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 precision, no. composition measd concn, ppm % 1 2 3
4 5
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
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
could undoubtedly be obtained by taking special precautions suitable for work with volatile solvents. The absorption spectra shown in the figures display molar absorptivity, E, or absorbance, A, plotted against wavenumber. The MCD spectra show molar magnetic ellipticity [e], per unit magnetic field (deg.Ld.mol-' G-l), or magnetic ellipticity per meter and unit magnetic field 0 (degm-l.G-'), plotted against wavenumber. All pathlengths were 1 cm. RESULTS AND DISCUSSION For molecules derived from a coujugated (4N + 2) electron annulene perimeter, it has been shown that the MCD effects of the four characteristic low-energy transitions can be expressed as a sum of two contributions (2). The four transitions are the two L bands at low energy and the two B bands at somewhat higher energy in the Platt nomenclature (4). One of the contributions (p- contribution) is proportional to a small magnetic moment p- and is largely independent of the molecular structure. It provides a weakly negative or vanishing contribution to the MCD of each of the L bands, a stronger negative contribution to the MCD of the first of the two B bands, and a similarly strong positive contribution to the MCD of the second of the two B bands, in the order of increasing transition energy. The other contribution (p+ contribution) is proportional to a large magnetic moment p+ and unlike the former is very sensitive to molecular structure in a way which reflects the nature of the perturbation which produced the molecule in question from the parent (4N 2) electron annulene perimeter. Each such perturbation has the potential of splitting the originally degenerate pair of highest occupied molecular orbitals of the parent annulene by an amount which we shall refer to as AHOMO and also of splitting the originally degenerate pair of lowest unoccupied molecular orbitals by an amount which we shall refer to as ALUMO. If AHOMO = ALUMO, the p+ contribution to all four transitions in question vanishes. If AHOMO > ALUMO, the signs of the p+ contributions to the MCD effect are -, +, -, +, in the order of increasing transition energy. If AHOMO < ALUMO, signs of the p+ contribution to the MCD effect are +, -, +, -, in the order of increasing transition energy. The magnitudes of the p+ contributions are dictated by the size of the difference between AHOMO and ALUMO. Since the p- contributions to the L bands are particularly weak, the p+ contributions find it relatively easy to dominate the MCD signs of these transitions, even if AHOMO and ALUMO are almost equal. On the other hand, it is harder for them to dominate the MCD signs of the B bands, for which the p- contributions are more significant. Therefore, the long wavelength L bands are more likely to be of analytical significance. Simple theories of the Huckel(5) and Pariser-Parr-Pople (6)types suggest and a variety of experimental data confirm that in benzenoid hydrocarbons AHOMO ALUMO. The MCD spectra of benzenoid hydrocarbons and other molecules which share this property with them are therefore likely to be extremely sensitive to any further perturbation which will remove this equality. For this reason, they are referred to as soft MCD chromophores. This sensitivity provides the basis for the potential analytical usefulness of MCD for polynuclear aromatics. On the other hand, hydrocarbons in which AHOMO and ALUMO are quite different, such as many nonalternants (acenaphthylene, fluoranthene), have MCD signs determined by the sign of their AHOMO-ALUM0 difference. This is not likely to change upon weak perturbation such as methyl substitution, and such hydrocarbons are referred to as hard MCD chromophores. In order to determine whether the introduction of a substituent or heteroatom into a benzenoid hydrocarbon will cawe the difference AHOMO-ALUM0 to become positive or negative rather than zero, it is usually quite sufficient to use the simplest perturbation theory (3, as outlined elsewhere (I,2).
+
=
Table I. Substitution in Pyrene: Observed MCD Signs of L Bandsb substituent position typea L, N (aa)
1
D S D D S D D
2
S
4
D
1 2, 7
4 2"
1 2
4
COOC,H,
a
D = dominant, S = subdominant,
++-
+-
++
237
L,
-
+++-
+-
See ref 9 and 10.
Inspection of MO coefficients leads to the classification of substitution positions as dominant or subdominant. The action of a a-electron donor, such as a halogen or methyl, in a dominant position leads to an increase in AHOMO, and thus a negative MCD sign for the first aa* transition. In a subdominant position, weak a donors cause a decrease in AHOMO and will produce a positive MCD sign for the first an* transition, and the situation is somewhat more complicated for very strong donors. Attaching a a-electron-withdrawing substituent will have just the opposite effect on the relative size of AHOMO and ALUMO, and thus on the MCD signs. It is useful to remember that dominant positions generally are those of high reactivity in aromatic substitution, and subdominant positions are those of low reactivity, for reasons readily derived by an application of the frontier molecular orbital theory of chemical reactivity (8). In pyrene, position 1is strongly and position 4 weakly dominant; position 2 is subdominant (9). The observed MCD signs of strongly perturbed pyrenes are in perfect agreement with theory (Table 1) (9,10). 2
7 PY
Since the methyl group acts as a weak a-electron donor, we would expect the MCD signs of the L b band in methylpyrenes to follow those predicted from theory and verified experimentally for the aminopyrenes (Table I), that is, a negative MCD sign for the L b transition of 1-Py, a weaker MCD band of the same sign for that of 4-Py, and a positive MCD sign for the Lb transition of 2-Py. We feel that the methylpyrenes are a good test case for the potential analytical usefulness of MCD in the area of polynuclear aromatics, not only because such compounds are of some practical interest but also because the methyl substituent is one of the weakest in its perturbing ability, and if the theory works for it, it has a good chance of working for weakly interacting substituents in general. The spectra of the three methylpyrenes are shown in Figures 1-3 and are seen to conform perfectly to the theoretical expectations. The Lb transition near 27 000 cm-l has the expected MCD sign, negative in 1-Py, somewhat more weakly negative in 4-Py, and positive in 2-Py. In 2-Py, where we expect both the p- and the'M contribution to the MCD sign of the second band near 30 000 cm-l to be negative, the observed MCD sign is strongly negative. In 1-Py and 4-Py, in which the essentially structure-independent p- contribution to the L, band is also negative, it is expected to be counteracted by a positive p+ contribution and indeed the observed
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
238
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Flgure 3. 4-Methylpyrene (4-Py) In cyclohexane (room temperature):
Flgure 1. 1-Methylpyrene (1-Py) in cyclohexane (room temperature):
bottom, absorption: top, MCD.
bottom, absorption; top, MCD.
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Flgure 2. 2-Methylpyrene (2-Py) in cyclohexane (room temperature): bottom, absorptlon; top, MCD.
In cyclohexane (room temperature): bottom, absorbance per cm; top, magnetic ellipticity In units of deg-m-' 0-'; left, 1.5 X M in each component (nolse negligible);right, 1.5 X lo-' M in each component (nolse level lndicated).
spectrum has almost vanishing intensity in this region. The methyl substituent is apparently not sufficiently strong to perturb significantlythe considerably stronger p- contributions to the two B bands at about 36 000 cm-l and about 42 OOO cm-l. In this region, the spectrum resembles closely that of pyrene itself. This illustrates nicely the higher analytical interest of the long wavelength region of the L bands relative to the higher energy region of the B bands. Since the separation of positional isomers is frequently difficult, it is interesting to ask whether MCD would be useful for quantitative analysis of mixtures of the methylpyrenes. To test this, we have prepared a 1:l:l mixture of 1-Py:2Py:CPy and assembled a simple computer program using the
known MCD spectra of pure isomers to determine the least-squares fit of the measured spectrum of the mixture (Figure 4) in the wavelength region of the Lb band, 25 60028600 cm-l. With a relatively concentrated sample (1.5 X lo4 M in each isomer), the answer was 1.001.020.96 from a single 4-min scan. With a more dilute sample, 1.5 X lo4 M in each isomer, the answer was 1.06:0.95:1.00 from an average of 32 20-min scans. Considering that only about 2.5-3 mL of the solution was needed, this represents a simultaneous determination of ,.,100 pg or ,.,1pg of each isomer in the former and the latter measurement, respectively. The measurement on the concentrated solution was repeated several times, and the standard deviation was determined. The calculated
Figure 4. A 1:l:l mixture of le,2-Py, and 4-Py
J (103cm-1)
Anal. Chem. 1981, 5 3 , 2 3 9 - 2 4 2
concentrations were 1.51 X 1.53 X and 1.44 X lo-“ M for 1-Py, 2-Py, and 4-Py, respectively, with a standard deviation of 0.10 X M. Next, we were interested in the smallest percentage of each isomer which could be reliably detected as impurity in the presence of an excess (ca. 1 X 10“ M) of both other isomers. We found that it was less than 5% but more than 2%. At the 5% level, the quantitative determination of the concentration of “impurity” isomer was accurate to within about a factor of 2. It is of interest to compare these results with those of absorption spectroscopy, since Figures 1-3 show that the absorption spectra of the three methylpyrenes also differ in the fine structure of the Lb band, even though to a lesser extent than their MCD spectra. By use of the same solutions, wavelength region, and computer program, the determination of the isomers in the 1:l:l mixture at the 1.5 X M level was only accurate to within about a factor of 2. A determination of the “impurity” isomer at the 5% level was not possible. It should also be noted that there is no simple relation between molecular structure and the vibrational fine structure in absorption bands. We conclude that the large differences among the methylpyrene positional isomers evident in Figures 1-3 can be used advantageously for quantitative analysis of the mixtures at microgram levels. The above outlined theory suggests that similar situations will be encountered with heteroanalogues and derivatives of other benzenoid hydrocarbons but not with those of nonalternant hydrocarbons such as fluoranthene. In summary then, it appears quite likely that even a grand mixture of methyl-substituted benzenoid hydrocarbons could be analyzed by chromatographic separation into fractions containing several positional isomers each, followed by MCD spectroscopy of the fractions. An extension of the technique toward higher sensitivities can be readily envisaged.
239
Fluorescence-detected MCD measurements have already been demonstrated (3)and would permit a lowering of the detection limits, particularly if laser excitation were used. Moreover, by providing the freedom of choice of the monitoring wavelength, they would permit selective analysis of only those components of a mixture which emitted at that wavelength. The resulting analytical tool could find application in the analysis of air pollutants, combustion and pyrolysis products, oil spills, etc.
ACKNOWLEDGMENT We are grateful to A. Berg for a gift of the methylpyrenes, to J. Michl, the father of one of us, for technical assistance, and to John W. Downing for producing the computer program.
LITERATURE CITED (1) Michl, J. Pure Appl. Chem. 1980, 52. 1549-1564. (2) Mlchl, J. J. Am. Chem. SOC. 1978, 100, 6801-6824. (3) Sutherland, J. C.; Low, H. H. froc. Natl. Acad. Sci. U.S.A. 1976, 73, 276-280. (4) Platt, J. R. J. Chem. fhys. 1949, 17, 484-495. (5) Heilbronner, E.; Bock, H. “Das HMO-Modell und seine Anwendung”; Verlag Chemie: Weinheim, Germany, 1968. (6) Salem, L. “The Molecular Orbital Theory of Conjugated Systems”; W. A. Benjamin: New York, 1966. (7) Dewar, M. J. S.; Dougherty, R. C. “The PMO Theory of Organlc Chemistry”; Plenum Press: New York, 1975. (8) Fleming, 1. “Frontier Orbitals and Organic Chemical Reactions”; WileyInterscience: New York, 1976. (9) VaSBk, M.; Whipple, M. R.; Berg, A.; Michl, J. J. Am. Chem. SOC. 1978, 100, 6872-6877. (IO) Thulstrup, E. W.; Downing, J. W.; Michl, J. Chem. fhys. 1977, 23, 307-319.
RECEIVED for review May 19,1980. Accepted October 14,1980. This work was supported by the U.S.Public Health Service through Grant GM 21153. It was presented a t the 180th National Meeting of the American Chemical Society, Las Vegas, NV, Aug 24-29, 1980.
Line Profile Distortions in Laser-Induced Impedance Change Signals for Wavelength Determination of Tunable Dye Lasers G. J. Beenen and E. H. Piepmeier” Department of Chemistry, Oregon State University, Corvaliis, Oregon 9733 1
Spectral profiles recorded by use of [optogaivanic] laser-lnduced impedance change (LIIC) signals, with hollow cathode lamps operating In either the pulsed or dc mode may exhibit spectral dlstortions including line broadening, line shifts, and line reversal under varlous conditions. These spectral distortions, which have been observed to shift the wavelength at which the LIIC signal reaches a maximum value by as much as 0.055 A, can give erroneous results if LIIC signals are used to determine the wavelength of a tunable dye laser. A distinction is made between those distortlons that are optical in origin and those that result from space charge effects within the dlscharge. The experimental condltlons under which these dlstortions are observed, and can be avoided, are dlscussed.
The uses of [optogalvanic] laser-induced impedance change signals (LIIC) for wavelength determinations and stabilization
of tunable dye lasers have been reported previously. Green et al. (1) showed how LIIC signals could be used as part of a feedback network for frequency stabilization of a continuous wave (CW) dye laser. King et al. (2) demonstrated the use of LIIC signals for bandwidth measurement of a CW dye laser. In addition they showed that LIIC signals could be used to accurately tune a tunable dye laser to an atomic transition. Keller et al. (3)have recently published an atlas comparing hollow cathode emission intensities to LIIC signals as an aid to wavelength calibration of tunable dye lasers. Their study covered the wavelength region of 3848-9085 A by use of uranium and neon transitions. In their study they noted that LIIC signals for neon transitions did not correlate with emission intensities and that saturation effects can be observed for laser powers in excess of 100 mW (CW). In addition they provided tabulated values of the wavenumbers for the stronger transitions with an estimated accuracy of 0.005 cm-l. They do not report any observed shifts in the LIIC signal profiles compared to the hollow cathode emission profiles. Nonlinear
0003-2700/81/0353-0239$01.00/00 1981 Amerlcan Chemical Soclety
