1602
JOHNJ. SURASH AKD DAVID M. HERCULES
Vol. 6G
STUDIES ON PHOTO-INDUCED ELECTRODE POTENTIALS’ BY JOHNJ. SURASH AND DAVID &I.HERCULES~ Departments of Chemistry, Lehigh University, Bethlehem, Pennsylvania, and Juniata College, Huntingdon, Pennsylvmia Received February 2, 1.968
Photo-induced changes in the potential of a platinum electrode in contact with a solution of an organic compound in ethanol have been studied. Upon irradiation in the absence of air, the potentials rapidly became more negative (relative t o an Ag/AgCl reference electrode), reached a limiting value, and decayed slowly when the exciting radiation was extinguished. The electroactive species is interpreted as being a free radical formed by a photochemical reaction of the solute with the solvent. Negative potentials are consistent with the Nernst equation. The effects of solute concentration, photoreduction product, viscosity, and wave length of exciting radiation have been studied. It has been demonstrated that the electroactive species is produced independent of the electrode. Absoibtion spectra and polymerization data are submitted as evidence that the intermediate is a free radical.
Introduction The production of a potential difference between two electrodes immersed in a suitable electrolyte, when only one of them is illuminated, has been called the photovoltaic effect. Since Becquerel’s original investigation^,^ this effect has been observed for different typee of electrolytes and electrode systems. Work involving coated electrodeelectrolyte system^^-^ and semiconductor-electrolyte systems*-’o indicated that direct irradiation of the electrode-solution interface was essential for the production of a photovoltaic effect. The effect described in the present communication can be produced without direct irradiation of the electrode-solution interface and therefore should be distinguished from the photovoltaic effects of earlier investigator^.^ -lo Levin, White, and co-workers investigated potentials produced by irradiation of platinum electrodes immersed in alcoholic solutions of organic ~ompounds.~1-~3 Their studies indicated that photopotentials were produced by the organic solute. Because Levin, et al., failed to exclude oxygen from their solutions, thereby complicating interpretation of their data, the present investigation was begun to establish the nature of the physical processes giving rise to photo-induced electrode potentials. Experimental Materials.-Eastman Kodak White Label (or equivalent) chemicals were used throughout, They were furthcr purified by one or a combination of the following methods: vacuum distillation, vacuum sublimation, and recrystallization from absolute ethanol. (1) (a) Presented before the 138th National Meeting of the American Chemioal Society, New York, N. Y., September 15, 1960: (b) this paper represents a part of the dissertation submitted b y John J. Surash to the faculty of the Graduate School of Lehigh Univelsity in partial fulfillment of the requlrernpnts for the Degree of Doctor of Philosophy. (2) De&mznt of Chemistry, Juniata College, Huntingdon, Penna. Address all correspondence to this author. (3) E. Recquerel. C o m p t . rend., 9, 144, 561 (1839). (4) V. I. Veselovskii, Zh. Pia. Khim.. 15, 145 (1941). (5) V. I. Veselovskii, ibid., 20, 269 (1946). (6) V. I. Veselovskii, ibid., 23, 1095 (1949). (7) V. I . Ginzburg and V. I. Veselovskii, ibid., 24, 366 (1950). (8) W. H. Brattain and C. G . B. Garrett, B e l l S y s t e m Tech. J . , 34, 129 (1956). (9) 1%. Gobrecht, R. Kulinkies, and A. Tausend, 2. Eleklrochem., 63, 541 (19,59). (10) R. ’Williams, J . Chem. P h y s . , 32, 1505 (1960). (11) I . Levin and C . E. White, ibid., 13, 417 (1950). 112) I. Levin and C . E. White. ibid., 19, 1079 (1951). (13) I. Levin. J . It. Wiebush. Xi. B. ISnsh, and C. E. White, ibid., a i , 1654 ( 1 9 ~ 3 ) .
U. S. I. (pure grade) absolute ethanol wm employed as a solvent in most cases. No photopotential was obscrved for the solvent. Apparatus .-The apparatus used in this investigation consisted of the following: Vycor, T-Shaped Cell.-This is shown in Fig. 1 ; it was made by joining together three 24/40 standard-taper Vycor joints to give a cell having a volume of about 30 ml. The entire cell was covered with black “Scotch” electrical tape except for an area of 1.5 cm.2 where the exciting radiation entered the cell. Platinum Indicator Electrode.-This electrode, 0.5 mm. in diameter, wm sealed in ,a standard-taper Pyrex joint leaving approximately 2 mm. exposed. The electrode was supported in the cell against the irradiated side. Silver-Silver Chloride Reference Electrode .-This electrode was made by anodizing a I-mm. diameter silver wire in 2 M HC1 for 5 min. a t 5 ma. The reference electrode was placed in the long arm of the cell in a position where radiation could not strike it. Having the electrode in direct contact with the solution eliminated the necessity for a salt bridge. All potentials were measured relative to this electrode. Hanovia, Air-Cooled, High-pressure D.c. Xenon Arc (1000 Watts).-This served as a source of ultraviolet radiation and was focused on the sample cell by means of a quartz lens (focal length 14 c p . ) . The lamp to cell distance was 56 cm. Filter System.-A Corning No. 7-54 filter and Corex glass No. 9863 (3.05 mm. thick) were used to isolate the 2400-4200 A. region. A shutter system was used t o control exposures. Recording D.c. Electrometer.leThis instrument had an input impedance of 1012ohms and a 1-see. full-scale response time. Procedures.-The following procedure was used to obtain potential time curves. A sample ,O,f 25.00 ml. was added to the Vycor cell a t 25.0 f 0.5”. I h e solution was flushed for 10 min. with oxygen-free nitrogen, prepared by passing tank nitrogen through separate containers of vanadous chloride,’s water, ascarite, and calcium chloride. (An atmosphere of nitrogen was maintained above the eample at all times.) With the recorder running, radiation was made incident upon the indicator electrode by means of the camera shutter, and the potential-time curve was recorded. To record potential-time curves as a function of wave length, the filter system was replaced by a Bausch and Lomb 500-mm. grating monochromator. The photopotentials recorded were corrected for variations in intensity of the xenon lamp as a function of wave length. Absorption spectra were obtained using a Warren Spectracord with matched, 1.OO-cm. glass-stoppered fused-silica cells. A special cleaning procedure was necessary for the platinum electrode in order to ensure reproducible results. The electrode was immersed in hot (110’) chromic acid cleaning solution for 3 min., rinsed with distilled water, cathodized for 10 min. in 1 M sulfuric acid a t 5 ma., and rinsed with absolute ethanol. The electrode was always cleaned just prior to use. (14) R . It. €1. Miron, Ph.D. Thesis, Lehigh University, l95S. (15) L. Meites, “Polarograpbic Techniques,” Interscieneo, Now York, N. Y . , 1955. I?. 34.
Results and Discussion Photopotentia1s.-Beoause Bolland and Cooper16 studied the photochemical reaction of the a nthraquinone system, 9,10-anthraquinone (hereafter called A&) was selected as the model compound for most of our investigations. A typical photopotential-time curve for A& is shown in Fig. 2. It is evident that, the potential rapidly became more negative during illumination, reached a maximum negative potential, and slowly returned to a value near the initial potential after the exciting radiation was extinguished. Similar curves were observed for a variety of organic compounds in absolute ethanol, amd their maximum negative potentials are recorded in Table I. The following mechanism is proposed to account for the production of photopotentials in oxygenfree solutions of A& in ethanol AQ
+ h 6 +A&*
A&* -+
1603
STUDIES o s PHOTO-IKDUCED ELECTRODE POTENTIALS
Sept., 19G2
I
il
12
(1)
Fig. 1.-Vycor cell diagram: (1) Ag-AgC1 electrode; (2) inlet for maintaining nitrogen atmosphere above solution; (3) inlet for bubbling nitrogen into solution; (4) opening to TABLE I atmosphere; (5) platinum point electrode; ( 6 ) glasR stopper, PHOT'OPOTEWTIALS OF ORGANIC COMPOUNDS IN ABSOLUTE T 24/40. ETHANOL~
A&*'
Compound
(2)
Photopotential in r n ~ . ~
Acetophenone -434 4-Aminoaoetophenone 43 4-Bromoacetophenone -210 4-Hydroxyacetophenone - 62 4-Methoxyacetophenone - 350 4-Nitroacctophenone 42 9,lO-Anthraquinone (0.0005 M ) - 395 Anthraquinone 2,6-disulfonic acid disodium salt (0.005 M ) -250' Benzaldehiyde - 290 0 Benzhydro1 -409 Benzoin Benzophenone -400 4-Rromobenzophenone -350 4,4 '-Dimethylbenzophenone -510 2-€lydroxy-5-chloro-benzophenone 0 Benzopinacol (0.005 M ) -33Od p-Benzoquinone 60 Benzoyl peroxide 150 2,2 '-Bipyridyl 240 Diphenylamine - 465 Fluorescein 10 Tetrabromofluorescein (0.0001 M ) - 200" - 95 1,4-Naphthoquinone (0.001 M ) Phenazine (0.001 M ) - 345 Tet raphenylhydrazine 130f a Potentials reported were measured against Ag/AgCl ceference electrode. Excitation used was the 24004200 A. component of the xenon lamp. Solutions were purged with nitrogen for 10 min. prior to measurement. Concentration is 0.01 M unless otherwise specified. I, Reproducibility =Id'%Solvent . mixture u ~ e d - 6 0 ~water ~ and 40% absolute ethanol by volume. d Used unfiltered radiation from xenon lamp. e Solution was 0.001 M in aecorbic acid. Also used unfiltered radiation from xenon lamp. J Solution used was the filtrate from a saturated solution of tetraphenylhydrazine in absolute ethanol. A 0.01 M solution of tetraphenylhydrazine in chloroform gave a photopotential of 1-165mv. --
-
-
-
+
(10) J. L. Bolland a n d H. R. Cooper, Proo. Roy. S O ~(London), . A286. 405 (1964).
L C L _ L . .
0
40
80
120
160 TIME
IN
200 240 SECONDS
28
Fig. 2.-Photopotential-time curve for 9,lO-anthraquinone (0.0005 M ) in absolute ethanol in a nitrogen atmosphere.
A&*'
+ RCHzOH +AQH.* + ReHOH (34
+ RCHzOH +AQH. + RCHOH (3b) AQH. + h6' (4) AQH. * AQ*'
The primary process, eq. 1, must involve absorption of radiation in the 2400-4200 A. region. Because the most intense absorption band for A& in that region corresponds to a T+T* transition it may be assumed that T+T* absorption is the predominant primary process. The correlation between photopotentials and the absorption spectrum of A&, shown in Fig. 3, enhances this point of view. However, the much weaker n+T* transition cannot be ruled out, and probably contributes somewhat to the primary process. Equation 2 indicates conversion from the initial excited state of A& to some other excited state prior to reaction with the solvent. Generally, it has been assumed
JOHN J. SURASH AND DAVID nl. HERCULES
1604
1.8
41 -10
I.6
-9 -8 -7
ra 5!
4
5 &
.5
e
n
-4 -3
0
E
L
t
i!
-2
T'ol. 66
The shape of the curve shown in Fig. 2 is readily understood if one assumes the mechanism given above to hold. Prior to irradiation, the predominant species in the vicinity of the electrode surface is AQ. Upon irradiation some of the A& undergoes photochemical reaction to form AQH- which begins to accumulate a t the electrode surface, changing the potential of the electrode. As irradiation proceeds, a steady-state is established and the potential becomes constant. When the radiation is interrupted, chemical reaction of AQH. and diffusion remove it from the electrode surface, and the potential slowly falls to a value near the starting potential.lg If one assumes the Xernst equation to hold for an electrode indicating the potential of the A&/ AQH. system, the fact that the potential of the irradiated electrode becomes more negative relative to the Ag/AgCl electrode is quite reasonable. The Nernst equation for this system would be
0 4 -1 0
02
that photochemical reactions proceed via the triplet state (which in this case would be AQ*') although the validity of this assumption has been questioned re~ent1y.l~The excited A& molecule then abstracts a hydrogen atom from the solvent (alcohol) to form a semiquinone radical and the corresponding alcohol radical as shoxm in eq. 3a and 3b. Evidence for such a reaction is supported by the study of Bolland and Cooper16 on the photosensitized oxidation of ethanol, using anthraquinone-2,6-sulfonic acid, disodium salt, as the sensitizer. At present it is not known whether eq. 3a or 3b is the predominant mode of radical production in the A& system, although 3b seems more likely. If excited radical production does occur, one of the mays AQH. * could lose its excess energy would be the emission of radiant energy, as depicted in eq. 4. The process described is one of chemiluminescence. Although irradiated solutions were observed to fluoresce green during photopotential measurements, this cannot be considered support for process 3a because the radical AQH. is fluorescent under the condition of excitation.18 We have assumed that the electroactive species is the monomeric radical AQH.. The possibility exists that the ,4QH. radical undergoes reaction with an unexcited A& molecule to produce a radical dimer, or that AQH. radicals partially dimerize to produce an equilibrium mixture of monomeric radicals and dimer.
When both A& and AQH. are present in solution, the ratio [A&]/ [AQH. ] will determine the potential of the electrode. During the course of irradiation, AQ is being converted into AQH-, which will make the ratio (A&]/ [AQH. 3 become smaller as a function of time. Such a behavior would tend to produce a negative potential according to eq. 5 . When a steady-state concentration of AQH. is reached a t the electrode surface, the ratio [A&]/ [AQH.] will be constant and the potential of the electrode will become constant as a function of time. When the radiation is extinguished, AQH- will be removed from the electrode surface (and AQ will diffuse into the previously irradiated area) causing the potential to become more positive. The interpretation presented above is consistent with the observation that the electroactive species can be produced in the absence of an electrode, but will cause a change in the potential of an electrode brought into contact with the solution after the exciting radiation is extinguished (see discussion below). The interpretation also is consistent with the observation that a larger initial concentration of A& gives a larger maximum potent ial. The major difierence between the results of Lerin, et and the present study is that the former investigators obtained positive photopotentials while we obtained negative photopotentials. This discrepancy arises because Levin, et al., measured photopotentials of solutions in equilibrium with atmospheric oxygen, while we measured solutions that had been purged with nitrogen. When we equilibrated our solutions with air, positive photopotentials were obtained, as shown in Table 11. I n the presence of air, it is quite probable that AQH. undergoes further reaction immediately after it is formed AQH. 0 2 --+ AQ HOz. (6)
(17) R. Livingston a n d V. S.R a o , J . P h p . Chem., 68, 794 (1959). (18) D. M. Hercules a n d J. J. S w a s h , to be published elsewhere. Investigation of the degree of chemiluminescence In the A& system will be the subject of a continuing investigation.
(19) Since some A& apparently is used up in a. photochernioal reaotion. the final potential is more poeitive than the initial potential.
0.a 280
290
300
310
320
330 340 350 360
0
WAVELENGTH IN MILLIMICRONS
Fig. 3.-Correlation of photopotential and light absorption m a function of vvave length of radiation for 9,lOanthraquinone (0.003 M ) in absolute ethanol: curve A, light absorption-1.00 cm. cell: curve B, photopotential.
+
2H02
+€ E 2 0 2
+
+
0 2
(7)
Bept., 1962
1605
8TGDIEG ON PHOTO-INDUCED ELECTRODE POTEKTIhLS
Therefore, in the presence of air the ratio of [A&]/ [AQH ] will not determine the potential of the electrode, but it probably will be determined by the 0t/He02 or the HtOe/HzO couple, Apparently, the potentials of these couples are more positive than the initial potential of the platinum electrode. This was confirmed experimentally by the addition of small amounts of 3% HzOzto an ethanolic s)olution of A& (non-irradiated, in air) in the vicinity of the point electrode. The potential became more positive during addition of the H202. From this one may conclude that Levin, et ai., were really measuring the relative efficiencies of compounds to photooxidize ethanol, and that their changes in potent>ialswere largely due to peroxide formation. TABLE I1 I
E:FFECT Compound
OF
AIR
OK PHOTOPOTENTIALS
Molar concn. in absolute ethanol
Photopotentials in Airb Nitrogen
Acetophenone 0.01 $10 -434 CBromoacetophenone .Ol $60 -210 4-Methoxyacetophenone .Ol 0 -350 9,10-Anthraquinone .0005 +70 -395 .Ol $47 -409 Benzoin Benzophenone .01 $60 -400 a Measuret3 against Ag/AgCl reference electrode; reproducibility 1 3 % . b Same experimental arrangement was used as for the nitrogen-flushed samples except that a 50-ml. Pyrex beaker was used as the cell.
It was necessary to determine if photopotentiah might be caused by accumulation of photoreduction products at the electrode surface. The benzophenone-benzopinacol system was chosen for this study because the photochemical synthesis of benzopinacol from benzophenone is well known. Photopotentials were recorded for solutions containing varying amounts of benzopinacol with constant amounts of benzophenone. The results are tabulated in Table IV. From these data it is apparent that benzopinacol had no effect on the photopotential of benzophenone. TABLE IV EFFECTOF BENZOPINACOL ox THE PHOTOPOTENTIAL OF 0.001 M BENZOPHENONE (IN ABSOLUTEETHANOL) Molar conon. of benzopinacol in absolute ethanol X 101
---Photopotential Filtered light (2400-4200
I.)
in mv.a---Unfiltered light
0 - 200 - 305 0.1 200 300 0.5 - 196 - 305 1.o 204 -310 a Measured against -4g/AgCl reference electrode; producibility 1 3 % .
-
-
re-
Figure 4 illustrates the effect of viscosity on the photopotential-time curves for benzophenone in ethanol. It is apparent that the maximum photopotential decreases as a function of viscosity, and that the rate of decay of the photopotential decreases as viscosity increases. The latter pheEffects of Variables on Photopotentia1s.-The nomenon is readily explained if we assume diffusion effect of solute concentration on maximum photopotentials for A& and diphenylamine is summarized to be the principal mechanism for removal of the in Table 111. It! is evident that the maximum intermediate from the electrode surface after the photopotentials vary with concentration of solute, exciting radiation is extinguished.20 The decrease approaching a lirniting value. On the basis of the in maximum photopotential with increasing solproposed mechanism, it is reasonable to expect vent viscosity probably results from collision of the the maximum photopotential to increase with two radicals formed in eq. 3 within a Franckconcentration due to the increased absorption of Rabinowitch cage.21 As the solvent viscosity radiation in the more concentrated solutions and the increases, the radicals will encounter a slower rate corresponding increase in production of interme- of diffusion and an increased probability of rediate species per unit time. The limiting value of combination. This will tend to reduce the number the photopotential probably occurs a t the point of of free radicals a t the electrode surface. Earlier investigatorsI2 attempted to correlate total absorption of the exciting radiation by the the magnitude of the photopotential with absorpsolution in the vicinity of the electrode. tion of radiation by the compound. However, TABLE I11 their data were not conclusive. Figure 4 shows that the photopotentials definitely follow the EFFIWTOF SOLUTECONCENTRATION OF PHOTOPOTENTIALS absorption spectrum for A&. (Similar results Molar conon. in Photopotentials were obtained for benzoin and benzophenone.) Compound absolute ethanol X 102 in The first T+T* absorption band was chosen for 9,lO-Anthraquinone 0.5 - 395 irradiation studies because the n-m* band in A& 9,lO-Anthraquinone .I - 285 was too weak for study. It is reasonable to antici9,lO-Anthraquinone 05 - 225 pate a correlation of photopotentials with the 9,lO-Anthraquinone .01 -200 absorption spectrum because increased absorption 9,lO-Anthraquinone .005 - 195 results in a greater number of molecules being 9,lO-Anthraquinone ,001 Ob excited per unit time. These can produce a greater Diphenylamine 10.0 - 465 number of electroactive species a t the electrode Diphenylamine 1.0 - 465 surface and consequently a larger photopotential. Diphenylamine 0.1 - 320 In order to distinguish the photo-induced elecDiphenylamine .05 - 160 trode potentials of the present study from a true Diphenylamine .Ol - 15 a Measured against Ag/AgCl reference electrode; reproducibility &3%. b 17isual observation revealed the presence of zi very small amount of yellow intermediate around the irradiated electrode, but the detection system was not sensitive enough t o show any photopotential.
(20) T h k assumes diffusion is rapid relative to a n y chemical reaction b y the intermediate. (21) This type of process has been studied recently b y L. Herk, M. Feld, a n d &.I.Sawarc, J. Am. Chem. Soc., 88, 2998 (1961). and R. K. Lyon a n d D. H. Levy, %bid.,88, 4290 (1961).
1806 a new diffuse band appears in the electronic absorption spectrum from 350 to 460 mp. The maximum is a t 380 mp. When an alcoholic KO12 solution of A& is irradiated, an additional band appears having its maximum a t 480 ~ p In . their flash photolysis studies, Bridge and PortorZzreported a group of bands a t ca, 360-390 rnw which they attributed to the semiqiiinonc radical and a band at 480 mp which they attributed to the semiquinone ion. The maxima reported here, as well as those of Bridge and Porter, are consistent with the normal spectral shift that would accompany the ionization OH I
is
TIME
IN
SECONDS
Fig. 4.-Effect of solvent viscosity on the decay curve of irradiated benzophenone solutions (0.01 M ) : curve 1 , 100% absolute ethanol; curve 2, 80% absolute ethanol plus 20% glycerol; curve 3, 60% absolute ethanol plus 40% glycerol; curve 4, 40% absolute ethanol plus 60% glycerol.
photovoltaic effect, it was necessary to prove the electroactive species was produced in solution independenlly of the electrode surface, and if an electrode was placed in contact with the electroactive species in the absence of radiation, the electrode potential would become more negative. To accomplish this, the platinum point electrode was rotated 18OOso it was not in the path of the incident radiation. With the recorder running, radiation was made incident on the cell. During several minutes of irradiation, no change in potential of the electrode was recorded, even though formation of the yellow intermediate was observed visibly. Then the exciting radiation was shut off, and immediately the platinum electrode was rotated 180’ to its normal position against the cell wall in the area previously irradiated. The potential of the electrode became more negative, reaching a maximum value and decaying to a final potential very near the starting potential. Nature of the Intermediate Species.-Although the intermediate giving rise to photopotentials was assumed to be a free radical, an attempt was made to acquire some direct evidence to support this assumption. Electron spin resonance studies on ethanolic solutions of the intermediate in the photoreduction of A& were inconclusive, due to loading of the e.s.r. spectrometer cavity by the alcohol. However, other studies indicate that the intermediate is a free radical. Upon irradiation of a solution of AQ in ethanol,
is
Studies have been reported on the photoreduction of eosin with accompanying potential measurem e n t ~ . ~Electron ~ - ~ ~ spin resonance studies on the photoreduction intermediate indicated the presence of a free radicaL26 We have obtained typical photopotential curves for the systems studied by these investigators. Additional evidence that the intermediate in the photoreduction of A& is a free radical was provided by polymerization studies similar to those of Cooper, et ~ 1 It. was ~ ~demonstrated that the yellowcolored intermediate could initiate polymerization of styrene and methyl methacrylate. Furthermore, photopolymerization of styrene has been initiated by benzoin,27-29 while benzoyl peroxide30*31 and phenazinea2 have been reported as sensitizers. All three compounds gave photopotentials in the present study. Berg, et aL133have postulabed free radical intermediates for a number of compounds in their photopolarographic studies. They were concerned with irradiating a dropping mercury electrode and measuring the changes in limiting current produced. It is quite probable that the electroactive species in the work of Berg, et al., are similar to t8hoseof the present investigation. (22) N. K. Bridge and G. Porter, Proc. Rag. Soc. (London), A244, 259 (1958). (23) V. F. T. Tsepalov a n d V. Ia. Shliapintokh, Dokl. Akad. Nauk S S S R , 116, 641 (1957). ( 2 4 ) V. F. T. Tsepalov a n d V. Ia. Shliapintokh, Izu. Akad. Nauk S S S R . Otd. Khim. Nauk, 4,737 (1959). ( 2 5 ) N . N . Bubnov, L. 8. Kibalko, V. F. Tsepalov, a n d V. I a . Shliapintokh, O p t . i Speklroskopiya, 7 , 7 1 (1959). (26) W. Cooper, G. Vaughan, S. Miller, a n d M. Fielden, J . Polymer S e i . , 54, 651 (1959). (27) B. R. Chinmayanandam and €1. W . Melville, Trans. Faraday Soe., 60, 73 (1954). (28) R. M . Joyce, U. S. Patent 2,647,080, July 28, 1953. (29) C. M. McCloskey and J. Bond, Ind. Eng. Chem., 41, 2125 (1955). (30) S. G. Cohen, R. E. Ostberg, D. B. Sparrow, a n d E. R. Elout, J . Polymer Sei., 8 , 264 (1848). (31) M. Burnett and H. W. Melville, Proc. Roy. Soc. (London) Al89, 456 (1947). (32) G. -4. Schroter, K u n s t o f e , 41, 291 (1951). (33) (a) H. Berg, Collectzon Czech. Ckem. Commun., 2 6 , 3404 (1960): (b) H. Berg, Naturwiss., 47,320 (1960); (c) H. Berg and H. Schmeim, tbzd., 47, 513 (1960); (d) H. Berg and H. Schmeiss, Nature, 191, 1270 (1961).
