J . Phys. Chem. 1987, 91, 2408-2415
2408
a lower data density and we were not able to measure premicellar data for those surfactants with longer alkyl chains. In conclusion, we have measured osmotic coefficients for the C 10, C 12, C 14, and C 16 alkyltrimethylammonium bromides in aqueous and in aqueous sodium bromide solutions at concentrations ranging from 0.01 to 0.30 mol kg-'. We have shown the
feasibility of using a vapor pressure osmometer to obtain osmotic coefficients subject to some limitations in precision and accuracy, especially at low concentrations. We have also shown that the model discussed above can be used to describe our osmotic coefficients. Osmotic coefficients have also been calculated as a function of temperature.
EPR Characteristics of Separated Fractions of Mesophase Pitches L. S. Singer,*+ I. C. Lewis, D. M. Riffle, Union Carbide Corporation, Carbon Products Division, Parma Technical Center, Cleveland, Ohio 441 01
and D. C. Doetschman Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13901 (Received: May 13, 1986; In Final Form: December 24, 1986)
Carbonaceous pitches are complex mixtures of aromatic hydrocarbonswith wide distributions of molecular weight and structure. The solid materials are isotropic or nematic glasses which contain neutral odd-alternate doublet free radicals with radical concentrations of 101*-10L9spins/g. The spin-spin and spin-lattice relaxation properties, which are presented for various fractions separated by both high-temperature centrifugation and solvent extraction, span the range between inhomogeneous and homogeneous behavior. Studies are presented of the EPR CW saturation and spin-echo of several molecular weight fractions of pitches derived from naphthalene, petroleum, and coal tar. The EPR saturation and line shapes and the spin-echo T I ((1-6) X lo4 s) and T2 ((2-7) X lod s) values are compared with one another. The CW and pulsed EPR indicate the detailed nature of the spin relaxation and of the transition from heterogeneousto homogeneous behavior as the samples increase in molecular weight and aromatic character. The results also indicate that the diffusion of saturation from spins near the pumping frequency into the rest of the EPR line occurs via exchange interactions between the free radicals, and that relaxation to the lattice may involve the nuclear spin system. Comparison of the pulsed and CW EPR saturation parameters indicates that, whereas the EPR results give the average parameters over a wide distribution of molecular species, the pulsed EPR selects out the species with particular spin-spin relaxation times within the time response of the instrument.
I. Introduction Carbonaceous pitches are complex mixtures containing thousands of primarily aromatic hydrocarbon and heterocyclic components. Although, generally, pitches are by-products derived from the pyrolysis of petroleum and coal fractions, they can also be prepared from polymers and single aromatic hydrocarbons such as anthracene and naphthalene.' The thermal reactions involved in the transformation of organic materials to pitch are exceedingly complex and include bond cleavage, hydrogen transfer, molecular rearrangement, and polymerization processes.2 Pitches behave as single-phase eutectic glasses, exhibiting glass transitions and melting over broad ranges of t e m p e r a t ~ r e . ~The individual constituents of pitch vary in molecular weight from several hundred to several thousand.] Pitches are also known to contain significant concentrations of stable free radicals. There is substantial evidence to support the contention that these stable radicals are neutral odd-alternate aromatics produced by thermally induced molecular rearrangement and polymerization proce~ses.~Although individual oddalternate hydrocarbon radicals have been studied in considerable detail by both EPR and ENDOR?s6 very little attention has been directed to complex mixtures of these radicals such as those which occur in pitches. EPR has, of course, been observed in many coals and char^^,^ and presumably the radicals are related in structure to those present in pitch. As pitches are heat treated, the aromatic components polymerize and the pitch is transformed to an infusible carbonaceous solid known as coke. The free-radical content continually increases during the transformation of pitch to coke. Further heat treatment Present address: 525 Race Street, Berea, OH 44017.
0022-36541871209 1-2408$01.50/0
above 1000 O C leads to the formation of carbon and ultimately graphite. At these latter stages, the EPR signals originate from the conduction electrons rather than from aromatic free radicals. During the thermal transformation to coke, pitches exhibit a liquid crystalline or mesophase state.9 The development of pitch mesophase is attributed to the polymerization of aromatic molecules into larger disk-like molecules which then associate and separate out from the lower molecular weight isotropic phase. A recent publicationlo has described the separation of the phases in mesophase pitches by high-temperature centrifugation and the chemical characterization of the separated phases. Limited EPR data, including spin concentrations and line widths, were also included. It was found that the higher molecular weight mesophase fractions had higher free-radical concentrations than the lower molecular weight isotropic ( I ) Greinke, R. A.; Lewis, 1. C. Carbon 1984, 22, 305. ( 2 ) Lewis, I. C. Carbon 1982, 20, 519. (3) Rand, B.; Shepherd, P. M. Fuel 1980, 59, 814. (4) Singer, L. S.; Lewis, I. C. Appl. Spectrosc. 1982, 36, 52. (5) Broser, W.; Kurreck, H.; Oestreich-Janzen, S . ; Schloemp, G.; Fey, H. J.; Kirste, B. Tetrahedron 1979, 35, 1159. (6) Lewis, I. C.; Singer, L. S. Magn. Reson. Chem. 1985, 23, 698. (7) Lewis, I. C.; Singer, L. S. In Chemistry and Physics of Carbon; Walker, Jr. P. L.; Thrower, P. A.; Eds.; Marcel Dekker: New York, 1981; Vol. 17, pp 1-88. (8) Singer, L. S. Proceedings of the 5th Carbon Conference; Pergamon: New York, 1963; Vol. 2, p 37. (9) Brooks, J. D.; Taylor, G. H. In Chemistry and Physics of Carbon; Walker, Jr., P. L., Ed.; Marcel Dekker: New York, 1968; Vol. 4, p 243. (IO) Singer, L. S . ; Lewis, I. C.; Greinke, R. A. Mol. Cryst. Liq. Cryst. 1986, 132, 65. (11) Singer, L. S . ; Lewis, I. C.; Riffle, D. M. Exrended Abstracts, 17th Biennial Conference on Carbon, Lexington, KY, June 16-21, 1985; American Carbon Society: University Park, PA, 1985; pp 155-156.
0 1987 American Chemical Society
Separated Fractions of Mesophase Pitches In this paper, the EPR results and their interpretations are discussed in considerable detail for the centrifuged fractions and solvent extracts of mesophase pitches derived from petroleum, coal tar, and naphthalene. Since the free radicals present in pitches have been shown to be neutral odd-alternate hydrocarbon radicals7 which exhibit EPR power saturation at very low microwave power, particular emphasis is given to the saturation behavior and its rationalization in terms of molecular structure, free-radical concentration, and spin-lattice interactions. Apparatus and other experimental details for both the EPR saturation and spin-echo measurements are discussed in section 11. The relevant theories of EPR relaxation and saturation are discussed in section 111, with emphasis on the transition from inhomogeneous to homogeneous saturation behavior. The experimental results and their discussion are presented in the final sections of the paper. 11. Experimental Apparatus and Procedures A . EPR Equipment for CW Experiments. Room temperature C W EPR measurements (300 f 1 K) were made with an IBM Instruments Model ER/200D-SRC EPR spectrometer equipped with the ASPECT 2000 computer. The low-power (90 dB) bridge was of particular importance in these measurements, since microwave powers in the nanowatt range were required for the long relaxation times of the radicals present in the pitches. For convenience and reproducibility, the field-frequency lock mode of operation was used. B. Curve Generation and Storage. An EPR curve was generated at each of 30 different microwave power levels ranging from 0.4 p W (57 dB) to 200 m W (0 dB) for each material. A scan width of 50 G and scan times of 100 or 200 s were used with time constants at 0.05 and 0.10 s, respectively. Under these conditions, the ratio (SW)(TC)/(ST)(SF) was approximately 0.005, which is well below the critical 0.1 value for observable line distortion. In this ratio, SW is the scan width in gauss, TC is the time constant in seconds, ST is the scan time in seconds, and SFis the peakto-peak line width of the EPR derivative curve in gauss. Magnetic field modulation frequencies of 375 Hz and 100 kHz were both used. The dependence of saturation behavior on modulation frequency will be discussed further in subsection D. A peak-to-peak modulation amplitude of 0.5 G was employed, which is approximately one-tenth of the observed 4-6 G line widths SFand is thus well below the amplitude which would give rise to line-shape distortion. A total of 2000 points was taken for each spectrum which corresponds to a more-than-adequate 200 points per line width. The curves were then stored on the 24 megabyte hard disk for later retrieval. C . Spin Concentration. Areas were determined by double integration using the IBM software which employs appropriate base-line corrections. Since the EPR line shapes were between Gaussian and Lorentzian and the signals dropped off fairly rapidly in the wings, the 50-G scan width was more than adequate to include essentially 100% of the area. This fact was verified experimentally. The areas were converted to spin concentrations assuming spins of one-half and Curie’s law, using small single crystals of CuS04-5H20as primary standard^.^ All spin concentrations were measured at 52 dB (1.3 pW) to avoid significant saturation errors. D. Saturation Measurements. Saturation measurements were made by determining the peak-to-peak amplitude ( D ) of the first-derivative EPR curve as a function of microwave power. The first step in displaying the data was to plot a BPP (Bloembergen, Purcell, and Pound)I2 plot as shown in Figure 1 . The ratio R, = (D/P1/2)/(D,/P,,’/2), where P is the variable microwave power and P , is one power level in the unsaturated regime, linearizes and normalizes the data so that in the absence of saturation, R, = 1 and is independent of power. The linearization is the result of the use of a homodyne system in which the crystal detector is microwave biased (200 PA) to become an accurate linear detector, (12) Bloembergen, N.; Purcell, E. M.; Found, R. V . Phys. Rev. 1948, 73, 679.
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2409 I
0.6
a* 0.4
0.2
-I
1
J
Figure 1. A typical saturation plot as presented by Bloembergen , Purcell, and Pound (BPP Plots).12The parameter P I j zis the microwave power at which the normalized amplitude Rs falls to one-half its unsaturated value.
that is, one in which the signal amplitude in the unsaturated regime is strictly proportional to the square root of power. The parameter P l l 2is defined as the microwave power at which the normalized EPR amplitude falls to one-half of its unsaturated value. There are alternative ways of expressing saturation behavior. One can measure the amplitude of the derivative curve as a function of power at constant field as More et al. have done.” Or one can measure the maximum amplitude of the absorption curve (first integral of the measured derivative curve) as a function of power.I4 The latter yields saturation values proportional to x” (coo), which does not depend explicitly on changes in line width during saturation and is somewhat amenable to theoretical calculation. We have determined for the petroleum mesophase pitch that the first two methods yield almost identical saturation curves, while the peak absorption method differs only slightly. Such behavior might be expected for EPR curves which change very little in line width or line shape as a function of microwave power. Bloembergen et al.Iz also discussed the variation in saturation behavior with field modulation frequency, w,. Since it was not clear at the outset whether the T 1 values for pitches and their fractions would be in the range such that w,TI > 1, most of the saturation curve data were obtained at both 375 Hz and 100 kHz. As Bloembergen et pointed out, the saturation effect becomes evident at a lower power for case 1, in which w,T1 6 In this analysis, the widths (in units of SF)at various fractional heights on the derivative curve are plotted against the corresponding widths at the same fractional heights for a calculated Gaussian or a Lorentzian curve. Linearity or deviations from linearity can then be used to estimate the line shape for different portions of an EPR curve. For example, Figure 8 shows line-shape analysis plots for the separated isotropic and anisotropic phases of the petroleum mesophase pitch. It is clear that the line shapes are similar for both phases, with the higher molecular weight mesophase curve lying slightly closer to the Lorentzian correlation line and, of course, farther from the Gaussian correlation line. The rapid drop-off of EPR intensity in the wings (Gaussian shape in the wings) had been ascertained by varying the integration (26) Singer, L..S.;Kommandeur, J. J . Chem. Phys. 1961, 34, 133.
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2413
Separated Fractions of Mesophase Pitches A-ANISOTROPIC PHASE .-MESOPHASE PITCH
6-4-MESOPHASE
MICROWAVE POWER (pw)
Figure 9. BPP-typeI2saturation plots for the naphthalene mesophase pitch and its centrifuged and solvent extracted fractions (modulation frequency 100 kHz). Note that the abscissa is power rather than HI.
fi/$ Figure 10. PortisI4 plots for four different fractions of the petroleum mesophase pitch (modulation frequency 375 Hz).
TABLE II: Saturation P l j l Values for Pitches and Their Fractions (Microwatts)
sample petroleum
PS isotropic whole
mesophase
PI
moduln freq
2600 1380 975 510 12000 5900
2600 1410 2100 1250 14500 6650
25000 15400 6600 4400 32000 18300
100 kHz 375 H z 100 kHz 375 H z 100 kHz 375 H z
340 135 naphthalene 95 40 coal tar 310 130
440 185 280 145 1950 1035
ranges during the spin concentration determinations. B. Saturation Behavior. Figure 9 shows BPP-type plots of the saturation behavior at 100 kHz magnetic field modulation of the various fractions of the naphthalene mesophase pitch. As expected, the difficulty of saturation increases with increasing molecular size and spin concentration as one proceeds from PS to isotropic phase to mesophase pitch to anisotropic phase to PI. The saturation results for all of the pitches and their fractions are summarized in Table 11, in which the Pl12values, microwave power at which the normalized EPR amplitude R, falls to one-half of its unsaturated value, are listed at two different field modulation frequencies, 375 H z and 100 kHz. It is clear, as pointed out by Bloembergen et a1.,I2that at lower modulation frequencies, when presumably U T ,< 1, the spin system appears to saturate more easily. Moreover, the ratios of values a t the two modulation frequencies for a given pitch fraction are between 1.5 and 2.5, which are reasonably close to the value 2.39 predicted for the ratio of the high- and low-frequency limits in a homogeneously broadened line.'2 Although the saturation curves in Figure 9 are relatively similar in shape, there are significant quantitative differences at high microwave powers. As described in the work of Portis,I4 a saturation plot as shown in Figure 3 not only emphasizes differences at high powers, where there is a high degree of saturation, but also clearly differentiates between homogeneous and inhomogeneous relaxation mechanisms. Such Portis plots for the four different fractions of the petroleum mesophase pitch are shown in Figure 10 for 375 H z modulation frequency. Note that there is a gradual trend from inhomogeneous to homogeneous relaxation behavior as one proceeds from the PS to isotropic phase to mesophase to PI, that is, from the lower molecular weight to the higher molecular weight fractions. The other pitches behave similarly. The fact that the relaxation behavior is neither purely homogeneous nor purely inhomogeneous, but changes gradually between these extremes, is understandable. As the molecular weights of the fractions increase, the relaxation rates also increase. That is to say, the homogeneous line widths of the spin packets broaden while the inhomogeneous line width of the overall line envelope narrows, caused in part by the changes in radical concentration and the distribution of molecular sizes and structures in the various pitch fractions.
4 A-1' '"''160'
""'l'bl ' ""'lbz'
"11111b3' "'"l'o4'
'""l'05'
"'To6
MICROWAVE POWER, P (MICROWATTS)
Figure 11. Line width (peak-to-peak)vs. the degree of saturation for the isotropic and mesophase fractions of the coal tar mesophase pitch.
Most of the pitch fractions contain some distribution of relaxation behaviors between the two extremes shown in Figure 3, as can be seen by comparing the Portis plots with the intermediate curves of Ca~tner.2~ For example, the Portis plot for the PI fraction of the petroleum pitch in Figure 10 conforms very closely to purely homogeneous relaxation, but the other three petroleum pitch fractions in Figure 10 do not conform to any particular ratio of homogeneity and heterogeneity. The comparison appears to indicate that there is a distribution of many ratios present in each fraction. In particular, the low molecular weight fraction has the widest distribution of ratios with a substantial fraction near the heterogeneous limit. The distribution of ratios in the fractions appears to approach the purely homogeneous limit with increasing molecular weight of the fraction. As mentioned previously, there is also a difference in line shape and line width vs. the degree of saturation for the two relaxation mechanisms (see Figure 4). A comparable line-width plot for the isotropic and anisotropic phases of the coal tar mesophase pitch is shown in Figure 11. One may assume that the line-width increase with power beyond the low power limit in Figure 11 should be approximately equal to the peak-to-peak SFof a homogeneously broadened line, ( 2 / 3 ) ( P / P l 1 2 ) T 2The . slope of an SFvs. P / P l j 2plot, below the extremely high power narrowing region to be discussed next, is then proportional to T2-'. A plot for the isotropic coal tar pitch fraction gives a reasonable T2 estimate of 1.5 ~ s for , which a direct spin-echo measurement, 2.17 hs, is available (see subsection C). Note that the line-width increase with saturation is much greater for the mesophase than for the isotropic phase, consistent with the more homogeneous behavior and shorter T2of the higher molecular weight mesophase. The tendency for the line widths to decrease at extremely high microwave powers can be explained by the presence of a few very high molecular weight radicals in the tails of the distribution. That
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The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
is, when the EPR signals of most of the smaller molecules have been obliterated by saturation, the narrow EPR signals of the few larger molecules can then become apparent. Selective detection of the spectrum of these molecular species with narrower lines and shorter T2 times is also observed at short decay times with the spin-echo and free induction decay signals (see subsection C ) . Participation of dipolar interactions in the spin-lattice relaxation, according to the Provotorov theory,21may be contributing to the observation (in Figure 11) that the line width does not increase indefinitely with microwave power. Assuming Ti = 200 ps, (Hi,2)’/2 = 2 G, and Gaussian or Lorentzian line widths of about 5 G, one finds that T,’ must be around 100 ps fot the Provotorov theory2’ to contribute to the increase in a reasonable way. However, the asymptotic approach of the line width to ( Tl/Ti‘)(H,N2)1/2 predicted by the theory is not consistent with the observed decrease of the line width at still higher power. The theories of Provotorovziband Wind et al.24predict that biexponential saturation recovery curves should be observed at fields off the center of the EPR line. No indication of biexponential recoveries is found when experiments are performed off the centers of the EPR lines. C. Spin-Echo Results and Relaxation Times. Electron spin echoes were observable in the naphthalene and petroleum pitches and in all of their fractions except the pyridine-insoluble fractions. Echoes were observable in only the pyridine-soluble and isotropic fractions of the coal tar pitch. T2 measurements were made for these samples and the results indicate that the remaining samples have T2 values shorter than the instrumental time limit. T, measurements were possible on all of the above except the mesophase fractions, whose echoes were too weak for reliable T , determinations. Some of the results relevant to the present EPR study will be given here. A full description of the spin-echo study will be published separately. The two-pulse electron spin-echoes decay with a Gaussian dependence on pulse interval. This dependence is predictedi8 for the types of spin-spin interactions to be expected at the high radical concentration and high hydrogen content of the pitches. In the whole petroleum pitch, there is clear evidence of a mixture of species with a distribution of T2 values. The T2 values, which range from 2.01 f 0.13 ps for the mesophase fraction of the petroleum pitch to 6.44 f 0.03 ps for the PS fraction of the naphthalene pitch, increase in inverse order of radical concentration. There is, however, no consistently monotonic variation with line width or proton density throughout the range of pitches. The observed trend with radical concentration appears to imply that increasing interactions between the radicals as spin concentration increases lead to decreasing T2values. The decreasing T2values are, in turn, expected to vary inversely with the homogeneous or spin-packet line width as predicted in the scheme of Figure 5. The observed relationship between T2 and spin concentration is clearly not an f 3(dipolar) dependence on distance between the radicals, but it does appear to be consistent with an exponential (exchange) dependence on r. The echo intensities measured during recovery from saturation generally follow the simple exponential form predicted by the Bloch equations. Although the whole pitches did not display obvious distributions of T, values, the pyridine-soluble fraction of the naphthalene pitch did show evidence of a component with longer T I values. The Ti values range from 188 f 24 w s for the isotropic coal tar pitch fraction to 562 f 8 ps for the isotropic naphthalene pitch fraction. The T , values are shorter in the more aromatic coal tar pitch than in other pitches and shorter in the pyridine-soluble fractions of a given pitch than in other fractions of the same pitch, except for the coal tar pitch. The spin-lattice relaxation time measurements clarify the variation of EPR saturation behavior with field modulation frequency. For 1 W k H z modulation, the eight pitches measured give 118 < u,T1 < 353, a range that is clearly consistent with w,Tl >> 1 for Bloembergen’s case 2. For 375-Hz modulation, the range 0.44 < u,Tl < 1.32 determined for the eight pitches, however, is intermediate between cases 1 and 2. We note that for the pitches
Singer et al. TABLE III: Comparison of Relative H1,2aValues Measured with 100-kHzModulation and the Saturation Factor samDle HI,, mG ( Y ~ T , T , ) - mG ~/~,
coal tar isotropic PS
petroleum isotropic PS
naphthalene isotropic PS
22.5 9.0
2.81 1.41
10.7 9.4
1.31 1.48
8.5 5.0
1.23 1.os
‘Determined from P I l 2 with an estimated instrument calibration factor of 0.259 mG2 pW-I, based on the observation of 5% saturation of a DPPH sample at 0.10 W using case 1 of Bloembergen et all2 with measured Pij2(100 kHz)/P1l2 (375 Hz) 2 2.38, where 2.39 is the theoretical rat0 of Pljzvalues for cases 1 and 2, the range w,Ti I1.18 is found. This suggests that, from the experimental point of view, case 1 behavior is already being closely approached with the 375-Hz modulation frequency. The inverse of the T I T 2product should be a measure of the microwave power required to saturate the EPR transition. Table I11 compares the microwave field at half-saturation H I l 2determined from (100 kHz), with the saturation factor (-y2T1T2)-1/2.The correlation over the narrow range for which spin-echo measurements could be made shows that the quantities differ by factors of 5-8. From simple homogeneous saturation theory, these two quantities should be equal. It would appear that the echo method is selecting out radicals with generally longer T , and T2from the very wide distribution of molecular relaxation times. (This is a practical limitation imposed by the time response of the echo spectrometer employed and is not inherent in the method itself.) On the other hand, the H I l Zvalue from the EPR saturation measurement represents an average over the whole distribution of species. The magnitudes of the distributions of relaxation rates indicated by these results are substantial. The results indicate that the geometric mean relaxation time ( T IT2)i/2 varies over the distribution by at least a factor of 5-8 times the average value of the relaxation time.
V. Discussion The EPR studies of mesophase pitches and their fractions give a remarkably self-consistent progression of resonance characteristics with increasing average molecular weight. The free radicals appear to be associated to a large extent with species whose molecular weights are greater than 1500 in the naphthalene and petroleum pitches and considerably smaller than 1500 for the coal tar pitch. In low molecular weight materials with small free-radical molecules, the spin is localized and the proton hyperfine interactions are relatively large. In the higher molecular weight materials, the spin is more delocalized in more aromatic structures with fewer protons and, thus, the relatively smaller and fewer hyperfine interactions narrow the EPR line. As the radical concentration increases and the distances between the radicals decrease with higher average molecular weight, the spin relaxation is faster. The character of the relaxation is dominated by interactions between the electron spins of the free radicals. It is predominantly a spin-spin relaxation or spin diffusion process. The spin-spin relaxation rate increases with average molecular weight and the form of the echo decay is characteristic of interactions between electron spins of different resonance frequencies in solids. The dependence of the spin relaxation rates on free-radical concentration suggests that the nature of the interaction is of the exchange type rather than dipolar. Once the free-radical saturation has diffused or “burned a hole” to spins of different frequencies within the EPR line by spinspin exchange interactions, it is then transferred to the lattice by spin-lattice relaxation. The spin-lattice relaxation does not correlate with free-radical concentration, but may depend on proton content and chemical structure. Thus, the free-radical
J. Phys. Chem. 1987, 91, 2415-2422 saturation may be relaxing to the lattice via proton hyperfine interactions. Independent evidence exists for energy transfer between free radicals and the proton spin system in carbonaceous materials. In coals, the nuclei may be polarized by pumping the free-radical EPR transition and the nuclear Tl and Tz times are of the same order of magnitude as those measured here for electron spin-lattice relaxation.*' Moreover, strong matrix ENDOR lines have been found for the pitches under present discussion.11*z8 Finally, the EPR line shape and saturation behavior also demonstrate the interplay between the static or heterogeneous line broadening by hyperfine interactions and the dynamic or homogeneous line broadening by spin-spin interactions, as the average molecular weight of the pitch increases. In the low molecular weight materials with small radicals, the spins are dilute and the hyperfine interactions are large. These free radicals with their particular nuclear spin states tend toward an overall Gaussian line shape and saturate independently of one another. In high molecular weight materials, the free radicals are more concentrated (27) Wind, R. A.; Duijvestijn, M. J.; Lugt, C. V. D.; Smidt, J.; Vriend, J. In Magneric Resonance. Introduction, Advanced Topics, and Applications ro Fossil Energy; Petrakis, L.; Fraissard, J. P., Eds.; D. Reidel: Dordrecht, The Netherlands, 1984; pp 461-484. (28) Singer, L. S.;Lewis, I. C. Carbon 1984, 22,487.
2415
and the hyperfine interactions are smaller. The free radicals interact with one another, exchange saturation and exhibit more Lorentzian line shapes. The degree of dynamic broadening, as gauged by TZ-l, is observed to increase with molecular weight. Thus, the Portis plots of saturation exhibit a clearly increasing ratio of homogeneous to heterogeneous broadening as molecular weight increases. Although the pitch fractions generally display properties characteristic of a chemically homogeneous material, there is, nevertheless, some evidence of chemical heterogeneity of the pitch fractions. The Portis plots show that the samples comprise radicals with a range of ratios of homogeneous and heterogeneous broadening. The distribution of ratios appears to be widest for the lowest average molecular weight pitches and narrows toward purely homogeneous behavior in the highest average molecular weight pitches. The discrepancy between the average saturation parameters measured by EPR and the selective values measured by the echo method also points out a surprisingly wide range of TITzvalues. Also, the pyridine-soluble fraction of the naphthalene pitch actually displays a range of T , values. Acknowledgment. We gratefully acknowledge the assistance of Dr. Devkumar Mustafi in making the electron spin-echo measurements of T , and T2.
Complexation of Diaza Crown Compounds with Some Alkali Metal Ions in Acetonitrile and in Methanol at 25 OC Alessandro D'Aprano* Institute of Physical Chemistry, University of Palermo, 901 23 Palermo, Italy
and Bianca Sesta Department of Chemistry, "UniuersitB La Sapienza", 00185 Rome, Italy (Received: September 9, 1986)
Conductometric measurements of lithium and potassium picrate in pure methanol, in pure acetonitrile, and in the presence of 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane and N-methyl-N'-dodecyl- 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane macrocyclic ligands have been carried out at 25 ' C . The analysis of the results obtained with the different systems shows that the interaction forces, correlated with the molecular details of ions, solvents; and ligands, are, in most cases, superimposed on the ion-dipole forces acting between cations and macrocyclic cavities to such an extent as to prevent the cation macrocyclic ligand complexation. The effect of side chains attached to the main diazo crown ether ring on the complexation process is also discussed.
Introduction Complexation of alkali metal cations with cyclic polyethers (crown compounds according to Pedersen's' nomenclature) in both aqueous and nonaqueous solvents has recently received increasing interest as shown by the several review articles"' published in the (1) Pedersen, C. J. J . Am. Chem. SOC.1967, 89, 7017. (2) Izzatt, R. M.; Bradshaw, J. S. Nielsen, S.A.; Lamb, J. D.; Christensen, J. J.; Sen, D. Chem. Rev. 1985, 85, 271. (3) Christensen, J. J.; Eatough, D. J.; Izzatt, R. M. Chem. Rev. 1974, 74, 351. (4) Lisegang, G. W.; Eyring, E. M. In Synthetic Multidentate Compounds; Academic: New York, 1978. (5) Lamb, J. D.; Izzatt, R. M.; Christensen, J. J.; Eatough, D. J. In
Coordination Chemistry of Macrocyclic Compounds; Plenum: New York, 1979. ( 6 ) Poonia, N. S.; Bajaj, A. V. Chem. Rev. 1979, 79, 389. (7) De Young, F.; Reinhaudt, D. N. Adu. Phys. Org. Chem. 1980,17,279.
past 15 years. Equilibrium constants, enthalpy changes, entropy changes, rate constants, and activation parameter data on the complexation of cations with a variety of macrocycle compounds are reported. The implications of such a process in industrial and biological fields such as phase transfer,8-10 ion-selective electrodes,' '-I3 membrane separation processes,14chelation therapy,15 and tran(8) Pedersen, C. J. J . Am. Chem. SOC.1967, 89, 2945. (9) Pedersen, C. J.; Frendorff, H. K. Angew. Chem. 1972, 84, 16.
(IO) Landini, D.; Montanari, F.; Parisi, F. M. J . Chem. Soc. Chem. Commun. 1974, 879. (11) Fyles, T. M.; Melik-Diemer, V. A,; Whitfield, D. M. Can. J . Chem. 1981, 59, 1734. (12) Kale, K. K.; Cussler, E. L.; Evans, D. F. J . Phys. Chem. 1980, 85, 593. (13) Shono, T.; Okohara, M.; Ikeda, I.; Kimura, K. Tonura, H. J . Electroanal. Chem. 1982, 132, 99. (14) Hong Qi, 2.; Cussler, E. L. J . Membr. Sci. 1984, 19, 259.
0022-365418712091-2415$01.50/0 0 1987 American Chemical Society
