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TABLE V Kinetic Barriers in Model Compounds"
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'AMl/UHF, kcal/mol. Numbering systems shown in Figures 8, 10, and 1 1 .
Results of the calculations of the UHF enthalpies of activation for the models are shown in the figures and listed in Table V. There is variation among the bonds, but qualitatively it may be said that the computed AM1 bamers are significant. In one class of reactions, at least, AM1 activation enthalpies are a lot better, and generally lower, than those from MNDO, MIND0/3, or
many ab initio methods.'* If AM1 gives a reasonable representation of the thermodynamic and kinetics of N-O compounds, our results are probably valid.
Conclusion The results indicate the possible existence of metastable polycyclic (ONNO), and suggest that it would be comparable in thermodynamic and kinetic stability with its open-chain isomers. Realization of the polymer would be expected to require extreme pressures, in which case the existence of the known cis-ONNO in the solid state would probably favor a polymerization to the polycyclic rather than the open-chain material. Acknowledgment. This study was sponsored by SDIO/IST and managed by the Naval Surface Warfare Center. The writer is indebted to Dr. Richard D. Bardo for helpful discussions, and to reviewers for useful comments. Registry NO. 2, 140928-94-5; 3, 140928-95-6; N20, 10024-97-2; Hz(N202)2N2H2, 140928-96-7; H ~ ( N z O ~ ) ~ N140928-97-8; ~H~, Hz(N202)4N2H2, 140928-98-9;HI(N202)5NzHz, 140928-99-0; H2(N,O,),N2H2, 140929-00-6; H2(N202)7N2H2, 140929-01-7; (ONO),NN(ON0)2, 140929-02-8; (ONNO),, 140929-04-0; (N202)3N2, 140929-03-9.
References and Notes ( 1 ) Jones, W. H.J . Phys. Chem. 1991, 95, 2588-2595. (2) Jones, W. H.J . Phys. Chem. 1992, 96, 594-603. (3) Jones, W. H.;Bardo, R. D., to be submitted for publication. (4) A possible solid-state mechanism for this conversion is described in ref 3, wherein it is shown that (SNNS), may be a better superconductor than its precursor. (5) QCPE Program No. 523 (IBM Mainframe Version), Indiana, University Chemistry Department, Bloomington, IN. (6) Dewar, M. J. S.; Thiel, W. J . Am. Chem. Soc. 1977, 99, 4914. (7) Peterson, M. R.; Poirier, R. A. MONSTERGAUSS; Department of Chemistry, University of Toronto, Canada. ( 8 ) Pauling, L. The Nature of rhe Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; p 260. (9) Gimarc. B. M. Croat. Chim. Acra 1984.57. 5 . 955-965. (lO)~Lipscomb,W. N.; Wang, F. E.; May,'W.'R:; Lippert, E. L. Acta Crysrallogr. 1961, 14, 1100. (11) Harcourt, R. D. Leer. Nores Chem. 1982, 30, 144. (12) Spellmeyer, D. C.; Houk, K. N., J . Am. Chem. Soc. 1988, 110, 3412-34f6. L&, I.; Cha, 0. J.; Lee, B . 4 . J . Phys. Chem. 1990. 94. 3926-3930, and references therein cited.
Electrochemistry of Conductive Polymers. 11. Spectroelectrochemical Studies of Poly(3-methylthiophene) Oxidation Sally N. Hoier and Su-Moon Park* Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 871 31 (Received: September 30, 1991; In Final Form: February 27, 1992)
The oxidation of electrochemically grown poly(3-methylthiophene) and its other spectroscopic properties have been studied by in-situ spectroelectrochemical techniques as well as in-situ conductivity measurements, and the results are reported. An absorptivity of 1.1 X lo5cm-I is reported for the absorption band at 490 nm for a neutral, reduced polymer film grown in propylene carbonate. The oxidation of the neutral polymer to the cation radical, or polaron, and its further oxidation to the dication, or bipolaron, are shown to take place at about 0.45 and 0.80 V,respectively, and be controlled by the diffusion of counterions. In-situ conductivity measurements of the film show that both the polaron and bipolaron are charge carriers. We conclude from the results that at least two chemically and optically different species, radical cation and dication, and perhaps other products, are produced in different potential regimes upon oxidation of poly(3-methylthiophene).
Introduction Since polyacetylene was shown to have high electrical conductivities when properly doped,l many other forms of organic conducting polymers have been reported for the past decade and a half. Recent advances in this area have been compiled in review articles, books, and/or proceedings volumes.24 Of the many conducting polymers, polythiophene and its derivatives have been
receiving a great deal of interest due to their stabilities in their undoped and doped states in Optical properties of doped and neutral (undoped) forms of polythiophenes have been reported by several investigators.18-25 Bridas et a1.2628concluded, from their ab-initio studies on the geometry and electronic structure modifications resulting from the doping process, that bipolarons are the charge carriers in the
0022-3654/92/2096-5 188%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5189
Electrochemistry of Conductive Polymers
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conducting regime of polythiophene. Kaufman et al.29concluded from their in-situ spectroscopic and ESR studies of doped and undoped polypyrrole that polarons, generated upon removal of an electron from 4-5 monomeric units, evolve further into bipolarons with an increase in the doping level. They further concluded that polarons cannot be present in the conducting form of polythiophene because of the effective evolution of bipolaron through an equilibrium reaction of polarons. As a result, two bands observed at longer wavelengths upon oxidation of polythiophene must arise from one species, bipolaron. There is only one species, bipolaron, in oxidized polythiophenes, Le., bipolaron, which is the origin of two absorption bands observed at around 750 nm (1.65 eV) and 1500 nm (0.83 eV) upon oxidation of polythiophenes, and this is the only charge carrier in their conducting form. On the other hand, polarons can exist in conducting forms of polypyrrole because of slow kinetics for the conversion of polarons into bipolar on^?^^^^ and thus three absorption bands were reported at longer wavelengths upon its oxidation. In this report, we present results that three rather than two absorption bands are observed and at least two optically and chemically different species are generated upon oxidation of poly(3-methylthiophene). The generation of the first oxidized species and its further transformation into the second are shown to have two different redox potentials and be limited by the diffusion of counterions.
Experimental Section Poly(3-methylthiophene) (P3MT) films were grown electrochemically by cycling the potential in propylene carbonate (Fischer, reagent grade) solution containing approximately 1.O M 3-methylthiophene (3MT; Aldrich, 99%) and 0.50 M tetran-butylammonium trifluoromethanesulfonate(TBATFMS; Fluka, 99+%) or 0.50 M lithium perchlorate (Fluka, 98+%). 3MT was used as received. Potentials were cycled between 0.0 and +1.7 V or -0.4 and + 1.3 V vs silver wire pseudo-reference electrode depending on whether the solution contains TBATFMS or LiClO, as supporting electrolyte. The silver wire pseudo-reference electrode was used in all experiments to avoid problems resulting from electrolyte contamination. Films were grown at a reflective platinum disk working electrode for subsequent spectroelectrochemical measurements. In-situ conductivity measurements were made according to the procedure published by Paul et ala3'using films grown on interdigitated gold line electrodes with their width of 120 F m and 60-pm spacing between them on a quartz disk (Figure 1). The interdigitated electrode was fabricated by electroplating in a Dequest 2000 gold cyanide bath on a quartz disk, 1 in. in diameter and 1/16 in. thick. Leads A and B in Figure 1 were connected together as the working electrode during the film growth. Films were grown until two electrodes, connected to leads A and B, are electrically shorted by polymer films. In-situ conductivity measurements were made by holding the film at a given potential with one potentiostat at one electrode, e.g., electrode A, and recording
the current-potential (I-y) curves at another, Le., electrode B, with a second potentiostat by scanning the potential from 0 to 110 mV at 10 mV/s, which is superimposed on the potential applied at electrode A.31 A Princeton Applied Research (PAR) Model 173 potentiostat-galvanostat along with a PAR 175 function generator was used to record cyclic voltammograms and to control potentials at the electrode surface. The near-normal incidence reflectance spectroelectrochemicalsetup, described was used for spectral recording and derivative cyclic voltabsorptometric (DCVA) e~periments.3~9~~ Bifurcated optical fibers with different optical specifications were used in two different spectral regions, Le., UV-vis and near-infrared (NIR), for spectral measurements. Data acquisition and instrument control were accomplished by using an IBM PC-XT computer with a Keithley 570 interface board. A Perkin-Elmer thermogravimetric analyzer, TGA-7, was used to determine the mass of the films by pyrolysis at 600 OC in air. The mass thus determined was then used for the calculation of the polymer film thickness.
Results and Discussion Figure 2 shows a family of spectra of P3MT films recorded in UV-vis and NIR regions in 0.5 M LiC104 propylene carbonate solutions at various applied potentials. These spectra were taken after the equilibrium had been reached at a given applied potential. The spectra shown in Figure 2a-c are in excellent agreement with those published previously in the l i t e r a t ~ r e . ~ ' The , ~ ~ band * ~ ~ at about 490 nm, which represents the band gap of this polymer, is the result of the m*transition, whereas one at about 720 nm has been assigned to be one of two bands arising from the bipoThe NIR spectra shown in Figure 2d can be characterized as having two broad absorption peaks at about 1200 and 1600 nm depending on the applied potential. At lower applied potentials, the absorption peak at 1600 nm is dominant, while the one at 1200 nm becomes important at higher applied potentials. To our knowledge, the transition at about 1100 or 1200 nm has not been described for P3MT in the literature, although a similar absorption peak has been described as that for the polaron during the oxidation of polypyrr~le.*~J~ As mentioned, the two bands at 720 and 1600 nm were assigned to arise from electronic transitions from the valence band to two energy states within the band gap of the bipolaron. According to the theoretical analysis of Fesser et al.,35there should be four signatures for optical absorption bands due to bipolarons: (1) there should be only two optical transitions within the band gap, (2) the sum of energies for these two transitions must be equal to the band gap energy, (3) the transition at 1500 nm must have a higher oscillator strength than that at 720 nm by about 4 times, and (4) as the degree of doping increases, the interband gap transition should undergo a blue shift. All the spectra for P3MT reported thus far, including ours reported previously el~ewhere?~ appear to meet these requirements except for the relative oscillator strengths. Thus, these two bands have been assigned to two transitions of the bipolaron within the band gap. This assignment requires that both of these transitions must arise from one species, bipolaron. However, our observation shown in Figure 2 and DCVA results to be described below indicate that three absorption bands originate from at least two optically, and thus chemically, different species within the band gap depending on the potential regime. We first examine the problem of the "chemical" reversibility of the transformationof the polymer by examining imbestic points in a series of spectra shown in Figure 2a-c:. We will call the first oxidized species a radical cation, which is equivalent to a polaron but electrons are more localized. An isosbestic point is observed near 620 nm between 0 and 0.60 V (Figure 2a); another is observed at a wavelength a little shorter than 590 nm between 0.7 and 0.95 V (Figure 2b). Above 1.0 V, there is no further isosbestic point (Figure 2c). Isosbestic points indicate that there is no stoichiometric change involved in these conversions other than just an electron.
5190 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 1.21 al
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We also note that the rate of increase in absorbances at 720 nm decreases at higher electrode potentials than 0.80 V, indicating that the radical cation is converted to some other species or the film may undergo degradation reaction(s) above 1.O V, or both. These observations suggest that the initial oxidation of the polymer into the radical cation and its further transformation into another species, a dication, are reversible through an electron-transfer process for a given number of monomeric units. At more positive potentials than 1.0 V, no more reversible transformation of the dication to another species is observed. From the limited result, we reach a conclusion that the “chemically reversible” redox transformation of the polymer to the first oxidized species, radical cation, is completed below 0.65 V, and its further conversion into the dication takes place between 0.70 and 0.95 V. Above this potential, the reaction becomes complicated as the dication may undergo chemically irreversible degradation reactions. The dication is equivalent to the bipolaron, but its more rigid structure approximates the quinoid structure through electronic conjugation. The second isosbestic point should not have been observed, if the polaron underwent an equilibrium reaction to the bipolaron as was concluded by Bredas et a1.26-2S and Kaufman et al.29 Our results suggest that two species with the same stoichiometry are generated in sequence upon oxidation of P3MT at different potentials. The only difference between the two species should then be the number of electrons on them. In order to examine the validity of the above conclusion further, we conducted DCVA experiments for three absorption bands observed upon oxidation of P3MT shown in Figure 2, and the results are shown along with a corresponding cyclic voltammogram in Figure 3 for comparison. While the C V (Figure 3d) shows a total, convoluted current signal resulting from both faradaic and nonfaradaic processes, DCVAs (dA/dt vs E plots; Figure 3a-c) at given wavelengths represent net chemical changes taking place as a function of the potential ~ c a n n e d . ~ ~ . ~ ~ Examining the DCVAs at both 720 and 1500 nm (Figure 3, a and b) as well as the CV (Figure 3d), we first note that a large capacitive current observed in the CV beyond about 1.0 V is completely eliminated. This capacitive current results from charging a large capacitor, the polymeric film. Thus, the DCVAs recorded at these wavelengths represent only the faradaic processes for oxidizing the relevant chromophore as a function of applied potentials. The rate of generation of the radical cation absorbing a t 720 nm reaches a maximum value at an applied potential of about 0.50 V. The rate becomes negative beyond about 1.O V,
indicating that the species is being destroyed or converted into some other species. The DCVA behavior recorded a t 1500 nm is somewhat similar to that at 720 nm in that the threshold and half-wave potentials are very close, but not exactly the same in that both rise and decay patterns of the signal are different. Furthermore, the DCVA signal at 1500 nm never reaches negative values, while that at 720 nm does. If both bands had originated from a single species, the DCVAs at two wavelengths must be identical. It is possible, however, that some experimental artifacts might have affected the DCVAs recorded a t the two wavelengths, as both optical fibers and detectors used in these regions were different. Whether the DCVA signal may reach negative values or not is also shown to be dependent on the film thickness (Figure 4). Furthermore, peaks in the NIR region are so broad that the DCVA signal recorded a t 1500 nm could have some component originating from the absorption tail of the 1100-nm band. The species absorbing at 1100 nm gives positive DCVA signals beyond about 0.80 V (Figure 3c) and may thus add up to the negative values at 1500 nm to make them close to 0 as shown in Figure 3b. Although the difference in wavelengths is relatively large for these bands in the NIR region, the difference in energy is relatively small at only 0.30 eV. It is reasonable therefore to conclude that the bands at both 720 and 1500 nm might have originated from one species as was originally assigned from theoretical s t ~ d i e s . 2 ~ ~ ~ On the other hand, a little different assignment consistent with our observations should not be ruled out and is thus discussed below. The DCVA signal recorded at 1100 nm (Figure 2c) is, however, clearly different from those at 720 and 1500 nm in that threshold, half-wave, and peak potentials are correspondingly more positive. The potential where the band at 1100 nm begins to increase is approximately where DCVA signals at 720 and 1500 nm begin to recede. This indicates that the species absorbing at 1100 nm is produced by further oxidation of the one absorbing at 720 and 1500 nm. The only difference between these two states is, most likely, in the number of positive charges per given monomer units. This conclusion is also consistent with the isosbestic point observed near this potential region. We thus attribute this band to a dication; two charges must be spread over 6-10 monomer units to make a quinoid structure through a conjugation. Although the current remains large even beyond 1.0 V to the solvent background potential (Figure 3d), the DCVA signal at 1100 nm begins to decrease beyond about 0.95 V, indicating that
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5191
Electrochemistry of Conductive Polymers 4
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the dication undergoes either a further oxidation or some other reaction. The product(s) generated by these reactions do not absorb photons at 720, 1100, or 1500 nm, as DCVAs at these wavelengths are either 0 or negative values at potentials above 0.95 V. We have not investigated the products that might be generated beyond this potential. From these observations, we conclude that the polymer is oxidized to the cation radical first, which is transformed to the dication at higher potentials. Another supporting evidence for this is that the redox potentials for two absorption bands at 720 and 1200 nm, determined from Nernst plots of absorbances at
these wavelengths, were reported to be 0.45 and 0.80 V,25 respectively. Theae are in excellent agreement with those estimated from half-wave potentials of the DCVA curves in Figure 3a-c. The dication produced in the second step may undergo a further oxidation to produce degradation products, which may not absorb photons within the spectral regions we have studied here. This conclusion assumes that the radical cation absorbs photons at both 720 and 1500 nm. The two bands, however, may arise from different species as in the case of polyaniline (PA).36 For the case of PA, the peak observed at 430 nm upon oxidation of neutral PA was assigned to be the "localized" radical cation, whereas the broad a h r p t i o n peak observed at about 800 nm at slightly higher electrode potentials was attributed to the "delocalized" radical cation, which is equivalent to the polaron. We believe that this might be the case for P3MT also. The absorption peak at 720 nm may be from the localized radical cation produced initially upon oxidation of the polymer, which may undergo a reasonably fast equilibrium to the partially delocalized radical cation absorbing at around 1500 nm. This may explain why the two bands behave slightly differently. Although the potential region where the signal rises and decays is somewhat similar for these two transitions (Figure 3a,b), their spectroscopic behaviors are reported to be differentaZ5 DCVA signals at 720 nm recorded for various film thicknesses are shown in Figure 4a. While the shapes of DCVA signals are similar to each other, the potential, at which maximum DCVA signals are observed, becomes more positive from about 0.50 to about 0.75 V as the thickness increases. At thinner films, the DCVA signal becomes negative as the potential increases. The negative DCVA signal indicates that the radical cation is indeed ~~ns~m upon e dfurther oxidation to produce the dication. Upon reversal of the potential, the radical cation is seen to be regenerated from the dication. The potential, at which the DCVA signal of the opposite sign crosses, becomes more positive also, as the thickness increases. These observations indicate that the net chemical change upon increasing the potential is limited by the transport of the counterion, perchlorate, into the polymer matrix. These features are not seen in CVs, for the CV current represents convoluted signals of more than one process, the rise and fall of both species, as well as capacitive currents of the film. Similar behaviors are seen in Figure 4b, when DCVA signals arc recorded in solutions containing various amounts of supporting
5192 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
40
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electrolyte at a polymer film with a constant thickness. Here the resistance to mass transport is large and becomes a limiting factor at lower electrolyte concentrations. CVs recorded concurrently with these DCVAs (not shown) do not show these characteristics, however. In the CVs, the peak current is barely seen only in a 0.50 M LiC104 solution, and broad, featureless currents are observed in both 0.050and 0.0050M solutions with no peak signals. High capacitive currents observed in solutions at lower electrolyte concentrations obviate CV peak currents. The observation that the oxidation of the radical cation to the dication is diffusion limited also leads us to draw a conclusion that the interconversion between these two species must be of electrochemical nature as in R+ * R2+ e-
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This is because reaction 1 requires the diffusion of counterions in or out of the polymer film, whereas reaction 2 requires a rearrangement of them only within the film. One may argue that similar effects could be observed for the electrochemical conversion of the neutral species to the polaron, if the equilibrium reaction as shown in reaction 2 is fast enough not to present any activational barrier and thus that simply more radical cations per given monomer molecules are generated at more positive potentials. However, the overpotential required to reach the maximum electrochemical rate at about 0.95V from the initiation of the electrochemical process at about 0.1 V is about 0.85 V as seen in Figure 3d, and this represents just too large an overpotential, i.e., activation energy, for a single electron-transfer process at reasonably thin films. The rate of a single electrochemical reaction must have been limited by the rate of mass transfer long before the applied potential reaches about 0.95 V, and thus, more than one electrochemical process must be taking place in a single CV curve shown in Figure 3d. In order to determine whether the bipolaron is the only charge carrier and the concentration of the supporting electrolyte would affect the conductivity, we made the conductivity measurements of the film in situ as a function of the applied potential; the results are shown in Figure 5. The conductivity, measured in current across the insulating gaps between interdigitized electrodes (Figure 1) covered with P3MT, is affected slightly by the concentration of supporting electrolyte, as shown in Figure 5a-c. Also shown in this figure is that the increase in the conductivity levels off when dications are generated. It is clear from the figure that the film becomes electrically conducting when only polarons are generated. In the case of polyaniline,31~37~38 the film becomes a very poor semiconductor when the bipolaron or its isoelectronic form, i.e., a quinoid structure, is generated. However, the dication is shown to be a better conductor than radical cation in the case of P3MT as shown in Figure 5. This indicates that the dication is highly
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delocalized to act as a bipolaron rather than a rigid quinoid structure. While this is in agreement with the conclusion of Br€das et a1.,2&28our result shows that both the polaron and bipolaron are charge carriers. Finally, we have also determined the extinction coefficient of the band gap transition at 480 nm by determining the thickness and absorbance of polymer films. The extinction coefficient of the ?rr* band of polythiophene has been reported to be about 2.1 X lo5 cm-' in the which was estimated from the film thickness estimated using the charge spent for the polymer growth assuming 100% of a Coulombic efficiency. The Coulombic efficiency is not expected to be loo%, however, because the growth rate is not expected to be constant over time even under the constant-current electrolysis condition due to different overvoltage requirements for the polymer growth, which depend primarily on what is on the electrode surface. We therefore conducted experiments to determine the extinction coefficient by measuring absorbances for various film thicknesses. Film thicknesses were determined thermogravimetrically after CVs were recorded. The calibration curve for the thicknessanodic CV peak current relation is shown in Figure 6a. The CV peak current was measured at a scan rate of 50 mV/s during the oxidation of the polymer in a propylene carbonate solution containing 0.10 M LiC104. The thickness, L,of the film was then calculated from the equation J5 = M / ( P 4 (3) where M is the amount of the film in grams obtained from thermogravimetric measurements, A is the surface area of the electrode on which the film is grown, and p is the density of the film. The density was determined to be 1.50 g/cm3 by the Archimedes principle. The slope of the line shown in Figure 6a is 0.43 pm.cm2-mA-'. This relation allows one to estimate polymer film thicknesses from CV peak currents, if recorded in 0.10 M LiC104 electrolyte solution at a scan rate of 50 mV/s. Figure 6b shows the absorbance at 490 nm plotted as a function of the film thickness as determined above. An extinction coefficient of 1.1 X lo5cm-' is calculated from the slope of the plot for P3MT in its fully reduced state. Our extinction coefficient is in good agreement with the one reported for polythiophene by Chung et a1.22at approximately half of that estimated from their report. The oscillator strength of P3MT is expected to be larger than that of polythiophene due to the methyl group attached to the 3-position, which is weakly electron donating. The larger
J. Phys. Chem. 1992,96, 5193-5196 extinction coefficient in their report could have resulted from a number of experimental parameters such as less than 100% of the current efficiency for the electrochemical polymerization based on which the thickness was estimated, different solvents used (acetonitrile) in two studies, the difference in morphology due to different methods of preparation, swelling of the polymer, etc. Extinction coefficients of other bands can be calculated in relation to this by simply measuring the relative absorbance values.
Conclusion We have studied changes in spectral, electrochemical, and electrical properties upon oxidation of electrochemically grown P3MT films, employing in situ spectroelectrochemical and conductivity measurements. From the results, we conclude that the evolution of the second oxidized species from the first one is a potential activated process rather than just a chemical equilibrium reaction. The two processes have different redox potentials at about 0.45 and 0.8 V, respectively, which were first determined from Nemst plotsz5and also shown by our current DCVA study. The doping process, through which two oxidized states are generated at two electrode potentials, is shown to be controlled by the diffusion of counterions. Finally, both species generated upon oxidation of neutral P3MT are shown to be charge carriers. From these observations, we question the validity of the previous conclusion that the conducting form of P3MT has only one charge-carrying species, Le., bipolaron. We conclude that three absorption bands observed from oxidized P3MT have origins in at least two rather than one different chemical species, bipolaron, and both of them are stable in different potential domains. Our DCVA observations suggest that the absorption band at 720 nm could be originating from the localized radical cation, which turns into the delocalized polaron absorbing at 1500 nm through an equilibrium reaction. Registry NO. P3MT, 84928-92-7; 3MT, 616-44-4; TBATFMS, 35895-70-6, LiClO,, 7791-03-9; C104-, 14797-73-0.
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Enhanced Stability of Potassium Solutions in Tetrahydrofuran Containing 15-Crown-5 Zbigniew Grobelny, Andrzej Stolarzewicz,* Maria Sokbl, Janusz Grobelny, and Henryk Janeczek Institute of Polymer Chemistry, Polish Academy of Sciences, 41 -800 Zabrze, Poland (Received: June 28, 1991; In Final Form: March 9, 1992)
The process of potassium metal dissolution in tetrahydrofuran containing 15-crown-5and the properties of the solution were studied. The changes in concentration of potassium anions and electrons as a function of time were determined by means of 39KNMR and ESR spectroscopies. Enhanced stabilityof the solution with 15-crown-5 relative to that containing 18-crown-6 or 12-crown-4 was revealed.
Introduction Potassium solutions in linear or cyclic ethers containing cation complexing agents have usually been obtained a t low temperatures.14 Above 220-250 K a gradual decomposition accompanied by a discoloration of the blue solution takes place.
* Author to whom correspondence should be addressed.
The dissolution of potassium in tetrahydrofuran (THF) con( 1 8 c 6 ) as well as the decomposition of the taining 18-"-6 solution were recently studied.'-'O In a short time of contact with a potassium mirror (t, =: 1 min) at 298 K,the resulting solution contains mainly solvated electrons, e;, and potassium cations complex4 by the crown ether, K+(18C6), whereas the contribution due to potassium anions is small. At longer t , the con-
0022-3654/92/2096-5193%03.00/0 0 1992 American Chemical Society
