Anal. Chem. 1890, 62, 1619-1623
14.0
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analysis of technological materials with SIMS. Investigation of additional systems will be needed to further confirm the relations found.
1
1
LITERATURE CITED (1) Newbury, D. E. Scanning 1080, 3 , 110-118. (2) Newbury, D. E. In QuanfnElthre Swface Analysis of Meted&, ASTM STP 643; McIntyre, N. S., Ed.; ASTM: Philadelphla, 1978; pp 127-149. (3) Wilhartitz, P.; Virag, A.; Friedbacher, G.; Grasserbauer, M.;Ortner, H. M. Fresenius’ 2.Anal. Chem. 1987, 329, 228-236. (4) Virag, A.; Friedbacher, G.; Grasserbauer, M.;Ortner, H. M.;Wilhartk, P. J . Mater. Res. 1988, 3 , 694-704. (5) Friedbacher,G.; Vkag, A.; Grasserbauer. M.;Bubert. H.; Palmetshofer, L.; Wilhartii. P.; Ortner, H. M.;RGdhammer, P.; Rltter, W. SIA, Surf. Interface Anal. 1988, 12, 165-167. (6) Scherer, V.; Hirschfeld, D. €rzmetalll98S, 39, 251-253. (7) Scherer, V.; Hirschfeld, D. €rzmetalll987, 40. 611-613. (8) Andersen, C. A.; Hinthorne, J. R. Anal. Chem. 1973, 45, 1421-1438. (9) Wilson, R. G. J . Appl. phvs. 1988. 6 3 , 5121-5125. (10) Lapides, L. E. SIA, Surf. Intefface Anal. 1985, 7 , 211-216. (11) Lapides, L. E.; Whlternan, G. L.; Wiison, R. 0 . Mater. Res. SOC. Symp. Roc. 1984. 25, 657-662. (12) Galuska. A. A.; Morrison, 0.H. Int. J . Mass Spectrom. Ion Processes 1984, 8 1 , 59-70. (13) Stevie, F. A.; Kahora, P. M.;Cochran, 0.W. J . Vac. Sci. Techno/. 1989, A7. 1539-1544. (14) RMenauer, F. G.; Steiger, W.; Rledel, M.;Beske, H. E.; Holzbrecher, H.; Diistemijft, H.; Gerlcke, M.; Richter, C. E.; Rieth, M.; Trapp. M.; Giber, J.; Solyom, A.; Mai, H.; Stingeder, G. Anal. Chem. 1985, 57, 1836-1643. (15) Ramseyer, G. 0.; Morrison, 0.H. Anal. Chem. 1983, 55, 1963-1970.
i
6 4.0 .0o I
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Flgure S. - d / k (from Figures 4 and 5) of the log RSF versus E , regression lines (Figure 1) versus ionization energy of the matrix reference element (Ti was used for the matrix WTi 10% ). The regression line is also plotted in the diagram.
regression lines were plotted versus the ionization potentials of the matrix elements. Figure 5 shows the corresponding pattern, which is rather similar to the d versus Ei pattern of Figure 4. The relatively good linearity of the -d/k versus Ei plot (Figure 6) supports the conclusion that the relations demonstrated in Figures 4 and 5 are not caused bv casual scattering of the measured values. We do not yet have an explanation for the relation found, but we think that it is worth further investigation, because it bears the potential for a more accurate transfer of RSF versus Ei patterns from one matrix to another, allowing semiquantitative analysis. This would be an important advancement in multielemental ultratrace
RECEIVEDfor review January 11, 1990. Accepted April 12, 1990. Support of this work by the Austrian Science Foundation, the Federal Ministry of Research, the Austrian National Bank, and the University Jubilee Fund of the City of Vienna is gratefully acknowledged.
Fabrication of Band Microelectrode Arrays from Metal Foil and Heat-Sealing Fluoropolymer Film David M. Ode11 and Walter J. Bowyer* Department of Chemistry, Hobart and William Smith Colleges, Geneva, New York 14456 We descrlbe a technique for easlly preparlng arrays of band microelectrodes by seallng metal foil and film of Tefrel fluoropolymer In “multldecker” sandwiches. Electrodes have been prepared from gold, platlnum, nickel, and sliver foils with thlcknesses ranglng from 4 to 100 pm. Double-layer capacitance measurements suggest that the seal between the film and the metal Is very good. Results of voltammetry experiments uslng arrays, single-band electrodes, and a conventional disk electrode are compared. Using anthraqulnone(O/-) In acetonltrlle as a reversible couple, we demonstrate three dlffuslon regimes ( h e a r diffusion to each dement, hemlcyllndrlcal ditfuslon, and h e a r ditfuslon to the array) at a slngle array. Results are compared to theoretlcal descrlp tlons. With the electrode arrays, relatlvely undlstorted cycllc voitammograms can be recorded at Scan rates up to 1000 V/s In aprotlc solvents wlth no compensation or correction.
INTRODUCTION There has been much interest in arrays of microelectrodes. Shortly after microelectrodes became popular, arrays were used to increase the current while maintaining microscopic *To whom correspondence should be directed. 0003-2700/90/0362-1619$02.50/0
dimensions of the electrode. Arrays have many advantages over both microelectrodes and macroelectrodes including improved signal-to-noise ratio and lower limits of detection (1-13). Cylindrical, band, and dual-band microelectrodes also have been studied (14-17). Theoretical descriptions of the voltammetric response at microelectrode arrays have been well developed (18-20). Amatore et al. (18)described three regimes. At very short times (high scan rates), diffusion to each microelectrode element in the array is linear. At longer times, when diffusion is no longer linear, a plateau, rather than a peak, is recorded by cyclic voltammetry. Finally, a t very long times, voltammetric peak currents are the same as those that would be recorded at a macroelectrode, but the measured electrontransfer rate is predicted to decrease proportionally to the fractional area that is active microelectrode. Experimental work has been done to support the theoretical results (e.g. 3,5,18,19).However, the construction of arrays is difficult. Composite electrodes have complex and irregular geometries, and lithographic techniques are not available in most labs. Furthermore, lithographically prepared electrodes are not sufficiently robust to withstand polishing or rigorous electrochemical cleaning. Robust band electrode arrays of regular geometry have been described (12,13).Magee and Osteryoung (12)describe arrays 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62. NO. 15. AUGUST 1. 1990 foil Strips for electrical sonnection exposed
Array
\ Tefzel wafer
Flpure 1. Schematic diagram of electrode wafer before being fixed to glass lube.
fabricated from a glassy carbon plate using a dicing saw; bands are as narrow as 60 pm. DeAbreu and Purdy used gold ink to construct arrays of bands 400 pm wide (13). In this paper we describe a simple technique for construction of band microelectrode arrays. Platinum, gold, silver, or nickel foil can be sealed between layers of heat-sealing film made of Tefzel. We have prepared arrays of band microelectrodes by making "multidecker sandwiches" of metal foil and film. The sizes of active (foil) and inactive (film) areas is easily controlled by choosing the proper thickness of metal foil and Tefzel film. Active areas range from 2% to 50% of the total; band widths range from 4 to 100 pm. We have characterized these electrode arrays . by . microscopy and electrochemistry. Tefzel film has been used nreviouslv in thin-laver cell construction (21) as well as fo; fabricat& single-band microelectrodes (22). EXPERIMENTAL SECTION Electrode Construction. Metal foil was cut into ten strips 2.5 X 30 mm with a scalpel and straight edge. Glass microscope slides (1.0 mm thick, Clay-Adams) and Tefzel film (Type LZ, American Durafilm) were cut into rectangles 13 X 20 mm. The assembly was begun with a glass slide on which were laid four layers of Tefzel film. Then, a strip of foil was laid lengthwise along the top Tefzel film so that 5 mm of the foil extended over each end of the assembly. Alternating layers of Tefzel and foil were used until 10 metal strips had been included. The assembly was capped with four layers of Tefzel and a glass slide. A t this point, great care was required to ensure that the metal strips were aligned one over the other as closely as possible. After alignment, the assembly was light clamped with a large (4 em) paper clip bent to a gap only slightly smaller than the thickness of the array. (We estimate the force applied to he about 0.02 N.) Heat-sealing was at 300 OC for 9 min in a Syhron Thermolyne 1500 furnace. The assembly was then removed from the oven, allowed to cool, and cut into two halves 13 X 10 mm with a glass cutting saw. This exposed a cmss section of the sandwich resulting in an array of 10 microelectrode hands 2.5 mm in length. Two arrays can be constructed from each assembly. The glass was removed from the assembly, and the face of the electrode was hand polished with 400 and 600 grit grinding paper (Buehler) and then with 6.0-and 0.05-pm alumina on polishing cloth (Buehler). Figure 1is a schematic diagram of the wafer at this point in the fabrication. The polished wafer was affixed to a 6-mm-i.d. glass tube with 2-ton Devcon epoxy so that the foil extending from the sandwich entered the tube. Electrical connection to a copper wire was made with silver epoxy (Epoxy Technology, Inc.). The electrode was polished with 0.05-pm alumina for 5 min immediately before each experiment. Electrodes were constructed of platinum (25- and 4-pm thickness), gold (10 pm), silver (100 and 25 pm), and nickel (6 rm) foil, all purchased from Aesar. The distance between microelectrodes could be controlled by varying the thickness of the Tefzel film between the foil strips; we used film thicknesses of ~~
Flgure 2. SEM of two bands in a Ni array. Nickel foil was nominally 6 pm and Tefzel film was 125 pm. 25,50, and 125 pm. For larger separations, we used double and triple layers of 125-pm Tefzel film. Microscopy measurements indicated that separations between bands were slightly less than the thickness of the Tefzel film before heating. A single layer of 125-pm film yielded a separation of 100 rm; a double layer of this film yielded a separation of 140 rm; a triple layer yielded a separation of 260 pm. For comparison to the array, single-band electrodes were prepared by sealing a single foil strip in the middle of 12 layers of Tefzel in a fashion very similar to that described above. We have constructed over 20 electrodes using this technique. Electrochemical Measurements. Voltammetry was recorded by using a potentiostat (BAS CV-1A or an in-house constructed potentiostat with a time constant of 2 ps) and a three-electrode configwation. All potentials were measured relative to a saturated calomel electrode (SCE) segregated from the test solution by a porous Vycor frit. Solutions were purged with nitrogen. Voltammograms at scan rates greater than 0.5 VIS were recorded on a Nicolet digital oscilloscope (Model 310) and stored on disk. A BAS platinum electrode (0.82-mm radius) was used as the macroelectrode for comparison and was polished with 0.05-pm alumina before each experiment. Acetonitrile (ACN, Fisher Optima), potassium nitrate, anthraquinone, luminol, and potassium ferricyanide were used as received. Tetra-n-hutylammonium hexafluorophosphate (TBAHFP, Aldrich) was recrystallized three times from 95% ethanol and dried for 24 h at 100 "C. Chemiluminescent experiments were performed as described except with a luminol concentration of 2.0 mM (23). A 30-mL Teflon PFA sample bottle (Cole-Parmer) was used as the cell. To view the chemiluminescence, a hole was drilled in the bottom of the cell, and the electrode array was inserted from below. Plumbers putty (True Value) was used to seal the electrode in the bottom of the cell. A B a w h and Lomb SV-1070 S t e r e o h m microscope was used for inspection of the electrodes. Scanning electron micrographs were recorded on a Hitachi S-530 SEM after sputter coating the arrays with a thin layer of gold. RESULTS Microscopic Examination. At 70X magnification the seal between the metal and the Tefzel appeared continuous. Metal hands were straight and appeared parallel. Thin sections of the electrode assembly cut with a microtome and viewed at 2OOX magnification showed no evidence of any layers remaining in the Tefzel from incomplete sealing. A t 8000X, scanning electron micrography revealed no gaps in the seal between the metal and the Tefzel. See Figure 2 for an electron micrograph of a nickel electrode. Theoretical treatment of microelectrode band arrays (9) suggests that a regular geometry-uniform gaps, parallel bands, and constant band lengths-is optimal. T o quantify
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
1821
A
B
C
T
I
Flgure 4. Plot of log [capacbnce/@F/cm*)] vs log [scan rate/(mVls)]
I 0.4
0.2
0.0
-0.2
POTENTIAL/U us. S.C. E .
Flguro 3. Cyclic voltammogram backgrounds recorded with platinum
ten-band array (Ptl)in aqueous 1 M KNO,. A: scan rate = 20 VIS, Y-axis scale bar = 500 pA. B: scan rate = 500 VIS, bar = 2 mA. C: scan rate = 2000 V/s, bar = 2 mA.
the geometry of our arrays, a micrometer eyepiece and a light microscope were used to measure band length and separation between metal bands. For a gold electrode constructed with single layers of Tefzel (Aul), the average separation between bands was 100 pm with a relative standard deviation (rsd) of 3%. A platinum electrode (Ptl) was prepared from 25-pm thick foil and double layers of Tefzel film for separation. Using two layers of Tefzel increased the final average separation between bands to 140 pm (rsd = 12%). During the sealing step, some Tefzel is squeezed out the sides of the assembly. A gold electrode (Au2) constructed with triple layers of 125-pm-thick Tefzel had an average separation between bands of 260 pm (rsd = 4%). The greatest irregularity in the array geometry occurs a t the end of the bands. It is very difficult to cut strips of foil precisely 2.5 mm wide. Thus, some bands extend beyond the others, and some do not extend far enough. For the gold electrode Aul, the relative standard deviation of the band lengths was 8%. For the gold electrode Au2, the relative standard deviation of band lengths was 6%. T o confirm that all bands of the array were electrically connected to the electrode lead, electrogenerated chemiluminescence was observed at 70X magnification (vide supra). For all arrays tested, all bands were active. These observations, along with quantitative agreement between theory and experimental peak current values in the linear diffusion regime, allow us to conclude that all bands are electrically connected. Capacitance Measurements. Capacitance measurements are a good indication of the quality of the seal between the Tefzel and the metal (10, 24, 25). Electrodes with a large perimeter:area ratio typically show high specific capacitance at low frequencies. At higher frequencies, the capacitance is closer to that of a large electrode. Wightman and others (IO, 24,25) have proposed that the higher capacitance results from an imperfect seal between the electrode and the insulator. This would allow a film of electrolyte to contact part of the embedded metal. At higher frequencies, higher capacitive current leads to large iR loss in the film effectively preventing charging of the embedded metal. Capacitance data for our Tefzel/metal arrays indicate a good but not perfect seal. Typical cyclic voltammetry background scans are illustrated in Figure 3. These scans were recorded with electrode Ptl (described above) at 20, 500, and 2000 V/s in aqueous 1.0 M KNO,. The specific double-layer capacitance (C/A in pF/cm2) of the electrodes was measured
in aqueous 1 M KNO,. Capacitance measured at +0.4 V. Solid circles: Platinum 10-band array (band = 2.5 mm X 25 pm). Open circles: Platinum disk macroelectrode (diameter = 1.6 mm).
from the charging current over a wide range of scan rates (24). These data are illustrated in Figure 4 by the closed circles; the log of the specific capacitance is plotted vs the log of the scan rate. Capacitance of the platinum electrode Ptl was measured a t +0.4 V in aqueous 1.0 M KNO, between 0.02 and 4000 V/s. Over this range, the capacitance decreased by a factor of only 30. At 2000 V/s, the capacitance is not significantly different from that a t a macroelectrode (100 pF/cm2; data for the macroelectrode are illustrated in Figure 4 by the open circles). We measured the capacitance for all of our electrode arrays in H,0/1.0 M KNO, or in ACN/0.3 M TBAHFP, and similar results were obtained in every case. These data suggest that a good seal is being formed between the Tefzel and the metal foil. These are comparable to the data obtained by Tallman et al. (IO) for the seal between Kel-F and silver in the Kelsil composite electrodes. The technique of Saveant et al. (26) was used to estimate the time constant, RC, of the electrode array from the curvature of the capacitive current just after the switching potential. At 2000 V/s, a value of 50 ps was determined for Ptl in H 2 0 / 1 M KNO,. The capacitance was determined from the magnitude of the charging current to be 6 X lo-' F. Dividing RC by the capacitance indicates a solution resistance of about 80 0. Thus, these electrodes should be suitable for relatively high-speed voltammetry (27). Voltammetric Measurements. We used the reversible couple of anthraquinone(O/-) [AQ(O/-)] in acetonitrile to study the voltammetric behavior of these electrodes between 0.005 and 4000 V/s. Ferri/ferrocyanide in water (not reversible over the entire range) and ferrocene/ferrocenium in ACN were also studied in less detail. In Figure 5 are illustrated three voltammograms of the reduction of anthraquinone in ACN/O.3 M TBAHFP. At 500 V/s, the voltammogram recorded with the array has the shape characteristic of linear diffusion: the peaks are of approximately equal size in both the forward and reverse scans (Figure 5A). A voltammogram of a very similar shape is recorded a t the single-band electrode (not pictured). Different behavior is observed at slower scan rates. At 0.05 V/s, the voltammogram recorded by using the array is peak-shaped, but the peak on the reverse scan is much smaller than on the forward scan (Figure 5B). At the single band, the voltammogram has a plateau shape, and there is nearly no anodic current on the reverse scan (Figure 5C). The differences between the voltammograms B and C suggest that at slow scan rates diffusion layers of the elements of the array overlap significantly. A plot of log (current) vs log (scan rate) quantitatively supports this interpretation of the voltammogram shapes. In Figure 6, the solid circles represent faradaic peak currents
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ANALYTICAL CHEMISTRY, VOL. 62,
NO. 15, AUGUST 1, 1990 I
I
Table I. Peak Separation ( E , - Epc)of Anthraquinone(O/-) Recorded by Cyclic Voltammetry with a Gold Array in ACN/0.3 M TBAHFP
scan rate,
4 -0 4
-0 6
-0 8
POTENTIAL/V
-1 0
VS.
-1 2
s. c . E .
Flgure 5. Cyclic voltammograms of anthraquinone(O/-) in ACN/0.3 M TBAHFP. A: Ten-band gold array, v = 500 V/s, Y-axis scale bar = 45 pA. 6: Ten-band gold array, v = 0.05 V/s, bar = 2 p A . C: Single-band gold electrode, v = 0.05 V/s, bar = 0.2 pA.
1.0
2.0
3.0
4.0
5.0
6.0
1 Figure 6. Plot of log (cwent/concentrati) vs !q(scan rate) for cyclic voltammograms of anthraquinone(O/-) in ACN. Sdld circles: Ten-band gold array (band = 2.5 mm X 10 pm). Open circles: Single-band gold LOG [ (SCANRATE) / (mU/s)
electrode (currents multiplied by 10 for comparison). Solid line: Predicted function from eq 1 with linear diffusion to each band. Broken line: Predicted function with linear diffusion to total array.
recorded by using the 10-band gold array; the open circles represent data recorded by using the single-band electrode. For comparison, all current values have been divided by the anthraquinone concentration, and the currents recorded at the single band have been multiplied by 10. At high scan rates ( u > 100 V/s), log (current) is proportional to log (scan rate), and the slope of the line is 1/2. This is consistent with linear diffusion at each band. The solid line in Figure 6 is the dependence predicted by the equation for faradaic peak currents resulting from linear diffusion to each band (28): i, = (2.69 X 105)n3/2AD1/2u1/2C* (1) where D = 1.6 X cm2/s (measured independently by chronoamperometry (29)) and A is the nominal area of the gold bands ( A = 10 x 0.25 cm x 0.0010 cm = 0.0025 cm2). These high scan rate data also are consistent with SEM and capacitance data in indicating that the Tefzel is not smeared across the metal surface as is sometimes the case with epoxy (e.g., ref 30). In the scan-rate range ca. 2-50 V/s, the current at both the array and the single band are greater than that expected from linear diffusion. This indicates that edge diffusion is a significant contribution to the total flux in this time domain.
peak separation,mV
scan rate,
V/s
V/s
peak separation,mV
0.02 0.05 0.10 0.20 0.50 1.00
100 100 100 100 110 94
5.00 20.00 100.00 500.00 2000.00
90 105 115 170 310
Below 2 V/s the current density at the 10-band array is less than at the single band. This can be attributed to overlap of the diffusion layers of the individual elements of the array. The broken line in Figure 6 represents the values predicted from eq 1 when the area used is the area of the total array (gold bands plus Tefzel spacers). It can be seen that at the lowest scan rates data for the array are approaching the time domain where diffusion can be modeled as occurring linearly to the overall array. Similar plots were prepared by using platinum band arrays and single electrodes of both 4- and 25-pm thickness. All data were consistent with the interpretation described above for the gold electrodes. Of course, eq 1 applies only when the electron-transfer reaction is rapid. The heterogeneous electron-transfer rate of AQ(O/-) is rapid in N,N-dimethylformamide a t platinum electrodes (31). We measured peak separations for voltammograms at platinum and gold arrays over a wide range of scan rates. Interestingly, peak separations are 90-120 mV but show no consistent dependency on scan rate between 0.01 and 100 V/s. In Table I are peak separations for AQ(O/-) in ACN/0.3 M TBAHFP recorded with a gold array. We have reproduced this experiment at platinum and gold arrays, with 1.0 and 3.0 mM solutions, and with the ferrocene/ferrocenium couple (up to 10 V/s). Digital simulations (32) indicate that a t an ensemble of microelectrodes, the limiting peak separation should be ca. 90 mV for rapid electron-transfer kinetics. The peak separations observed at our arrays probably result from mixed diffusion (linear and hemicylindrical), heterogeneous electron-transfer kinetics, and ohmic distortion (at the highest scan rates). We are currently pursuing digital simulations to untangle these various contributions.
CONCLUSION We describe a technique for construction of arrays that for the first time contains all of the following advantages: (1)Simplicity of construction: This method requires only standard laboratory equipment. (2) Regular geometry: Bands are straight, parallel, and of uniform length; spacers are of uniform width. (3) Electrodes can be polished or cleaned electrochemically. (4) Band widths range from 4 to 100 pm. Gaps widths range from 25 to 250 pm. Thus, we have constructed electrodes with percent active area from 2% to 50%. Bands can be of Pt, Au, Ag, or Ni. We are currently pursuing application of these arrays to anodic stripping voltammetry analysis. Also we are constructing arrays with individually addressable bands for ring-disk type experiments ( I 7, 30). ACKNOWLEDGMENT We thank George Helfman for cutting the wafers and Brian Terhune for recording the SEMs. LITERATURE CITED (1) Petersen, S. L.; Tallman, D. E. Anal. Chem. 1988, 60,82. (2) Cheng, I. F.; Whitely, L. D.; Martin, C.R. Anal. Chem. 1989, 6 1 , 762.
Anal. Chem. 1990, 62, 1623-1627 (3) Sleszynskl, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 56, 130. (4) Bard, A. J.; Crayston, J. A.; Kittlesen, G. P.; Shea, T. V.; Wrighton, M. S. Anal. Chem. 1986, 58, 2321. (5) Chdsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601. (6)Thormann, W.: van den Bosch, P.; Bond, A. M. Anal. Chem. 1985,
57,2764. (7)Weber. S.G. Anal. Chem. 1989, 67,295. (6) Caudlll, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1982, 5 4 , 2532. (9) Fosdick. L. E.; Anderson, J. L. Anal. Chem. 1988, 58, 2481. (lo) Peterson, S.L.: Weisshaar. D. E.; Tallman, D. E.; Schulze, R. K.; Evans. J. F.; DesJarlais, S. E.; Engstrom, R. C. Anal. Chem. 1988, 60,
2385. (11) Stroehben, W. E.; Smith, D. K.; Evans, D. H. Anal. Chem., submitted for publication.
(12) Magee, L. J.; Osteryoung, J. Anal. Chem. 1989, 67,2124. (13) DeAbreu. M.: Purdy, W. C. Anal. Chem. 1987, 59, 204. (14) Singleton, S.T.; O'Dea, J. J.; Osteryoung, J. Anal. Chem. 1989, 61, 1211. (15) Kovach, P. M.; Caudill, W. L.; Peters, D. G.; Wightman, R. M. J . Electroanal. Chem. 1985, 185, 265. (16) Wehmeyer, K. R.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1985. 57, 1913. (17) Hill, H. A.; Klein, N. A.; Psalti. I . S. M.; Walton, N. J. Anal. Chem. 1989, 67,2200. (18)Amatore, C.;Saveant, J. M.; Tessier, D. J . Nectroanal. Chem. 1983, 746, 37.
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(19) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J . Electroanal. Chem. 1982, 738, 65. (20) Gueshi, T.; Tokuda, K.; Mats&, H. J. Ek%3manal. Chem. 1979, 707, 29. (21) Rhodes, R. K.; Kadish, K. M. Anal. &em. 1981, 53, 1539. (22)Fitch, A,, Personal communication. (23) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987. 59. 670.
(24) Wehmeyer, K. R.; Wightman, R. M. J. Electroanal. Chem. 1985, 796, 417. (25) Thormann, W.; Bond, A. M. J. Ekctroanal. Chem. 1987. 278, 167. (26)Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; Saveant, J. M. J. Electroanal. Chem. 1988, 243, 321. Krlstensen, E. W.; Deakin, M. R.; Wightman, R. M. Anal. (27)Wipf, D. 0.; Chem. 1988, 60, 306. (28) Bard, A. J.; Faulkner, L. R. Electrochembal Mefhods fundamentals and Appllcations; Wiley: New York, 1960;p 218. (29) Bowyer, W. J.; Engelman, E. E.; Evans, D. H. J. flectroanal. Chem. 1989, 262, 67. (30) Shea, T. V.; Bard, A. J. Anal. Chem. 1987, 59,2101. (31) Fitch, A.; Evans, D. H. J. Electroanal. Chem. 1986, 202, 83. (32) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1984, 767. 247.
RECEIVED for review February 20,1990. Accepted April 19, 1990. This research was supported by a grant from Research Corporation and by Hobart and William Smith Colleges.
Differentiation of Stereoisomers Using High-Resolution Electronic Spectroscopy Applied to Methyl-Substituted Tetrahydropyrans Timothy J. Cornish a n d Tomas Baer* Chemistry Department, University of North Carolina, Chapel Hill, North Carolina 27599-3290
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The n 3s Rydberg spectra of jet-cooled methyl-substltuted tetrahydropyrans (THPs) have been collected by the technique of 2 1 resonance-enhanced multiphoton ionization (REMPI) in the wavelength region 420-375 nm. As with the prevlously Investigated methyl-substituted cyclohexanones, the transltion energies of the THPs are hlghly sensltlve to both the positlon of substitution around the ring and its orientation (axial vs equatorial). Furthermore, these shlfts are addltlve. It Is thus readily posslble to distlngulsh on the basis of the spectra cls- and trans-dimethyl-THPs.
+
INTRODUCTION Ultraviolet spectroscopy has enjoyed a rebirth as a spectroscopic and analytical tool for intermediate size molecules because of efficient cooling of gaseous samples to a few degrees kelvin provided by pulsed molecular beam technology and by the use of pulsed lasers as excitation sources (1-14). The effect of cooling the rotational and vibrational energy has resulted in sharp absorption peaks and the simplification of the spectra due to hot band suppression. Of particular interest is that these methods are no longer limited to intermediate size molecules because techniques have been developed that allow large or nonvolatile molecules to be vaporized and entrained in a beam of rare-gas atoms (15-17). A particularly interesting electronic transition that we have recently investigated by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) for cyclic and linear ketones is that of the n 3s excitation (12-14). In many ketones, the 3s Rydberg state lies at an energy that is greater than two-
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thirds of the ionization energy. The n 3s transition is thus very convenient for REMPI studies because the laser wavelength lies in the easily accessible range of 420-375 cm-' and because the absorption can be efficiently monitored by collecting the ions resulting from the absorption of the third photon. A characteristic feature of the spectra are very sharp peaks, often exhibiting little vibrational excitation. We have found during the course of our investigations of many methyl-substituted cyclic ketones that the 3s Rydberg spectra are surprisingly sensitive to molecular stereochemistry. The n 3s transition origin for cyclohexanone was found to be 50717 cm-' (394.3 nm in a two-photon excitation scheme). The most stable conformation of the six-membered ring is a chair form, in which substituents are oriented in either the more stable equatorial or the less stable axial direction. When methyl groups are substituted in equatorial orientations at the 2, 3, or 4 positions, the n 3s transition origin shifts by -546, +109, and -7 cm-', respectively. These shifts depend strongly on whether the methyl groups are attached in axial or equatorial orientations. Thus, cis-3,5-dimethylcyclohexanone (diequatorial) and trans-3,5-dimethylcyclohexanone (equatorial-axial) have origins at 50 955 and 50 464 cm-I, respectively. Not only are these shifts sensitive to the location and orientation of the methyl groups, they are largely additive. This means that stereoisomers can be identified on the basis of their transition origin. The additivity applies as well to multiring compounds such as norcamphor derivatives (18). The standard spectroscopic method for structure determination of such stereoisomers is NMR (19). We recently compared the optical n 3s transition energy shifts with the carbonyl carbon 13C NMR shifts and found a remarkable
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0003-2700/90/0362-1623$02.50/0 0 1990 American Chemical Society
