766
Abraham Friedenberg and Haim Levanon
(37) H. S. Gutowsky, D. W. McCall, and C. P. Slichter, J . Chem. Phys., 21, 279 (1953). (38) H. M. McConnell, J . Chem. Phys., 28, 430 (1958). (39) D. E. Woessner, J . Chem. Phys., 35, 41 (1961).
(40) R. K. Gupta, T. P. Pitner, and R. Wasyiishen, J . Mhgn. Reson., 13, 383 (1974). (41) 6 . G. Cox, G. R. Hedwig, A. J. Parker, and D. W. Watts, Aust. J . Chem., 27, 477 (1974).
Electron Spin Resonance and Optical Electron Spin Resonance Studies of Alkali Metals-Tetrahydrofuran Solutions in the Presence of Dicyclohexyl 18-Crown-6' Abraham Friedenberg' and Haim L e v a n ~ n ' ~ Depatiment of Physical Chemistry, The Hebrew University, Jerusalem, Israel and Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received June 23, 1976; Revised Manuscript Received February 7, 1977) Publication costs assisted by the Division of Physical Research, U.S.Energy Research and Development Administration
An ESR and ESR-optical study of M-THF (M = K, Rb, Cs) solutions in the presence of dicyclohexyll8-crown-6, CR, is presented. In K-THF solutions at room temperature, the ESR signal intensity of the photoelectrons passes through a maximum at [CR] N lo9 M. The monomer radicals, K., exhibit an optimum ESR signal-to-noise ratio at [CR] M. In solutions which are not illuminated the ESR signal intensity of the dark solvated electrons increases monotonously with CR concentration (up to M), whereas its line width is almost unaffected (-180 mG) retaining its Lorentzian line shape. Above W3M CR the ESR line width becomes very narrow (-15 mG). Contrary to the case of potassium, the ESR line width of the solvated electron in Rb-THF systems is increased significantly on addition of CR. This observation is interpreted in terms of an exchange process of the type e + RbCR', e P RbCR', e + e. The rate constant for this exchange is 3 X 10' M-' s-'. The recombination process of the photoelectrons in M-THF systems follows a pseudo-first-order kinetics which depends on the CR concentration. This reaction is interpreted in terms of two competitive reactions, e + M' M- and e + MCR' e, MCR'. An activation energy for this recombination process is found to be in the range of 2-8 kcal/mol for low and high CR concentrations, respectively.
-
-
A. Introduction The introduction of the organic macrocyclic molecules such as crown ethers, CR, as complexing agents of alkali-metals cations, opened many interesting possibilities in the study of the blue solutions of alkali metals dissolved in ethers and amine^.^-^ Thus, by monitoring the CR concentrations, it is possible to control the elementary processes 2M,
* M+ + M-
M+ t C R 3 MCR'
It was shown by Dye and co-workers that in some cases, where the solutions consisted mainly of M- and MCR', the salt M'CRM- could be precipitated and identified.' It is generally accepted that, in addition to the anion and cation, there are also present in these solutions the solvated electron and the monomer radical. Regarding other species, there is experimental and theoretical evidence for the existence of e2,M2,and ion pairs between the electron and the cation or the solvent (e, M') or (M', S-), respectively.a-ll Solutions of alkali metals in amines have been extensively investigated by optical and magnetic resonance techniques.6i12i13 Regarding ethers as solvents, these have mainly been subjected to optical4p14 and, to a lesser extent, magnetic resonance experiment^.'^ The main difficulty in the latter experiments is the relatively low solubility of alkali metals in ethers. In a recent communication,16 we reported some preliminary results on the behavior of the photoelectrons and monomer radicals upon light excitation in the presence of low CR concentrations in M-THF solutions (M = K, Rb). The purpose of the present work is to study the solutions The Journal of Physical Chemistry, Voi. E l p No. 8 , 1977
of alkali metals over a wide range of CR concentrations by employing the technique of ESR and ESR-optical spectroscopy. In section C-I we describe in detail the behavior of the dark electrons and photoelectrons in K-CR-THF solutions. We find that the yield of the photoelectrons inM) and also creases with CR concentrations (up to on lowering the temperature. The monomer radical concentration, on the other hand, is affected only over a narrow range of temperatures and CR concentrations. Although the ESR line shape and line width are hardly affected on changing the CR concentration up to M in K-THF solutions, we find at high CR concentration a substantial change of the ESR line shape which is followed by a very narrow line width of the solvated electron due to exchange narrowing process. Unlike potassium, we find that the line width of the solvated electron in Rb-THF solutions is broadened with the CR concentration. This is analyzed in terms of exchange phenomenon. These effects are described in section C-11. In section (3-111,we describe the kinetics associated with the photoelectrons as a function of the CR concentration. Over almost the entire range of temperatures and CR concentrations we find a pseudo-first-order behavior for the recombination process of the photoelectrons. These observations are found in K-THF, Rb-THF, and Cs-THF systems. The effect of spin polarization reported previously16 was observed in all the systems described above and will be discussed elsewhere.
-
B. Experimental Section Dry solvents, prior to sample preparations, were prepared as described previously in the literat~re.'~~''Di-
Alkali Metals-THF Solutions in Dicyclohexyl 18-Crown-6
767
K-THF cyclohexyl 18-crown-6, CR, was purchased from Aldrich Chemicals and was used with no further purification. Rubidium and cesium were purchased from Fluka in 1-g ampules, and were divided into four capillary tubes of 3 mm i.d. under high vacuum. Each of these tubes was sealed off and was used separately. Alkali metal solutions were prepared on a vacuum line in two stages: (i) disK-THF tillation of the alkali metal through a glass side arm, di[CR],: 2xlQ5Y vided by constrictions, into the main glass tubing apparatus. On such an apparatus an ESR pyrex tube and an optical cell were attached; (ii) distillation of the dry solvent into the main tube whose volume was calibrated previously. This compartment was sealed off after a few freeze-thaw cycles. For additional details on sample preparation, the reader is referred to previous publicat i o n ~ .When ~ ~ CR was used, an additional pyrex glass compartment, separated by a break seal, was attached to the main glass tubing apparatus. The concentration of CR was governed by first dissolving a known amount of CR in DME solution. This solution served as a stock of CR Flgure 1. ESR spectra of the solvated electron e (g = 2.0021), and of the monomer radical K- at 293 K. The experimental conditions at from which a known volume was transferred into the which the spectra were recorded are indicated on each trace. Light separated compartment. The DME was evacuated, leaving spectra were recorded when the ESR sample was irradiated (in situ) behind only the oily CR. After an experiment was perwith all wavelengths above 650 nm. formed on the CR free compartment, the seal was broken and CR was added to the solution. In this way, two extemperatures while keeping the solvent in contact with the periments with different CR concentrations could be metal. Once the solution turns to pale blue, the color performed. persists for several hours also at room temperature. Before sample preparation, the glass apparatus was At room temperature, the paramagnetic species which rinsed with a detergent, followed by several rinses with is normally observed by ESR is the solvated electron, triple distilled water. Finally it was rinsed with analytical whereas the monomer radical in many cases escapes deacetone, degassed, and heated on a vacuum line for several tection. Upon irradiation of the sample at a wavelength minutes. It should be noted, however, that the stability which corresponds to the absorption peak of K- (900nm), of the solutions is markedly affected by the handling of the photoelectron, e, and the monomer radical, K., are the glass apparatus prior to the distillation p r o c e ~ s e s . ~ ~ expected ~~ to be observed by ESR according to the reaction All experiments were performed immediately after sample hv preparation. After 12 h the samples were discarded. K' -+K. t e Optical measurements were performed at room temESR detection of the monomer radical as described by perature on a Gary-14 spectrophotometer. The optical eq 1.1depends strongly upon the experimental conditions. path lengths of the cells varied from 1to 10 mm, depending In particular, the way of sample preparations is the main on the concentration of CR. factor responsible for the success or failure to obtain the ESR measurements were carried out on a Varian E-12 monomer radical. For example, we found that it is of spectrometer equipped with 100, 10, and 1 kHz field extreme importance that the solution should come in modulation units. The temperature of the samples in the contact with a highly active metal mirror. To test this, we ESR cavity was controlled by using a standard N2 flow dissolved a liquid alloy of Na-K, which is known to be system. The temperatures were measured using a cophighly active, in a THF solution. When the sample was per-constantan thermocouple. Part of the experiments irradiated continuously with all wavelengths above 650 nm, have been performed on a noncommercial ESR specboth the solvated electron, e, and the monomer radical, trometer with 10-kHz field modulation." K., could be observed easily. Nevertheless, under these The exciting light source was a high-pressure xenon arc experimental conditions, we could not observe the mo(150 W), which was modulated electronically. The exnomer radical in a reasonable signal-to-noise ratio when perimental procedure for time averaging of the ESR signal the ESR spectrum was taken under dark conditions. In intensity, and for phase sensitive detection coupled to the Figure 1 we present four ESR spectra taken at two difmodulated light frequency, was performed as described in ferent concentrations of CR, both in dark and light. As previous studies." Irradiation of the sample solutions was a comparison the ESR spectrum of K-Na-THF solution in situ in the ESR cavity using Corning cutoff filters. is also given (bottom trace). The signal intensity of the ESR lines was determined In the presence of small amounts of CR in K-THF by employing the expression for the relative line intensity solutions, we have invariably observed the ESR signal due CiAH:hi, where AH is the measured peak-to-peak sepato the monomer, both in the dark and in the light. Apration in the absorption derivative of the susceptibility, parently, the introduction of small amounts of CR initiates h is the signal height from peak to peak, and the sumthe dissociation of the metal in the nonpolar solvent. mation is over the hyperfine components. The relative Figure 2 shows the dependence of the signal intensity of intensities were always normalized to the same experithe monomer radical and the solvated electron vs. [CR], mental setup of the ESR spectrometer. (concentration of added CR). On increasing [CR], the C. Results and Discussion ESR signal of the monomer first increased then after I. K-THF Solutions at Low CR Concentrations. When passing through a maximum it started to decrease and potassium metal is dissolved under high vacuum in TMF finally disappeared at about lW4 M of [CR], (cf. triangles in Figure 2). At the same time the intensity of the color at room temperature, a colorless solution results. The solution turns into pale blue on rigorous shaking at low increased monotonously with [CRIoat all concentrations The Journal of fhyslcal Chemistry, Vol. 81, No. 8, 1977
768
Abraham Friedenberg and Haim Levanon 2
I
I
I
K-T H F
Figure 2. ESR signal intensities of the solvated electron (circles) and of the monomer radical (triangles) as a function of the [CR],, (concentration of CR added). Full circles and triangles are the experimental intensities measured under dark conditions. Open circles and triangles are the corresponding intensities measured under light irradiation conditions, after subtracting the dark contribution. The dashed curve indicates that above M of added CR the signal intensity is reduced.
Figure 4. The ratio R = [K-]/[e] as a function of [CR],. Open and full circles are the experimental points measured under dark and light conditions, respectively.
The dominant equilibria in nonpolar solvents are63l5 2M,
kl
M'
+ M-
k,/k, = K,
(1.2)
k2 k
M - a M . t e-
l z , / k , = K,
(1.3)
kJk,=K,
(1.4)
k4
k
M . b M' t ek6
k
M+ t CR
MCR'
k,/k, = K,
(1.5)
k8
The overall reaction may be summed up as 2M, t CR 2 2e' t M'
+ MCR+ 2 M- + MCR'
(1.6)
Thus, excess of CR will shift (1.6) to the right to give solvated electrons, M-, and to a lesser extent M+ and Me. This is reflected experimentally by increasing the solvated electron concentration and the intensification of the blue color of the solution. The latter was verified by monitoring the absorption peak due to the anion, K-, at 900 nm, on increasing the CR concentration. Alternatively, one can consider the ratio [KO]/[e] which in terms of (1.3) and (1.5) can be expressed as
193
213
233
253
273
293
ToK Flgure 3. ESR signal intensity of the solvated electron at two CR concentrations in light and dark conditions as a function of the temperature. The experimental conditions are indicated on each line.
of CR studied. At room temperature the ESR signal intensity in the dark always increased with increasing [CR], whereas in the light it showed a maximum at about M CR. In this range of CR concentrations, the ESR line shape was Lorentzian and its line width remained constant (180 mG). It was saturated by microwave power at about 2 mW. Figure 3 shows the temperature dependence of the ESR signal intensities of the solvated electron at two CR concentrations. For the low CR concentration, the signal intensity in the dark was too small to be drawn to scale. Also, it is worth noting that the difference in the signal intensities at low temperatures exceeds almost two orders of magnitude, whereas at high temperatures both signal intensities are of the same order or magnitude. The monomer radicals (observed in the range of 0 < [CR] < M) escaped ESR detection at temperatures below 230 K. The Journal of Physical Chemistry, Vol. 81, No. 8 , 1977
where [M']o= [M']
+ [MCR']
Since K4 N lo9 M-1,5the product K4[CR] will start to affect R at very low concentrations of free CR, Le., at [CR] > lo-' M. We thus expect R to decrease upon increasing the CR concentration. Figure 4 shows the experimental results of R vs. CR added at room temperature.20 From the experimental value of R at [CR] = 0 (lower trace), and taking K315as 5 X lo5 M-l we estimated the solubility of Kf (and K-) in THF as -5 X lo4 M in accord with optical determination.21This value is reasonable if it is compared M in DME at 193 K.16 to the reported value of 5 X The experimental observation that the monomer radicals M are not detected at higher CR concentration than is conspicuous and warrants some remarks. From eq 1.2 and 1.4, one should expect that the monomer concentration in a metal saturated solution should be independent of the CR concentration, i.e, [K.] = constant. This is in contradiction to ESR measurements (cf. Figure 2) and also to previous observations that the monomer radical at high CR concentration escapes detection.6 For example, the monomer radical in K-diethyl ether solution could be
Alkali Metals-THF Solutions in Dicyclohexyl 18-Crown-6 observed only in the presence of CR (5 X M).22 Such a discrepancy may be interpreted in terms of earlier suggestionsgthat part of the monomer radicals escape ESR detection due to ion-pair formation which exchange with the monomeric species. In view of the photoelectron kinetics (section 111)we propose that in addition to (e, K') at low CR concentrations, there exists also (e, KCR') at high CR concentrations. Both species have a broad ESR signal which is covered by the solvated electron signal. Inspection of the ESR spectra shown in Figure 1supports evidence for such a dynamic process. It is evident that the signal amplitudes of the inner hyperfine components exceed the corresponding outer components. (This phenomenon should not be confused with spin polarization.)16 Regarding Rb-THF and Cs-THF systems, the monomer radicals in the former system could be observed in a poor signal-to-noise ratio having a hyperfine splitting A = 150 G. 11. K-THF and Rb-THF Solutions at High CR Concentrations. A further increase of CR (up to 0.1 M) results in a deep blue color of K-CR-THF system, giving rise to a suspension type solution. When this solution is inserted immediately after its preparation into the spectrometer's cavity at room temperature, a single ESR line appears. Its line shape depends strongly on the sample tube diameter. In a -3-mm i.d. tube, the line shape of the first derivative spectrum is anomalous and is very similar to the ESR spectrum reported by Acrivos and c o - ~ o r k e r sfor ~~ Pb-Na-NH3 solutions. Such a spectrum is characterized by its broad wings and a very sharp break between the peaks of the first derivative spectrum. In an -0.5-mm i.d. tube, the ESR spectrum, although distorted by the inhomogeneity of the magnetic field, has a normal line shape and the peak-to-peak separation is 12 mG. The line shape and width change gradually with time, giving rise to a broader spectrum with a line width of 40-60 mG. After a 24-h period, the line width reaches a constant value of about 180 mG, retaining its normal Lorentzian line shape. The variation in time of the ESR line shape and width were always accompanied with a change in the solution from a suspension type into an homogeneous one. Apparently, the anomalous line shape having an extremely narrow line width originates from an experimental artifact due to the shift of the klystron frequency by the strong and narrow resonance itself.24 By reducing the sample volume the anomalous behavior disappeared. Also, when the AFC was turned off and the ESR spectrum was recorded using the large diameter sample, a normal line shape was obtained which was somewhat broadened (30 mG) . As mentioned above, very narrow ESR lines have been observed in Na-NH3 systems (dilute and concentrated solution^),^^-^^ where a fast exchange mechanism leads to a narrowing process. Such a mechanism may account for the narrow ESR lines in K-CR-THF solutions. The same experiment was carried out with Rb-THF in the presence of excess CR. Unlike potassium, the line width due to the solvated electron broadened appreciably on increasing the CR concentration and was saturated at relatively high microwave power (2-30 mW) for low and high CR concentration, respectively. Figure 5 shows the ESR line width of the solvated electron (peak to peak in the first derivative curve) vs. the CR concentration added, in a Rb-THF solution at 213 K. Comparing this observation to that discussed above for the K-CR-THF solution, it seems unlikely from the increasing of the line width with addition of CR that a spin exchange process between the solvated electrons may account for the ESR
709
-
, , , , ,,,
1.0 1.0.
01
,
~
(I , , , , , , ~I
,
, , , , .,~ ~
Rb-THF
0
4
10-5
10-3
10-4
Figure 5. ESR
[CRl, ( M )
10-2
peak-to-peak line width, A&,, vs. [CR],.
t
1 I 0 0 25
75 50 [CR],/AHO~IO~
Flgure 6. Plot of (&//+,)''* vs. [CR]olAHocorresponding to the solvated electron in Rb-THF solutions. The variables 4 and A,, are the normalized ESR signal amplitudes at [CR], = 0 and different [CRIo, respectively;AH, (140 mG) is the peak-to-peak line width of the solvated electron at [CR], = 0 (for details see ref 26).
line broadening. Neither a spin exchange of the type e t t Rb', e $
*
eJ t
Rb', et
(11.1)
since the [Rb'] decreases on increasing the CR concentration. Hence, either of the following exchange processes may account for the line broadening: et t RbCR', e$ 3 e $ t RbCR', e f
(11.2)
or e t RbCR', e
t RbCR+, e t e
(11.3)
Assuming that [CR], N [RbCR'] we calculated the second-order rate constant for the exchange process k,, following Ward and Weissman.26 From the slope S in Figure 6 we have k, = 4 3 X a X 2.8 X lo6 X S N 3 X lo8 M-' s-'. This value is smaller than the diffusion controlled rate. On the other hand, this value seems to be too high for the large cation transfer. It is likely that both processes contribute to the broadening effect. 111. Kinetics of the Photoelectrons. Inspection of Figures 1and 3 shows that strong photo-ESR signals are observed on decreasing the temperature and increasing the CR concentration. Employing pulse light excitation, we have followed the temporal behavior of the ESR signal intensity on varying the CR concentration and temperature. Typical kinetic curves are shown in Figure 7. These were taken under different conditions as indicated on each trace. Except for the high temperature region, all of the kinetic curves follow a pseudo-first-order recombination process. The effect of increasing the CR concentration on the observed first-order rate constant is analogous to that The Journal of Physical Chemistw, Vol. 81, No. 8, 1977
770
Abraham Friedenberg and Haim Levanon
i
3 ms
OFF 1
1
1
1
I
I
I
I
I
I
J ON Figure 7. ESR decay curves of the photoelectron in K-CR-THF solutions at two CR concentrations at different temperatures. Inserted are first-order plots as described in the text (cf. eq 1.4, 111.1, and 111.2). The distance between the ON and OFF is the duration of the light pulse. (s-') and Activation Energies A H (kcal/mol) for Potassium, Rubidium, and Cesium in THF at Different CR Concentrations [CRI,, M kKobsd AHK kRbobsd AHRb kCsobsd AH c s 0 400 2 -4 1470 2-3 3100 1-2 480 2 -4 147 3-4 1470 1-2 2 x 10-5 30 5 -6 173 3-4 1470 3-4 2 x 10-4 1 x 10-3 6 7-8
TABLE I: Pseudo-First-OrderRate Constants k o b d at 213 K
of decreasing the temperature. To account for these observations, we propose the occurrence of the process (see also section I): k9
e t KCR" -+ e , KCR+
(111.1)
m K-THF
in addition to k6
e t K" -+ K. (or e , K')
(1.4)
These two processes lead to a pseudo-first-orderdecay rate kobsdgiven by k 6 + k&,[CRI kobsd = [M+l0{ (111.2) 1 + K4[CR] where koM is the observed seudo-first-order rate constant. Figure 8 shows plots of k"! sd vs. [CR], at 233 and 293 K. It follows from (111.2) that when [CR], = 0, it is possible to determine k g , provided that [M+Iois known. In section C-I we have estimated the solubility of potassium in THF at room temperature ([M+IoN 5 X lo4 M). The observed rate constant at room temperature at [CR], = 0 was found to be kobsd = 4.0 X lo3 s-'. This value gives rise to k6 N 8 X 10' M-l s-l. The rate constant lies close to the diffusion-controlled limit, in agreement with previous rep o r t ~ . ' The ~ other limit of eq 111.2 enables us to estimate k9. When [CR], is in excess we have kobsd =
[M+Iok9
(111.3)
Assuming that [CRIoN [M'],, we calculated k9 to be -10 X lo6 M1 s-l at room temperature. We measured the activation energies for the electron recombination at The Journal of fhyslcal Chemistry, Vol. 81,No. 8, 1977
Figure 8. Plots of Pbsd vs. [CR], at 293 K (full circles) and at 233 K (open circles). For definition of Pbsd, see eq 111.2 in the text.
various CR concentrations, assuming a first-order process. The calculated activation energies lie between 2.0 and 8.0 kcal/mol for low M) and high M) CR concentrations, respectively. Considering the fact that the solubility of potassium metal in DME is greater than in THF, these results fit those reported by Glarum and Mar~ha1l.l~ The results of a series of experiments with potassium, rubidium, and cesium are summarized in Table I. It has been observed that at high CR concentrations M) and at low temperatures the kinetics of ([CR], >
77 1
Alkali Metals-THF Solutions in Dicyclohexyi 18-Crown-6
Acknowledgment. The valuable comments of the referees are highly acknowledged.
OFF
References and Notes ON
2.4Sec c-----------,
__c.
Figure 9. ESR decay curve of the photoelectron at 213 K and [CR], = 5 X lo-* M. Inserted is a second-order plot. The concentration of the electron was determined by comparing the observed signal intensity to standard solution of DPPH in benzene.
recombination deviates from first order (at high temperatures the photoelectron concentration is very low). The experimental observation (cf. Figure 3) that the photo-ESR signal intensity increases on decreasing the temperature, despite the fact that the dark ESR signal decreases, may be interpreted in terms of an additional reaction under these conditions. e t e+e, k, 1
(111.4)
On decreasing the temperature equilibrium 111.4 shifts to the right, resulting in a decrease of the dark ESR signal intensity. The strong enhancement of the ESR signal intensity upon irradiation is probably due to the photodissociation of the dielectron. Figure 9 shows a decay curve of the ESR signal intensity of the photoelectron in a K-THF solution at 213 K in the presence of [CR] = 5 X lo-’ M. The decay follows a second-order reacti0n.a The rate constant from a second-order plot (insert in Figure 9) has been calculated as 2.4 X lo6 M-’ s-’.
(1) This work was partially supported by a grant from US-Israel Binational Science Foundation (BSF), Jerusalem, Israel, and by the Radlatlon Laboratory of the University of Notre Dame (operated under contract wkh the US. Energy Research and Development Administration). This is Document No. NDRL-1712. (2) Hebrew University, Jerusalem. In part!!i fulfillment of the requlrements for a MSc. thesis to be submitted October, 1976. (3) University of Notre Dame. On leave from the Hebrew University, Jerusalem. (4) M. T. Lok, F. J. Tehan, and J. L. Dye, J . Phys. Chem., 76, 2975 (1972). (5) J. L. Dye, C. W. Andrews, and S. E. Mathews, J. Phys. Chem., 79, 3065 (1975). (6) J. L. Dye in “Electrons in Fluids”, J. Jortner and C. R. Kestner, Ed., Springer-Verlag, West Berlin, 1973, p 77. (7) F. J. Tehan, 6.L. Barnett, and J. L. Dye, J . Am. Chem. Soc., 96, 7203 (1974). (8) J. L. Dye, M. 0. DeBacker, J. A. Eyre, and L. M. Dorfman, J. Phys. Chem., 76, 839 (1972). (9) T. R. Tuttle, Jr., J . Phys. Chem., 79, 3071 (1975), and a private communication. (10) J. W. Fletcher and W. A. Seddon, J. Phys. Chem., 79, 3055 (1975). (11) S. Golden, J . Phys. Chem., 79, 2887 (1975). (12) R. Catterall and P. Edwards, J. Phys. Chem., 79, 3010 (1975). (13) A. Gaathon and M. Ottolenghl, Isr. J. Chem., 8, 165 (1970). (14) J. Eloranta and H. Linschitz, J. Chem. Phys., 38, 2214 (1963). (15) S. H. Glarum and J. H. Marshall, J. Chem. Phys., 52, 5555 (1970). (16) A. Friedenberg and H. Levanon, Chem. Phys. Lett., 41, 84 (1976). (17) T. R. Tuttle, Jr., and S. I. Weissman, J. Am. Chem. Soc., 80, 5342 (1958). (18) R. W. Fessenden, J . Chem. Phys., 48, 3725 (1968). (19) H. Levanon and S. I. Welssman, rsr. J . Chem., 10, 1 (1972). (20) Notice that the lower curve in Figure 4 was obtained after many unsuccessfulattempts to observe the monomer radical in solutions free of CR. (21) R. Catterall, J. Slater, and M. C. R. Symons in “Metal-Ammonia Solutions”, J. J. Lagowski and M. J. Slenko, Ed., Butterwwths, London, 1969, p 329. (22) J. L. Dye, M. 0. DeBacker, and V. A. Nicely, J . Am. Chem. SOC., 92, 5226 (1970). (23) J. V. Acrivos, J. Azebu, and S. Farmer in ref 21, p 145; J. V. Acvivos and J. Azebu, J . Magn. Reson., 4 , 1 (1971). (24) S. I. Weissman and T. R. Tuttie, private communication. (25) C. A. Hutchison, Jr., and R. C. Pastor, J . Chem. Phys., 21, 1959 (1953). (26) R. L. Ward and S. I. Weissman, J. Am. Chem. Sm., 79,2086 (1957). (27) Note that at thls CR concentration the ESR spectrum recorded immediately after sample preparationshows the anomalous line shape as described in section C-11. This kinetic curve was taken after the ESR signal gained a normal Lorentzian line shape (AH N 180 mG).
The Journal of Physical Chemlstry, Vol. 8 1 , No. 8, 1977
