232
Anal. Chem. 1981, 53, 232-236
summarized in Table 111. Indeed, the reduction potential of -0.5 V for FA indicates that it is a stronger reducing agent than humic acid and that it can clearly cause reduction of a variety of ionic and nonionic species which may be present in natural aqueous systems. It appears, for example, that O2 can be reduced by FA with subsequent reduction of the Hz02 formed (Eo= 1.77 V) thereby accounting, at least in part, for the minimal effects of aerobic conditions on the E,,measured for FA (33). Certainly, the ability of FA to reduce Fe and Hg, as well as H2Se03and H3As04and even humic acid, raises a number of possibilities of significance to numerous environmental and geochemical questions. These possibilities require further evaluation, however. Experiments designed to provide this information are in progress.
LITERATURE CITED Gamble, D. S.;Schnitzer, M. I n "Trace Metals and Metal-Organic Interactions in Natural Waters"; Singer, P. C., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1973. Reuter, J. H.; Perdue, E. M. Geochim. Cosmochim. Acta 1979, 41, 325-334. Stevenson, F. J. Soil Sci. 1977, 723, 10-17. Sposito, G.; Holtzclaw, K. M. SoilSci. SOC.Am. J. Ig79, 43, 47-51. Sposlto, G.; Holtzclaw, K. M.; LeVesque-Madore, C. S . Soil Sci. SOC. Am. J. 1978, 43, 600-607. Stevenson, F. J.; Krastanov, S.A.; Ardakoni, M. S . Geoderma 1973, 9 , 129-141. Guy, R. D.; Chakrabarti, C. L. Can. J. Chem. 1976, 54, 2600-2611. Buffle, J.; Greter, F. L.; Haerdi, W. Anal. Chem. 1977, 49, 216-222. Bresnihan, W. T.; Grant, C. L.; Weber, J. H. Anal. Chem. 1978, 50, 1675-1679. Chearn, V. Can. J. SoilSci. 1973, 53, 377-382. Brady, B.; Pagenkopf, G. K. Can. J. Chem. 1978, 56, 2331-2336. Whitworth, C.; Pagenkopf, G. K. J. Jnorg. Nuci. Chem. 1979, 41, 317-321. Chau, Y. K.; a c h t e r , R.; Lumrn-Shue-Chan, K. J. Fish. Res. Board Can. 1974, 37, 1515-1519. Mancy, K. H.; Prog. Water Technoi. 1973, 3 , 63. Hanck, K. W.; Dillard, J. W. Anal. Chim. Acta 1977, 89, 329-340. Chau, Y. K.; Lumm-Shue-Chan, K. Water Res. 1974, 8 , 383-388. Shuman, M. S.;Woodward, G. P. Anal. Chem. 1973, 45, 2032-2035.
(18) O'Shea, T. A.; Mancy, K. H. Anal. Chem. 1976, 48, 1603-1607. (19) Davison, W.; Whitfield, M. J. flectroanai. Chem. 1977, 75. 763-770. (20) Buffle, J.; Greter, F. L.; Nembrini, G.; Paul, J.; Haerdie, W. Z . Anal. Chem. 1976, 282, 339-345. (21) Nurnberg, H. W.; Valenta, P.; Mort, L.; Raspor. B.; Slpos, L. Anal. Chem. 1976, 282, 357-361. (22) Greter, F. L.; Buffle, J.; Haerdi, W. J. Electroanal. Chem. 1979, 707, 211-230. (23) Buffle, J.; Greter, F. L. J. Electroanal. Chem. 1979, 707, 231-251. (24) Brezonik, P. L.; Brauner, P.; Stumm, W. Water Res. 1978, 70, 605-612. (25) Siegerman, H.; ODom, G. Am. Lab. (Fairfieid, Conn.) 1972, 4 , 59-68. (26) Szllagyi, M. SoilSci. 1971, 1 7 7 . 233-238. (27) Szilagyi, M. Soli Sci. 1973, 775, 434-441. (28) Szalay, A.; Szilagyi, M. Geochim. Cosmochim. Acta 1967, 37, 1-14. (29) Alberts, J. J.; Schindler, J. E.;Miller, R. W.; Nutter, D. E. Jr. Science 1974, 184, 895-902. (30) Bloomfield, C.; Kelso, W. I. J. Soil Sci. 1973, 24, 368-375. (31) Goodman, 8. A.; Cheshire, M. V. Geochlm. Cosmochim. Acta 1975, 39, 1711-1716. (32) Sakatos, B.; Tibai, T.; Meisel, J. Geoderma 1977, 19, 319-323. (33) Wllson, S. A.; Weber, J. H. Chem. Geol. 1979, 26, 345-351. (34) Latimer, W. M. "The Oxidation States of the Elements and their Potentlals in Aqueous Solutions", 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1952. (35) Weber, J. H.; Wilson, S. A. Water Res. 1975, 9 , 1079-1084. (36) Wilson, S. A.; Weber, J. H. Chem. Geoi. 1977, 19, 285-293. (37) Wllson, S. A.; Weber, J. H. Anal. Lett. 1977, 10, 75-84. (38) Bresnlhan, W. T.; Grant, C. L.; Weber, J. H. Anal. Chem. 1976, 50, 1675-1679. (39) Saar, R. A.; Weber, J. H. Can. J. Chem. 1979, 57, 1263-1268. (40) Sandell, E. B. "Colorimetric Determinatlon of Trace Metals"; Interscience: New York, 1959. (41) Natusch, D. F. S.;Tucker; M. D. M.; Miller, J. A.; Schmldt, F. Q. Anal. Chem., in press. (42) Wilson, S. A.; Huth, T. C.; Arndt, R. E.; Skogerbce, R. K. Anal. Chem. 1980, 52, 1515-1518. (43) Sillen, L. G.; Martell, A. E. "Stability Constants of Metal-ion Complexes"; The Chemlcai Society: London, 1964. (44) Schnitzer. M.; Skinner, S. I. M. So// Scl. 1966, 702, 361-372.
RECEIVED for review February 19,1980. Accepted November 6,1980. Research supported by the EnvironmentalProtection Agency under Grant No. R805183-03.
Tensammetric Determination of Polyoxyethylenated Alcohols with Low Oxyethylene Content at Parts-per-Million Concentrations Milton J. Rosen," Xi-yuan Hua, Peter Bratln, and Anna W. Cohen Department of Chemistry, Brooklyn College, City University of New York, Brooklyn, New York 71210
Differential double-layer capacltance vs. potential measurements at the dropplng mercury electrode are used for the determinatlon of polyoxyethylenated n-dodecyl alcohols wlth low oxyethylene content at part-per-mllllon concentratlons. The method Is sensitive to materlals with 2 or more mol of ethylene oxide. The method has an accuracy of f2.5% for concentratlons down to 6 ppm and 3 5 % for concentratlons down to 3 ppm. The determlnatlon Is done directly on the surfactant solution without extraction or flltration steps and without the addition of any reagent except supportlng electrolyte. The potential of the hlghest desorption peak In the capacltance-potential curve can be used to estlmate the oxyethylene content of the polyoxyethylenated alcohol.
The literature on the analysis of polyoxyethylenated alcohols reveals that there are only a few methods available for the accurate determination of these materials at low con-
centrations (parts per million). A method involving formation of a cobaltothiocyanate complex, extraction of the complex into an organic solvent, and measurement of the absorbance of the solvent layer was proposed by Brown ( I ) and Kurata (2). Since then, the method has been investigated by several workers for various purposes under varying conditions (3-9). It is now regarded as one of the most convenient methods for the microdeterminationof nonionic surfactants which contain polyoxyethylene chains with three or more oxyethylene units. According to Nozawa et al. (3),however, a linear relationship between the concentration of surfactant and the absorbance is not found for pure polyoxyethylenated alcohols and for mixtures prepared by them from two pure compounds of this type. On the other hand, the modification of Buerger's Dragendorff reagent method (IO), in which the concentration of nonionic is measured by the difference in absorbance of a potassium tetraiodobismuth(II1)ate-barium chloride solution after precipitation of the nonionic, gave unreproducible results. This was due to the fact that the absorbance of the reagent
0003-2700/81/0353-0232$01.00/00 1981 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
233
II
LL
*
I;
1.5
I
I
r
potentiostot
\
1
Meosuring Circuit
\
I
I
Figure 2. Block diagram of the instrument.
u
I
-2
Potent,al ,"Ol,S
Figure 1. Capacitance-potential curve of hexaoxyethylenated n-dodecyl alcohol in 0.5 M LiCl (concentration = 11.2 ppm).
blank was large compared to the change in absorbance produced by the nonionic sample at these low concentrations and, as a result, small day-to-day variations in the reagent blank produced large changes in the measured concentrations of the sample. Many surfactants become specifically adsorbed at the dropping mercury electrode (DME) over a characteristic range of potentials around the potentials of electrocapillary maximum (EECM), the exact region of the adsorption-depending on the nature and concentration of the surfactant and the supporting electrolyte (11). The differential double-layer capacitance ( c d ) vs. potential curves resulting from this adsorption are characterized by two sharp peaks bordering a region of decreased capacitance. In our case, the peak near zero voltage is buried in the mercury oxidation peak (Figure 1). This differential capacitance lowering is a direct result of the adsorption of the surfactant on the DME and can be mathematically expressed through the appropriate adsorption isotherm (12).A quantitative analysis of surfactants is thus possible from differential capacitance lowering in the vicinity of the EEcM. Jehring and Weiss (13) reported that the height of the more negative desorption peak can be used for quantitative analysis of polyoxyethylenated nonylphenols with 9-50 oxyethylene units. Using a phase-sensitive ac polarograph with 1M LiCl as supporting electrolyte, they obtained linear calibration curves for concentrations up to about 30 ppm. We have modified their procedure to quantitatively determine polyoxyethylenated n-dodecyl alcohols with as little as two oxyethylene units at concentrations down to 3 ppm. Several supporting electrolytes were tested. LiCl was used in this work to permit comparison with the results of Jehring and Weiss. Since the more positive desorption peak of polyoxyethylenated alcohols is buried under the mercury dissolution peak, only the more complex peak a t a potential (Figure 1)around -1.5 V (vs. standard calomel electrode) was used for the analysis. We have tested seven individual compounds: polyoxyethylenated n-dodecyl alcohols containing two through eight oxyethylene units and a mixture of these having an approximate Poisson distribution (with an average oxyethylene content of 5.5 units). The procedure described here has an advantage. It permits the direct determination of the surfactant content of an aqueous solution without the need for precipitating reagents (tetraiodobismuth(II1)ate method) or extractions (cobaltothiocyanate method).
EXPERIMENTAL SECTION Materials. Diethylene glycol n-dodecyl ether (C,,EO,), triethylene glycol n-dodecyl ether (CI2EO3),tetraethylene glycol
n-dodecyl ether (C12E04),pentaethylene glycol n-dodecyl ether (C12E05),hexaethylene glycol n-dodecyl ether (C12E06),heptaethylene glycol n-dodecylether (C12E07),and octaethyleneglycol n-dodecyl ether (C12E08)were all from Nikko Chemicals,Tokyo, Japan. Mercury used was high-purity instrumental grade from Adrow Chemical Co. Supportingelectrolyteswere all of analytical reagent grade. Apparatus. Capacitance measurements were made with an automatic differential capacitance measuring instrument (14) based on the phase-null method (15). The instrument measures differential double-layercapacitance in the absence or presence of the faradaic reaction at frequencies of 1, 5 , 10, and 20 kHz. The instrument assumes that the double layer can be expressed by the equivalent circuit introduced by Randles (16) and is further simplified by experimental conditions to
where C d is the differential double-layer capacitance and R, is the uncompensated resistance of the electrolyte and the capillary thread. When ac current is measured by current to voltage converter with RC feedback (17)
If the phase angle of the double layer and the feedback impedance are kept equal by a servo network and photoresistor Rf (a condition referred to as phase-null),the output of the amplifier,e,, is directly proportional to the double-layer capacitance Cd
e, = -ei, Cf where Cf is the known capacitor placed in feedback and eh is the applied ac voltage. The phase difference was kept to better than 0.3' by the servo system and can be checked by an oscilloscope using a Lisajous figure, a commonly used method to detect phase shifts. Differential capacitance in the range from 50-1000 WF could be measured. The block diagram is given in Figure 2. The instrument makes extensive use of high-speed operational amplifiers (Analog Devices and Teledyne-Philbrick). Measuring and triggering ac potentials are supplied by Hewlett-Packard oscillators, while a staircase-likedc polarizing voltage is supplied by a capacitor pump circuit. Phase-null is enforced by a multiplier-based phase detector circuit controlling the photoresistor in the feedback circuit of the current-to-voltage converter. Output of the current-to-voltage converter is conditionedby an active rectifier circuit and recorded. A timing circuit is required for the stepping network, and the timing itself is based on the drop-fall detection, using a 150-kHz
-172-
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0 in 0 5 M LiCl -168-
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1
401
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20
-I 3 2 I
1
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25
30
Figure 4. The effect of concentration of supporting electrolyte and polyoxyethylene chain length on the initial, linear portion of the peak height-concentration curve.
From the data, the position of the desoprtion peak at a given concentration of supporting electrolyte appears to be a good indication of the oxyethylene content for both individual compounds and mixtures of these materials. Peak Height (Ah)-Concentration of Surfactant ( c ) Curves. For analytical purposes, it is most convenient to use a linear relationship. Therefore, conditions were sought to maximize the range over which a linear relationship would exist between peak height and surfactant concentration. The effect of the concentration of LiCl supporting electrolyte and the length of the polyoxyethylene chain on the Ah-c (in parts per million) curve is shown in Figure 4. The extent of the initial linear portion of the Ah-c curve depends upon the concentration of the supporting electrolyte. Electrolyte presumably competes with the polyoxyethylene chain for absorption onto the DME. Therefore, the smaller the electrolyte concentration, the greater the extent of the initial linear portion of the curve. The latter also depends upon the length of the polyoxyethylene portion of the molecule, as indicated by the curves for C12E03and C12E07 in 1 M LiCl (Figure 4). The longer the length of the polyoxyethylene chain, the greater the extent of the initial, linear portion of the curve. Since this implies greater tolerance for supporting electrolyte as the length of the polyoxyethylene chain increases, this is additional evidence for the stronger adsorption at the DME of the compounds with longer polyoxyethylene chains. There appears to be a change in the slope of the Ah-c curve in the 0-3 ppm concentration region, particularly at low concentrations of supporting electrolyte, in some cases, accompanied by a change in the shape of the capacitance peak. As a result, the linear portion of the curve above 3 ppm concentration in some cases does not pass through the origin. Figure 5 shows the effect of the drop time of the DME on the slope of the peak height vs. surfactant concentration curve for CI2EO7in 1M LiC1. The slope of the line increases with increase in drop time, indicating that sensitivity increases with drop time. This may reflect the greater time available for adsorption to occur. Careful control of drop time is therefore
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981 235
P
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60
-
60-
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j *
30-
20-
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d
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Flgure 5. The effect of drop time of DME on the peak height-concentration curve (CI2E0,,1 M LiCI).
P
80-
70-
5 S u r f a c l a n l Concentration, c x l b o ' ,
Figure 7. Peak height-molar concentration curves of various polyoxyethylenated ndodecyl alcohols in 0.5 M LlCl (drop time = 3.1 s).
-
60
1
15 mole dm-'
E
4f 50-
Table I. Desorption Peak Height vs. Surfactant Concentration for Poisson Mixture of Polyoxyethylenated n-Dodecyl Alcohols
(Y c s
40-
30
-
20-
10-
I
0
t
10 20 Surfoctont Concentration ,c ,p.p.m
30
Flgure 6. Peak height-concentration (pprn) curves of varlous polyoxyethylenated n-dodecyl alcohols in 0.5 M LiCl (drop time = 3.1 s).
surfactant concn, ppm
Ah, mm
4.99 9.99 14.98 19.97
13.5 30.7 49.3 66.1
Table 11. Accuracy of the Tensammetric Method for the Determination of Polyoxyethylenated n-Dodecyl Alcohols known accuracy, concn, measd concn, average, ppm % no. PPm PPm 1 2 3 4
4.71 6.28 7.85 10.99
C,,EO, (0.1 M LiCl) 4.50, 4.50 4.50 6.43, 6.38 6.40 f 0.5% 7.90, 8.02 7.96 f 0.8% 10.5,11.0 10.75 f 2.3%
-4.5 necessary for reproducible results. +1.9 Figure 6 shows Ah-c curves for some of the individual ~1.4 polyoxyethylenated n-dodecyl alcohols at drop times of 3.1 -2.2 s in 0.5 M LiC1. Figure 7 shows the peak heights vs. molar C,,EO, (0.5M LiC1) concentrations of the surfactants. It is apparent that, on an 5 3.37 3.12, 3.40 3.26 f 4.3% -3.3 equimolar basis, the compounds with longer polyoxyethylene 6 6.73 6.90,6.70 6.80 f 1.5% +1.0 chains show somewhat greater sensitivity to concentration 7 11.2 10.9, 11.5 11.2 f 2.6% 0 change than those with shorter chains in addition to having C,,EO, (1 M LiCl) an initial linear region which extends to higher concentrations. 8 3.18 3.50,3.20 3.35 f 4.5% f 5.3 Because commercial nonionic5 are mixtures of compounds 9 6.36 6.25, 6.40 6.32 f 1.2% -0.6 having a Poisson distribution of polyoxyethylene chain lengths, 10 7.95 8.15, 7.80 7.98 f 2.2% + 2.4 we prepared and analyzed a mixed sample containing six 11 10.60 10.67,11.00 10.84 f 1.5% +1.9 individual polyoxyethylenated n-dodecyl alcohols in approx12 13.25 13.10,13.00 13.05 f 0.4% -1.5 imately Poisson molar distribution (mole ratio 13 15.91 16.20,15.90 16.05 f 0.09% t1.2 20.40,21.10 20.75 f 1.7% -1.9 C 1 ~ E 0 ~ : C ~ 2 E 0 ~ : C ~ 2 E 0 ~ : C ~ 2 E O ~ : C ~ ~ E=O , : C ~142 E O21.20 ~ 0.206:0.485:1.00:1.04:0.523:0.197;average EO = 5.51). MeaC,,EO, (0.5 M LiCI) surements were made in 0.1 M LiCl at a drop time of 3.1 s. 15 13.69 13.60,13.50 13.55 f 0.4% -1.0 The initial, linear region of the ah-c curve extended to 20 16.43 16.60.16.85 16.73 f 0.8% 16 +1.8 ppm. Data are given in Table I. This Poisson mixture gave sharply defined desorption less defined peaks that were, however, measurable. peaks. Industrial nonionic surfactants consisting of mixtures Accuracy and Precision. The accuracy of the present of materials with a distribution of polyoxyethylene chain method is *2.5% for concentrationsdown to 6 ppm and k5% lengths and less well-defined hydrophobic groups gave broader, for concentrations down to 3 ppm. Data are given in Table
236
Anal. Chem. 1981, 53,236-239 (3) Nozawa, A.; Ohnuma, T.; Seklne, T. Analyst (London) 1978, 701, 543-548. (4) Morgan, D. J. Analyst (London) 1962, 87, 233-234. (5) Crabb, N. T.; Persinger, H. E. J . Am. Oil Chem. SOC. 1964, 47, 752-755. (6) Weber, J. R.; Degner, E. F.; Bahjat, K. S. Anal. Chem. 1964, 36, 678-679. (7) Greff, R. A.; Setzkorn, E. A.; Leslie, W. D. J. Am. 011 Chem. SOC. 1965, 42, 180-185. (8) Huddleston, R. L.; Alfred. R. C. J . Am. Oil Chem. SOC. 1965, 42, 983-988. (9) Courtot-Coupez, J.; le Bihan, A. Anal. Lett. 1969, 2 , 567-576. (IO) Buerger, K. 2. Anal. Chem. 1963, 796, 251-259. (11) B r e w , B.; Bauer, H. H. I n "Chemical Analysis"; Eiving, P., Kolthoff, I., Eds:; Interscience: New York, 1963; Vol. 13. (12) Maironovskii, S.G. "Catalytic and Kinetic Waves in Polarography"; Plenum: New York, 1968. (13) Jehring, H.; Weiss, A. TensMe 1989, 6, 251-257. (14) Bratin, P. Doctoral Thesis, City University of New York, in preparation. (15) Schwartz, R. Doctoral Thesis, City University of New York, 1974. (16) Randles, J. E. B. Dlscuss. Faraday SOC.1947, 7 , 11-19. (17) Nemec, L. Collect. Czech. Chem. Commun. 1966, 31. 1162-1171.
Table 111. Precision of the Tensammetric Method for the Determination of Polyoxyethylenated n-Dodecyl Alcohols sample no. 1 2 3
4 5
precision, composition measd concn, ppm C,,EO3 C12E03
ClZEO, ClZEO, CIZEO,
3.60, 3.80 a.35,a.vo 16.45, 16.70 13.60, 13.60 13.20, 13.40
% i. 2.7 %2.0 *1.2
tO.0
k0.a
'I- The precision Of the method is f3% for concentrations down to 3 ppm. Data are given in Table 111. ACKNOWLEDGMENT We are grateful to Joseph Glickstein and Orest Popovych of this department for assistance during the course of this investigation. LITERATURE CITED (1) Brown, E. G.; Hayes, T. J. Analyst (London) 1955, 80, 755-787. (2) Kurata, M. Yukagaku 1955, 4 , 293-298.
RECEIVED for review July 30, 1980. Accepted November 21, 1980. This material is based upon work supported by the National Science Foundation under Grant No. ENG-7825930.
Determination of Positional Isomers of Methylpyrenes and Other Polycyclic Aromatic Hydrocarbons by Magnetic Circular Dichroism Jacek W. Waluk and Josef Michl" Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
Recent theoretical advances make it probable that magnetic circular dichrolsm can become a useful tool In the determlnation of the derivatlves and heteroanalogues of polynuclear aromatics because of its sensltlvlty to posltional isomerism. A simple method for the predictlon of absolute MCD signs for molecules of this kind from first prlnclples Is illustrated on the three isomeric methyipyrenes. Their observed MCD slgns agree with predictions and thelr MCD spectra are sufficiently different that a quantltatlve determination of their concentrations in a ternary mlxture is posslble at microgram levels.
Qualitative and quantitative determination of polynuclear hydrocarbons, their substituted derivatives, and heterocyclic analogues is of considerable interest in view of the significant biological activity of many of them and of their widespread occurrence in the products of pyrolysis and combustion of organic materials. Fluorescence spectroscopy and gas chromatography-mass spectrometry are the two foremost analytical tools used for this group of compounds. Neither is particularly well suited for the differentiation of positional isomers. This is unfortunate considering that the biological activity of such isomers can frequently differ dramatically. In the present article we wish to point out the potential usefulness of magnetic circular dichroism (MCD) spectroscopy as a complementary analytical tool for this class of compounds. In particular, we wish to emphasize that recent theoretical advances (1,2)permit a prediction of the absolute MCD signs of the lowest or several of the lowest electronic transitions for positional isomers on the basis of molecular structure alone, without a need for elaborate calculations, and that these signs 0003-2700/81/0353-0236$01 .OO/O
are a sensitive function of positional isomerism. The analytical implications of this theoretical development are obvious, in particular since already weakly interacting substituents, such as methyl, are sufficient to induce different MCD signs when located in various positions of a polynuclear aromatic hydrocarbon. The reader is referred to ref 1for a simple nonmathematical description of the quantum mechanical model which permits MCD sign predictions for this class of compounds. A more rigorous description is found in ref 2. In the following, we illustrate the procedure on the case of the three isomeric methylpyrenes,whose MCD has not been studied before. We find that the MCD spectra show the expected absolute signs and that they differ sufficiently to permit the use of a commercial dichrograph equipped with a small electromagnet to analyze their mixture quantitatively down to the 1-pg level. For fluorescent samples such as these, the use of fluorescence-detected MCD, which would require somewhat more complex instrumentation (3), would provide several orders of magnitude in sensitivity, particularly if laser light were used for the excitation. EXPERIMENTAL SECTION Samples of 1-methylpyrene(1-Py),2-methylpyrene(2-Py),and 4-methylpyrene (4-Py) were obtained from A. Berg (Aarhus University, Denmark) and were purified by gradient sublimation. Spectra were run in spectrograde quality cyclohexane. Absorption was measured on a Cary 17 spectrophotometer and MCD on a JASCO 500C spectropolarimeter equipped with a 15-kG electromagnet, which was wavelength calibrated with a holmium oxide film filter and scale calibrated with the CD of d-camphorsulfonic acid and MCD of naphthalene. Standard volumetric procedures were used for sample preparation and dilutions; higher accuracy 0 1981 American Chemical Society
