaries (1 m m i.d.) which share a common wall, and emergesat the bottom to form a WE helix of two closely wound turns (seeinset). This WE capillary is sealedat both ends to prevent any electrochemical action occurring other than at the WE. This also has the effect of firmly attaching the electrode assembly to B. The SE consists of a m u ltitum helix wound around the twin capillaries and about 1.5 cm above the WE. A silver wire (0.27 mm) referenceelectrode, which is joined to a platinum wire sealedin the wall of A, slides down inside the openended capillary when A and B are fitted together. W h e n in place the tip of the silver * To whom correspondence shouldbe addressed. 551
0022-2364186 $3.00 Copyright 0 1986 by Academic press, Inc. All rights of reproduction in any form reserved
552
NOTES Vacuum
k P
Ag wire RE
B S E lead 3
WE
(Pt)
C 1 quartz
lead
(Pt)
L
Pyrex graded: seal ./
/’
/’
/’
0’
f
1.5
cm
(a 1
'\ '\
-\ '. '.
FIG. I. Combined electrochemistry/ESR cell for variable temperature measurements with low dielectric constant solvents.
wire is about 1 mm from the end of the capillary which is itself about 1 mm above the WE helix. The presence of a reference electrode as well as the normal working and secondary electrodes allows the WE potential to be measured much more accurately than for a two electrode cell. This is particularly important when low dielectric solvents such as ethers are used because these can give rise to large potential differences between the WE and SE. Although in this cell, the Ag wire acts as a pseudo-reference electrode (13) it simplifies the cell design, minimizes possible contamination that may arise
553
NOTES
from any reference electrode system, and provides adequate stability over a period of several hours (+-lo mV). Uncompensated resistance (14) is minimized by the close proximity of the reference electrode to the working electrode and the geometric arrangement of the working and secondary electrodes gives a uniform current density over the surface of the working electrode. The active area where electrochemical reactions occur is small enough to fit inside the Varian E-257 variable temperature insert. Hence both CVs and ESR spectra can be conveniently measured over a wide range of temperatures. Conversely the secondary electrode is located outside the active area of the microwave cavity and hence any reaction products electrochemically generated in this region will not be detected in the ESR spectrum. Although the inner twin capillary assembly is Pyrex the outer tube in C is quartz and hence any spurious signals from paramagnetic species in this tube are minimized. Oxygen can also be rigorously excluded in this cell design and this can enhance the resolution of radical ion ESR spectra. The procedure to use this cell can be summarized as follows: (1) Pretreat the Pt working electrode by soaking briefly in concentrated nitric acid, rinsing with distilled water, and then drying. It is then oxidized at + 1.O V vs saturated calomel electrode (SCE) in sodium perchlorate (0.1 mol dm-3) for 5 min and followed by reduction at -0.6 V vs SCE in the same solution for 10 min. (2) Assemble the parts in the order B, A, C, and D. Particular care is necessary in guiding the reference electrode wire into the twin capillary assembly. (3) Dry and check for any leaks by attaching to a vacuum line. (4) Insert into D preweighed substrate and supporting electrolyte to give final concentration typically 0.005 and 0.1 mol dme3, respectively. (5) Evacuate and then distil solvent into D from a storage bulb. Alternatively the solvent can be introduced at the time of adding the substrate and supporting electrolyte. It can then be rigorously degassedby freeze-pump-thaw cycles. (6) Remove from the vacuum line, dissolve the substrate and supporting elec-
1
I
I
I
I
0.4
0.6
0.8
1.0
I
E/V FIG. 2. Cyclic voltammogram at -30°C of phenazine (5 X 10e3 mol dmm3) in I ,2dimethoxyethane tetrabutylammonium perchlorate (0. I mol dm-‘). Scan rate 30 mV SK’with Ag reference electrode.
with
554
NOTES
FIG. 3. ESR spectrum of phenazine produced by electrolysis at -30°C under the above conditions. Time constant 0.128 s, scan time 4 min, 100 kHz modulation with 0.1 G amplitude, microwave power 1 mW.
trolyte, and tip sufficient volume from D into the lower section of C to cover the SE but not the top of the capillary tubes. (7) Insert the whole assembly in the variable temperature unit within the TEIo2 spectrometer cavity and connect the three electrodes to the appropriate terminals of a suitable electrochemical control system. We have used this cell successfully in studies of the radical ions derived from a number of heterocyclic aromatics (IS) and organometallic metal carbonyls (16). For example, a typical CV measured in this cell at -30°C for phenazine and TBAP in 1,2-dimethoxyethane is shown in Fig. 2. Allowing for the high uncompensated resistance which must necessarily arise whenever this solvent (dielectric constant of 7.2) is used even in a conventional CV cell, the observed peak separation of 160 mV for this wave is quite acceptable. The ESR spectrum shown in Fig. 3 was recorded at -30°C on this same sample with a Varian E- 104 spectrometer after electrolyzing at a potential of - 1.1 V for 5 min at this temperature. The resolution of the hyperfine structure in this spectrum is comparable to that obtained using traditional alkali metal reduction methods (17). We conclude that this cell provides a number of advantages over previous designs and enables satisfactory electrochemical and ESR measurements to be made in low dielectric solvents at variable temperatures using the Varian TEKU cavity. ACKNOWLEDGMENTS We thank G. P. Speck for assistance in the phenazine measurements. K.R.F. acknowledges the award of a Senior Demonstratorship from the University of Otago. REFERENCES 1. D. H. GESKE AND A. H. MAKI, J. Am. Chem. Sot. 82,267 1 (1960). 2. L. H. PIETTE, P. LUDWIG, AND R. N. ADAMS, J. Am. Chem. Sot. 83, 3909 (196 1); 84, 42 12 (1962); Anal. Chem. 34, 1084 (1963).
NOTES
555
3. J. E. HARRIMAN AND A. H. MAKI, J. Chem. Phys. 39,778 (1963). 4. M. T. JONES, E. A. LALANCETTE, AND R. E. BENSON, J. Chem. Phys. 41,401 (1964). 5. J. K. D~HRMANN AND K. J. VETTER, J. Electroanal. Chem. %I,23 (1969). 6. D. H. LEW AND R. J. MYERS, J. Chem. Phys. 41, 1062 (1964). 7. I. B. GOLDBERG AND A. J. BARD, Z. Phys. Chem. 75, 3281 (1971). 8. R. D. ALLENDOERFER, G. A. MARTINCHEK, AND S. BRUCKENSTEIN, Anal. Chem. 47,890 (1975). 9. F. GERSON, H. OHYA-NISHIGUCHI, AND C. WYDLER, Angew. Chem. Znt. Ed. 15,552 (1976). 10. H. OHYA-NISHIGUCHI, Bull. Chem. Sot. Jpn. 52,2064 (1979). 11. G. BOWMAKER, P. D. W. BOYD, G. K. CAMPBELL, J. M. HOPE, AND R. L. MARTIN, Znorg. Chem. 21, 1152 (1982). 12. C. LAMY AND P. CROVIGNEAU, J. Electroanul. Chem. 150,545 (1983). 13. R. R. GANGE, C. A. KOVAL, AND G. C. LISENSKY, Znorg. Chem. 19,2854 (1980). 14. A. J. BARD AND L. R. FAIJLKNER, “Electrochemical Methods-Fundamentals and Applications,” Wiley, New York (1980). 15. R. K. FERNANDO, Ph.D. thesis, Chemistry Department, University of Otago, Dunedin, New Zealand, 1984. 16. A. D~WNARD, Ph.D. thesis, Chemistry Department, University of O@o, Dunedin, New Zealand, 1984. 17. A. CARRINGTON AND J. D. SANTOS-VEIGA, Mol. Phys. 5,2 1 (1962).