NOTES
4402
spectively. The irradiation of pure liquid trans-1,3a weak e.p.r. resonance with unresolved hyperfine diphenylpropene revealed isomerization to the cis structure was reported for DPE adsorbed on silicaalumina4J and on the synthetic zeolites.6 Leftin, isomer (G 9.0 at 4% conversion and 5.4 at 12% conversion) but there was no detectable formation of et aZ., supposed that the paramagnetism stemmed from cyclopropane derivatives. In benzene solution a t 0.01 the blue species, but Rooney and Pink5 attributed it to M the yields of products from cis- and trans-l12-di- a band a t 330 mp because they observed that addition phenylcyclopropane, a t doses of 9-18 X 1019 e.v./g., of water to the sample eliminated the signal but did were as follows : cis-1,2-diphenylcyclopropane, , not affect the blue color. I n agreement with this, 0.70; trans isomer, 0.92, - ; cis-l13-diphenylpropene, Hall’ noted that paramagnetism, if present, was below his limit of detectability when the catalyst-hydro0.05, 0.07; trans isomer, 0.03, 0.04. These yields are carbon system was a deep blue-black. Hirschler* given as G values based on total energy absorbed. On summarized evidence supporting the view that the more prolonged exposure, 2 X 1O2I e.v./g., solutions of blue species is a carbonium ion formed from a dimeric either isomer tended toward a trans:cis ratio of 1.15. or trimeric reaction product of DPE, a hypothesis On the basis of product formation, it is evident that which is consistent with the kinetic observations of radiolysis represents a situation intermediate between Leftin and HalL3 Rooney and Hathawayg identified direct and sensitized photolysis. The radiolysis in the blue carbonium ion as dilute benzene solution is necessarily a benzene sensitized process, since it is a consequence of energy CH, absorbed primarily by the solvent, and the radiolysis of the neat liquids is perhaps best considered as self(CeH6)&-C-C(CeHs)z sensitized. I n benzene sensitized cistrans isomeri€+ zations of olefins by y-rays there is mounting e ~ i d e n c e ~ ~ ~ The first objective of the present experiments was to show that triplet benzene is the effective species. to confirm that the blue species is not paramagnetic. It seems highly probable that this will prove true also The second was to ascertain whether the weak paramagof the inversions on a cyclopropane ring and thus that netism found by others4+ was due to a radical formed there will be a close mechanistic correlation with the from DPE or from an impurity. I n air, DPE is quite sensitized photochemical reaction. The further conunstable. As received, it may contain up to 10% clusion is indicated that in radiolysis, and still more benzophenone and 1-2% biphenyl. To avoid compliin direct photolysis, some fraction of the reaction, cations due to impurities, freshly purified material corresponding to the yields of rearrangement products, must be used and contact with air avoided. takes place by way of a more highly energized interI n contrast with the data of Rooney and PinkI5 mediate or by a different mechanism than is involved et aZ.,4 reported that the e.p.r. signal from Leftin, in sensitized photolysis. chemisorbed DPE increased on addition of water vapor. They also noted that the signal intensity was enhanced by contact with hydrogen or argon but was decreased with oxygen. Effects of rare gases almost Electron Spin Resonance of 1,l-Diphenylethylene certainly must be of a physical nature. A similar Adsorbed on Silica-Alumina Catalysts apparent decrease in signal intensity on contact of perylene cation radicals with Oz5 has also been shown by Francis R. Dollish and W. Keith Hall
-
I
Mellon Institute, Pittsburgh, Pelzlzsylvalzia (Received June ,%?$ 1966)
Work on the optical spectra from 1,l-diphenylethylene (DPE) adsorbed on silica-alumina catalysts has been reviewed recently by Leftin and Hobsonl and by Terenia2 Both attributed the band at 607 mp (the blue species) to a cation radical. This assignment was first suggested by Leftin and Hall, who cited evidence supporting it but reported that they could not find the expected e.p.r. signal. However, shortly thereafter, The Journal of Physical Chemistry
(1) H. P. Leftin and M. C. Hobson, Advan. Cai!aZysis, 14, 116 (1963). (2) A. N. Terenin, ibid., 15, 227 (1964). (3) H. P. Leftin and W. K. Hall, J. PhUs. Chem., 64, 382 (1960); 6 6 , 1457 (1962). (4) H. P. Leftin, M. C. Hobson, and J. S . Leigh, ibid., 66, 1214 (1962). (5) J. J. Rooney and R. C. Pink, Trans. Faraday SOC.,58, 1632 (1962). (6) D. N. Stamires and J. Turkevich, J. Am. Chem. Soc., 8 6 , 749 (1964). (7) W. K. Hall, J . Catalysis, 1, 63 (1962). (8) A. E. Hirschler, ibid., 2, 428 (1963). (9) J. J. Rooney and B. J. Hathaway, ibid., 3, 447 (1964).
4403
NOTES
Table I : Spectral Data from 1,l-Diphenylethylene Adsorbed on Silica-Alumina Catalyst AAA A. Effect of purity and coverage Nominal coverage, DPE molecules/g.
x
1. 86.8% (as received) 2. 99.9% (first g.1.p.c. purification) 3. 99.99% (second g.1.p.c. purseation)
4. 86.8% 5. 86.8%
SpinColor
spins/g. X 10-16
Greenish yellow Lemon yellow Lemon yellow Yellow-green Dark blue-green
1.0 1.9 2.2 0.9
10-19
2.7 2.0 2.2 3. I 27.9
Catalysta lot
0.6
1 1 1 2 2
Color
Width: gauss
Nominal oonoentration, spins/g. x 10-1s
Lemon yellow Light green Lemon yellow Lemon yellow
8.40 10.13 10.59 8.59
B. Effect of pretreatmentb Nominal ooverage, DPE molecules/g.
x
1. Standard 2. Standard, sealed off with 5 mm. of
O2
3. Reduced 4. Reduced, sealed off with 8 mm. of Hz
10-19
3.15 2.% 3.1 2.9
0.8 6.3 0.03 0.03
The purity a Lot No. 1 consistently generated more radical ions than Lot No. 2 when tested with DPE, perylene, and pyrene. of DPE was 99.99%, and the catalyst used was Lot No. 2. ' Width between points of maximum slope, measured us. 0.1 M hydroquinone in alkaline ethanol in a dual cavity.
to be physicall0Jl; the radical-ion concentration is actually increased by 0%.The final objective of the present work was to determine the saturation properties of the paramagnetic signal from DPE in the presence of various gases in order to clarify the effects of environment on the e.p.r. spectra of chemisorbed radicals.
Experimental Section The equipment and high-vacuum procedures for the preparation of catalyst samples for electron spin resonance studies were given earlier.7 American Cyanamid Co. Aerocat ( A M ) cracking catalyst (22.1y0 A1203, surface area, 450 m."g.) was used. Unless otherwise specified, the 1,l-diphenylethylene reagent (from the Aldrich Chemical Co.) was twice purified by preparative gas-liquid partition chromatography. The purity of the first product was 99.9%, and after the second chromatographic separation it was 99.99%. The standard pretreatment of the catalyst consisted of treating the catalyst (1.5-g. charge) in flowing oxygen for 24 hr. a t 540" and then evacuating it for 24 hr. a t this temperature. When the O2 treatment was followed by evacuation for 6 hr. before treatment with flowing HZ for 24 hr. and a final evacuation for 24 hr., all a t 540°, the catalyst was said to be reduced. The DPE (-13 mg.) was weighed in a capillary tube and placed in a reagent reservoir where it was degassed by the freeze-pump-thaw technique and sealed under
vacuum. Transfer of DPE to the eatalyst was effected by rupturing a break-seal between the catalyst and reagent compartments and heating the assembly a t 62' for 1 hr. Most samples reached a constant radicalion concentration after 72 hr., and additional heating at 62" after this period did not lead to any change. The electron spin measurements were carried out with a Varian X-band spectrometer (Model V-4500) with the microwave bridge in the low-power configuration and a 12-in. magnet; the magnetic field was modulated a t 100 kc./sec. The Varian multipurpose cavity was used for the studies of the effects of added gases. For line-width determinations, the Varian dual cavity was used with an alkaline-ethanol solution of 0.1 M hydroquinone (UH = 2.368 gauss) as a reference. Spin-concentration measurements were made by comparison of the first moment of the ovennodulated derivative signals with those of 0.001 M l,l-diphenyl-2~ ~ 1M-l cm.-l picrylhydrazyl in benzene ( ~ 6 2 0 14,590 in C H C P ) using the same experimental conditions. The power entering the cavity ann was measured with a Hewlett-Packard 431B power meter using a 20-db. coupler.
(10) R.P.Porter and W. K. Hall, J . Catalysis, in press. (11) B. D.Flookhart and R. C. Pink, Talanta, 9,931 (1962). (12) J. W. Eastman, G. M. Androes, and M. Calvin, J . Chem. Phys., 36, 1197 (1962).
Volume 69, Number 18 December 1966
4404
Results and Discussion The results in Table I-A demonstrate that purity and coverage of DPE have a marked effect upon the color of the catalyst sample, indicating an increase in the 607-mfi band with increasing coverage and impurity level. The radical-ion concentration varied inversely. This behavior is consistent with the identification by Rooney and Hathawayg of the blue species with an allylic carbonium ion, provided that an oxidizing agent is included among the impurities. Leftin and Hall3 demonstrated that the formation of the blue species was strongly catalyzed by oxidizing agents. The effect of catalyst pretreatment upon the generation of radical ions is given in Table I-B. Treatment of the catalyst with hydrogen reduced the radical-ion concentration one order of magnitude. I n the presence of oxygen, the spin-concentration was increased one order of magnitude over that obtained with the standard pretreatment. These data agree with earlier results7* for polynuclear aromatic hydrocarbons adsorbed on the same catalyst. If it is assumed that the extinction coefficient of the radical ion is close to that reportedly3 for the methyldiphenylcarbonium ion in HZS04 (E 3 X lo4 M-1 cm.-l), then the maximum radical-ion concentration observed in the 02-treated sample would be sufficient to give an absorbance of only about 0.05. This suggests that the peak due to tlhe radical ion is obscured by the background in the absorption spectra of DPE on this catalyst. The extinction coefficient of the 607-mp banda was shown to be at least 2.2 X lo4; the absorbance of the 607-mp band under similar conditions is about 3. Evidently, a cation radical is formed from pure DPE, but it does not correspond to the blue species. I n order to clarify some of the contradictory res~lts*,5,7,10,11,13--1~ concerning the effects of oxygen and other gases on the e.p.r. signal from adsorbed radical ions, a careful study was made of the saturation properties and line shape of the e.p.r. signal from DPE on silica-alumina catalyst AAA in various environments. Results at two power levels are contained in Table 11. Reliable spin-concentrations can be calculated only from the integrated absorption area of curves taken under conditions of complete unsaturation. The effects of added hydrogen (C-1,2 and D-1,2) and argon (B-3,4,5) reported by Leftin4 can be accounted for mainly as changes in saturation level and not spinconcentration. The addition of small amounts of water vapor aIso results in a decrease in the saturation level (B-9 and D-3) ; however, large amounts of water do decrease the radical-ion concentration (B-10). These latter observations agree with those of Rooney and Pink.& A small apparent lowering in spin-concenThe Journal of Physical Chemistry
SOTES
tration may also occur at high power with evacuated samples due to temperature elevation by absorption of microwave energy. The addition of H2, Ar, and H20 did not lead to any change in signal width or shape under unsaturated conditions; the width did increase a t higher powers as the level of saturation increased. The addition of oxygen in all cases led to a reversible broadening of the signal width, e.g., AH = 10.1, 13.0, 15.2, and 10.1 gauss for B-5 to B-8, respectively. The effect of oxygen on e.p.r. spectra of DPE on silica-alumina is threefold. Exposure to oxygen unsaturates the signal (e.g., A-2 and B-6); it can also lead to a change in signal shape from gaussian to lorentzian (B-7,8 and C-3) and it effectsa real time-dependent increase in the spin-concentration (A-2 and (3-3). The results of Imai, Ono, and Keii,15 who reported that the peak height from the anthracene radical ion first increased sharply and then decreased more slowly with oxygen pressure, can be understood in these terms. The initial increase is due mainly to unsaturation of the sample but partly to a real increase in radical concentration; the decrease in peak height is caused by a change in signal shape. I n the absence of oxygen, the line shape of DPE adsorbed on silica-alumina catalyst AAA is gaussian, consisting of an envelope of many unresolved proton hyperfine components broadened by incomplete averaging of the anisotropic terms of the hyperfine interaction and also by any perturbations of the applied magnetic field by random local fields of the solid. Upon the addition of 10 mm. of 0 2 , the signal became lorentzian in the wings while still retaining the gaussian shape in the center. At 20 mm. of O2 and above, the signal width increased with increasing oxygen pressure, and the line shape was completely lorentzian. This broadening effect of oxygen has been observed in several carbonaceous materials,16-18but variations in the line shape from gaussian to lorentzian were not reported. Ingramlg states that the interaction of the electron with the paramagnetic oxygen molecule can be viewed as a form of collision broadening or as an increased spin-lattice interaction since both reduce the characteristic relaxation time. (13) J. X. Fogo, J. Phus. Chem., 65, 1919 (1961). (14) D. 11.Brouwer, Chem. Ind. (London), 77 (1961); J. Catalysis 1, 372 (1962). (15) H. Imai, Y . Ono, and T. Keii, J. Phya. Chem., 69, 1082 (1965). (16) D. E. G. Austin and D. J. E. Ingram, Chem. Ind. (London), 981 (1956). (17) L. S. Singer, Proc. Conf. Carbon, Sth, University Park, Pa., 1961, 2, 37 (1963). (18) A. J. Saracen0 and N. D. Cogpeahall, J. Chem. Phys., 34, 260 (1961). (19) D. J. E. Ingram, “Free Radicals as Studied by Electron Spin Resonance,” Butterworth and Co. Ltd., London, 1958, pp. 210-212.
NOTES
4405
Table 11: Effect of Added Gases on Spectra from 1,l-Diphenylethylene Adsorbed on Silica-Alumina Catalyst AAA -Power
input into cavity, mw. 7----4.3\
0.036-
Sample t r e a t m e n t - - - - - - - - - - - - - - ?
A.
1. Standard 2. 10 mm. of 0 2 added for 1 hr. 3. Oz pumped out and 10 mm. of Hz added
B. 1. Standard, sealed off with 5 mm. of 2. 3. 4. 5. 6. 7. 8. 9. 10.
c.
0 2
Oz pumped out 10 mm. of argon added 100 mm. of argon added Argon pumped out 20 mm. of 0 2 added 40 mm. of ID2added O2 pumped out 1 . 5 x 1018 H20/cm.2added 3.6 x 10x4 HzO/cm.Z added
1. Reduced 2. 8 mm. of Hzadded 3. H2 pumped out and 10 mm. of 02 added a. After 1 hr. b. After216hr. 4. O2 pumped out
D. 1. Reduced, slealed off with 8 mm. of HZ 2. Hz pumped off 3. Exposed to 10 mm. of HzO
Unsatd. spinconoentration, spins/g. X 10-18
Signal height, arbitrary units
saturation
Signal height, arbitrary units
%
1.3 2.1 2.4
19 33* 34
77 41 64
57 25gb 160
8.2 7.5 7.3 7.5 6.6 6.4 5.2 7.2 6.8 1.0
25 22 23 22 21 8" 4" 21 23 4
13 63 57 28 64 12 13 31 27
...
284 110 130 203 100 88" 46" 168 217
0.3 0.5
5 8
63 61
22 41
0.8 2.2 2.9
12b 28" 40b
11 2 16
142b 360" 222b
0.3 0.6 0.4
7 10 8
25 66 60
69 45 45
...
a Signal shape is lorentzian. a Signal shape is a mixture of lorentzian in the wings and gaussian in the center. All signal shapes without superscript are gaussian,
The use of relative signal height to investigate the effects of oxygen is misleading, as can be seen by comparing B-5 to B-7. The measured decrease in spinconcentration in 40 mm. of O2 was 23%, while the peak height decreased by a factor of 5. The signal, while enclosing approximately the same area, was altered in shape from gaussian to lorentzian. The situation was a little improved a t high power due to a partial compensation between unsaturation and broadening in 0 2 . The exact nature of the species responsible for the paramagnetic signal is not known; it may be the radical ion of the olefin or of a, (rearranged) dimer. Morigagi, et aZ.,20 published resolved e.p.r. spectra of to lo-* M DPE in tetra,hydrofuran (yellow solution) in the presence of sodium metal, which they attributed t o the D P E monomor anion with the approximate proton hyperfine splitting constants of spa, = aortho = 6 gauss, a,,,, = 1 gauss, and a@ = 3 gauss. After heating the solution for 2 days a t loo", they obtained a green solution with a nine-line spectrum (aparu= 5.4 gauss, aorlho= 2.7 gauss, amel, = -O), which was
ascribed to a polymer radical. Evans and Evansz1 obtained an e.p.r. spectrum similar to that of Morigagi, et al., for a solution of DPE in cyclohexane in contact with sodium-potassium alloy. Since the spin-concentration measured corresponded to 80% of the total DPE, they concluded that the anion radical was 0
cH2-cPh and they attributed a band at 630 mh to this species. It is well known that anthracene, which is quite similar to DPE from the spectroscopic viewpoint, forms both cation and anion radicals. It is not unlikely, therefore, that the monomeric cation radical is a t least partly responsible for the paramagnetism. Acknowledgment. This work was sponsored by the Gulf Resea:rch & Development Co. as part of the research program of the Multiple Fellowship on Petroleum. ~
~~~~
(20) K. Morigagi, K. Kuwata, and K. Hirota, Bull. Chem. SOC. Japan, 3 3 , 952 (1960). (21) A. G. Evans and J. C. Evans, Trans. Faraday SOC.,61, 1202 (1965).
Volume 69, Number 12 December 1966
