Using the measurement is more important than accuracy

Using the measurement is more important than accuracy

13C n.m.r. in aromaticity sample spinning at frequencies in which large effects on CP dynamics have been noted for representative organic samples, inc...

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13C n.m.r. in aromaticity sample spinning at frequencies in which large effects on CP dynamics have been noted for representative organic samples, including coals’,’ 5. Hence, even if one has the capability to spin fast enough to avoid spinning sidebands in a 13C CP/MAS experiment on coal, using a 200300 MHz spectrometer, one should expect major intensity distortions in the resulting spectra. One type of strategy for rendering high-speed MAS 13C detection compatible with CP is to conduct the experiment in a probe that allows one to use MAS during the detection period, whereas the cross-polarization segment of the experiment is carried out while the sample is static” or spinning at an angle in which the spinning modulation of the dipole4pole interactions is attenuated (e.g. spinning about an axis at 90” relative to B,) or eliminated (spinning about an axis colinear with B,)14. A related strategy in which the sample is static during CP is magic-angle hopping’ ‘. Another strategy that is used in an attempt to address the spinning sideband problem in high-field ’ 3C CP/MAS experiments is sideband suppressions’8,20, sometimes used in combination with chemical shift scaling schemes21. These approaches involve multiple-pulse sequences that, even when executed to ‘perfection’, are prone to introducing intensity distortions and, when executed imperfectly, can introduce artefacts into a spectrum. Hence, in the best case, one can be sure only of obtaining as simple a spectrum (absence of spinning sidebands) as in a lower-field spectrum obtained without sideband-suppression or chemical shift scaling. In the case of coal, with a distribution of relatively small T2 values, major intensity distortions can be expected. Furthermore, even if the intensity distortions and artefacts characteristic of these techniques could be overcome, what one would accomplish at best is to make a more expensive (higher field) spectrometer provide, in a relatively complicated experiment, approximately the same spectral results as yielded by a lower-cost (lowerfield) spectrometer in a simple experiment! In the higherfield case, a relatively complex experiment is carried out just to obtain a simple spectrum! If one wishes to add some spectroscopic complexity to the CP experiment (e.g. some kind of relaxation of 2-D measurement), then one is adding additional complexity to a technique that is complex, if sideband suprelatively already pression/chemical shift scaling is involved. A technique that is qualitatively useful in distinguishing among -CH, groups (x=0, 1, 2, 3) in coal is the interrupted-decoupling or dipolar dephasing experiment, based on the dephasing effects of 13C--lH dipolar couplings 22 While this technique is used extensively, it is based on the largely incoherent phenomenon of dipolar dephasing and lacks the capability of ‘crisp’ distinctions and spectral editing that are routinely available in liquidwhich are based on sample ’ 3C n .m.r. experiments, coherent behaviour under ’ 3C-1H J coupling. One can hope that continuing efforts directed to the development of superior solid-state ’ 3C spectral editing techniques for solids will ultimately be successful. Such developments will be highly important contributions to the goal of extracting more detailed information from the broad aromatic and aliphatic bands in the 13C n.m.r. spectra of coals. A solid-state n.m.r. technique that shows excellent promise for the study of coals, and especially the issue of aromaticity, is the ‘H CRAMPS technique23, in which

measurements

on coals:

A debate: C. Snape et al.

magic-angle spinning for averaging the chemical shift anisotropy is combined with technically difficult multiplepulse techniques for averaging the otherwise dominating ‘H-‘H dipolar effects. In its most straightforward form, this technique provides partially resolved peaks due to aromatic protons (hydrogen attached to aromatic rings) and aliphatic protons (hydrogen attached to sp3 carbon). Deconvolution of these peaks, which at this time is still plagued by base-line problems, shows potential for determining ‘hydrogen aromaticities’24. When used together with carbon aromaticities, these parameters provide highly useful constraints on structural interpretations25. The use of pyridine incorporation and/or dipolar dephasing dramatically enhances the aromatic/aliphatic resolution in the ‘H CRAMPS spectra of coal and provides information on structural mobility and solvent accessibilityz6.

Acknowledgement The author gratefully acknowledges support Department of Energy, under DOE Contract FG2245PC80506.

of the US No. DE-

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Pines, A., Gibby, M. G. and Waugh, J. S. J. Chrm. Phys. 1973, 59, 569 Schaefer, J. and Stejskal, E. 0. J. Am. Chem. Sot. 1976,98, 103 1 Lowe, I. J. Phys. Rev. Letters 1959, 2, 285 Bartuska. V. J.. Maciel. G. E.. Schaefer. J. and Steiskal Fuet 1978,56,354 Maciel, G. E., Bartuska, V. J. and Miknis, F. P. Fuel 1979,58, 391 Sullivan, M. J. and Maciel, G. E. An&. Chem. 1982,54, 1615 Wind, R. A., Duijvestyn, M. J., van der Lugt, C., Smidt, J. and Vriend, J. Fuel 1987,66,876 Botto, R. E., Wilson, R. and Winans, R. E. Energy & Fuels 1987, 1, 173 Gerstein, B. C. and Pruski, M. Fuel, submitted for publication Wind, R. A., Duijvestyn, M. J., van der Lugt,C., Maneschijn, A. and Vriend. J. Proa. NMR Soectrosc. 1985. 17. 33 Wind, R. A., Li, L.,“Lock, H. and Maciel, G.‘E. J. Mogn. Reson., 1988, 79, 577 Wind, R. A. and Yannoni, C. W. J. Mugn. Reson. 1986,68,373 Stejskal, E. O., Schaefer, J. and Waugh. J. S. J. Mugn. Reson. 1971, 28, 105 Sardashti, M. and Maciel, G. E. J. Magn. Reson. 1987,72,467 Wind, R. A., Dee, S. F., Lock, H. and Maciel, G. E. J. Magn. Reson. 1988, 79, 136 Zeigler, R. C., Wind, R. A. and Maciel. G. E. J. Magn. Reson., 1988, 79,299 Szeverenyi, N. M., Bax, A. and Maciel. G. E. J. Magn. Reson. 1985,61,448 Dixon, W. T. J. Chem. Phys. 1982, 77, 1800 Dixon, W. T. J. Magn. Reson. 1985,64, 332 Aue, W. P., Ruben, D. J. and Griffin, R. G. J. Chem. Phys. 1984, 80, 1729 Raleigh, D. P., Olenjniczak, E. T., Vega, S. and Griffin, R. G. J. Magn. Reson. 1987, 72,238 Opella,S. J. and Frey, M. E. J. Am. Chem. Sot. 1979,101,5854 Gerstein, B. C. and Pembleton, R. G. Awl. Chem. 1977,49,75 Bronnimann, C. E. and Maciel, G. E. Organ. Geochem., submitted for publication Davis, M. F., Quinting, G. R., Bronnimann, C. E. and Maciel, G. E. Fuel submitted for publication Jurkiewicz, A., Bronnimann, C. E. and Maciel, G. E. Fuel submitted for publication

USING THE MEASUREMENT IS MORE IMPORTANT THAN ACCURACY (by Michael A. Wilson) All analytical methods have inherent error, considerable amount of time must be expended

and a if it is

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13C n.m.r. in aromaticity

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necessary to reduce this error. Error may be experimental, or it may be because the particular samples under investigation may contain materials that interfere with the analytical procedure in some way. For example, paramagnetics are a particular problem in any analytical procedure involving n.m.r. measurements. They may cause nuclei to relax so quickly by spin-spin relaxation processes (T2s) that they are not observed by the spectrometer detector, or they may cause nuclei to resonate over such a large range of frequencies that detection is beyond the range of the spectrometer. Therefore, a prerequisite to obtaining a quantitative n.m.r. spectrum is often the removal of paramagnetics. all geochemical Since almost samples contain paramagnetics, the organicgeochemist usually ignores the problem, although inorganic’ or organic radicals’ may be removed or reduced in concentration by reduction. Because the carbon spin-lattice relaxation times (Tis) of solids are normally long (> 10 s), it is common to obtain 13C n.m.r. spectra ofcoals by cross-polarization. It is of course, also possible to obtain spectra by conventional techniques (Bloch decay methods), but it is generally presumed that this is too time consuming for most studies on coals, even though paramagnetics assist in reducing coal carbon Trs. The cross-polarization technique is a kinetic measurement in which signal intensity is a compromise between two opposing rates, the rate ofcross-polarization (Ten) and the rate of relaxation (‘H T,,). (Strictly, i”rpC should also be considered). Thus signal intensity is not proportional to the amount of carbon present, except in the limiting case of TCH+Oand ‘H TIP-+rc . Figure 5 shows data for two cases in which TCHis short and ‘H TIPis long, and TCHand ‘H TIP are similar. Clearly the amounts of carbon observed in both cases differ considerably, particularly as the contact time for cross-polarization between carbons and protons is varied. The problem with a heterogenous material such as coal is that a variety of TCHsand ‘H Tips may exist for different materials in the sample, and in addition, differences may be exaggerated by the effect of paramagnetic ions. Two components in a coal such as those shown in Figure 5 will not be detected quantitatively without correction for relaxation. There is a further problem. Magic-angle spinning (MAS) with high power decoupling is a technique that, by necessity, is needed to obtain high resolution solid state spectra of coals. However, this can also introduce a

I

10

20

Contact Figure 5 relaxation

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Relative signal intensities from two carbons and cross-polarization mechanisms

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further source of error. Motion at frequencies very close to the decoupling radio-frequency or the frequency of rotation of the MAS rotor will tend to undo the coherent averaging imposed by this technique, so that lines are still so inherently broad that they are virtually unobserved over the range of frequencies observed in the 13C spectrum 3,4. In summary, the analyst is left with the dilemma of estimating the error introduced in measuring aromaticity by CP/MAS methods and also with the problem of short T,s inherent in both Bloch decay and CP/MAS measurements. It is possible to approach this problem experimentally or theoretically. Because the limits on coal structure are not well defined, we have taken the former approach in our laboratory, and this is also the case for most other groups. A wide range of experiments have been performed to check quantitation: the removal of radical species’; comparison of solution and solid state data on coal extracts5-‘; cross-polarization from the electron*-* 3; demineralization14; introduction of additional carbon which can be monitored by other techniques and comparison of n.m.r. with predicted carbon content15; spin counting by adding a known weight of known compound6~15~19; measuring ‘H TIP directly through protons”; and Bloch decay measurements8-11~21-26. It suffices to say here that a wide variation in the amount of carbon observed by CP/MAS is found and this is invariably less than 100 %. The variation in aromaticity (f,) from that initially suspected from spin counting is much smaller. A good illustration is given in the paper by Botto et al.“j. For a hypothetical coal of 10 % exinite, 80 % vitrinite and 10 % inertinite they estimate f, as 0.69. N.m.r. analysis assuming only 25 y0 of the inertinite, 35 y0 of the vitrinite and 100 % of the exinite is observed gives fa = 0.65; only just outside operational experimental error obtained from repeat experiments ( f 0.03). Let us consider an extreme example. It is clear that it is the inertinite macerals that are particularly discriminated against. This is not surprising since they are: (a) most aromatic and contain more fused ring systems (particularly fusinites) and thus have more carbons further from protons with long T,,s; (b) contain the most mineral matter including paramagnetics; and (c) have largest organic radical counts. If we assign a count of 0 % to inertinite and estimate Af, from vitrinite as 0.4 at the extreme, then an f,of 0.80 for a 50:50 mixture will be observed as 0.60. Only 50 y0 of the carbon is observed, but the error in f, is only 25 %. If less carbon was observed the error would be no larger, and if some of the inertinite was observed the error would be considerably smaller. I think we can say that the error for coal analysis is always going to be lower than 25%. We also need to consider exinite groups and any material spread through the other macerals that cannot be observed directly under the microscope and could be described as ‘mobile’. The exinite macerals and probably any mobile material are less aromatic than other coal components. Some of the exinite materials are closer to waxes or rubbers than true solids and may exhibit the sort of molecular motion that undoes the coherent averaging by magic-angle spinning discussed earlier. Examples of this sort of behaviour have been exhibited by tars2’. Moreover, since TCHis influenced by molecular motion2*, some other mobile materials will have long TCH~29-31and will thus not be observed with full signal intensity. This

13C n.m.r. in aromaticity Table 4 Samples from which CP/MAS must be treated with caution

measurements

of aromaticity

12 13

Number

Sample description

Problem

1

Samples containing unusually large quantities of Fe3+ Carbonized samples including chars and coal liquefaction residues Mobile tars and pitches

Short

2

3 4

Samples containing hydrophilic groups

5

Distilled samples containing small molecules such as naphthalene

T,s

14 15 16 17

Long TCH. Motion close to decoupling frequency Water absorption makes part of samples mobile (M. A. Wilson, unpublished work) or affects tuning”. Paramagnetics selectively bond to hydrophilic groups reducing ‘H T,, of these groupsj3 Long T,s of protons in these samples make signal averaging experiments nonquantitative34

18 19 20 21 22 23 24 25 26 27 28 29 30

source of error will tend to increase the observed value of f, of coals containing these materials, and hence reduce actual error. We believe that f, measurements on coal by CP/MAS methods are accurate enough for most purposes, although they are normally slight underestimations. Rarely, however, is all the carbon seen. Unsuitable samples for CP/MAS will of course also exist, and we can list samples for which analyses should be regarded with caution (Table 4). Although there is virtue in accuracy of measurement, how that measurement is usued is invariably more important than accuracy itself. For most purposes, the coal chemist can be happy with CP/MAS n.m.r. as a valuable analytical tool, although the the physicist may find the spectroscopist or measurements too lax for their taste. Our advice to anyone needing a number of measurements from a spectroscopist, and who is not prepared to wait for a detailed relaxation study, is at least obtain spectra at two contact times (say l-2 ms and 5-6ms) and make comparisons. References 1 2 3 4 5

6 7 8 9 10

11

Mitchell, B. D. and McKenzie, R. C. Soil Sci. 1954, 77, 173 Muntean, J. V., Stock, L. M. and Botto, R. E. Energy & Fuels 1988,2, 108 VanderHart, D. L., Earl, W. L. and Garroway, A. N. J. Magn. Reson. 1981,44, 361 Suwelack, D., Rothwell, W. and Waugh, J. S. J. Chem. PhYs. 1980, 73, 2559 Retkofsky, H. L. in ‘Coal Science’, Vol. 1 (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, USA, 1982, pp. 43-82 Vassallo, A. M., Wilson, M. A., Collin, P. J. et ul. Anal. Chem. 1987,59, 551 Wilson, M. A., Collin, P. J., Pugmire, R. J. and Grant, D. M. Fuel 1982,61, 959 Wind, R. A., Trommel, J. and Smidt, J. Fuel 1982,61, 398 Wind, R. A., Trommel, J. and Smidt, J. Fuel 1979, 58, 900 Wind, R. A., Duijvestijn, M. J., Smidt, J. and Trommel, J. Proc. Int. Conf. Coal Sci., Dusseldorf, FRG, Verlag Gluckauf, Essen, FRG, 1981, p. 812 Wind, R. A., Duijvestijn, M. J. and Lugt, C. V. D. in ‘Magnetic Resonance: Introduction, Advanced Topics and Applications to Fossil Energy’ (Eds. L. Petrakis and J. P. Fraissard), Reidel Dordrecht, 1984, pp. 461485

31 32 33 34

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on coals: A debate: C. Snape et al.

Jones, R. B., Robertson, S. D., Clague, A. D. H. et al. Fuel 1986, 65, 520 Wind, R. A., Duijvestijn, M. J., van der Lugt, C. et a/. Fuel 1987, 66,876 Heng, S., Collin, P. J. and Wilson. M. A. Fuel 1987, 66, 1008 Hagaman, E. W., Chambers, R. R. Jr. and Woody, M. C. Anal. Chem. 1986,58,387 Botto, R. E., Wilson, R. and Winans, R. E. Energy & Fuels 1987, 1, 173 Kalman, J. R. in ‘Magnetic Resonance: Introduction, Advanced Topics and Applications to Fossil Energy’ (Eds. L. Petrakis and J. F. Fraissard), Riedel Dordrecht, 1984, pp. 557-567 Wemmer, D. E., Pines, A. and Whitehurst, D. D. Phi/. Trans. Rev. Sot. London Ser. A 1981,300. 15 van der Hart, D. L. and Retkofsky’, H. L. Fuel 1976,55,202 Packer, K. J., Harris, R. K., Kenwright, A. M. and Snape, C. E. Fuel 1983,62, 999 Dudley, R. E. and Fyfe, C. A. Fuel 1982,61, 1615 Erbatur, G., Erbatur, 0.. Coban, A. er al. Fuel 1986, 65, 1273 Yoshida, T., Nakata, Y., Yoshida, R. et u/. Fuel 1982,61, 824 Ohtsuka, Y., Nozawa, T., Tomita, A. et al. Fuel 1984,63, 1363 Sullivan, M. J. and Maciel, G. E. Anal. Chem. 1982, 54, 1606 Wilson, M. A. and Vassallo, A. M. Org. Geochem. 1985,8,299 Oflivier, P. and Gerstein, B. C. Carbon 1984, 22, 409 Cheung, T. T. P. and Yaris, R. J. Chem. Phq’s. 1980, 72, 3604 Soderquist, A., Burton, D. J., Pugmire, R. J. etal. Energy& Fuels 1987, 1, 50 Vassallo, A. M., Wilson, M. A. and Edwards, J. H. Fuel 1987,66, 621 Wilson, M. A., Batts, B. D. and Hatcher, P. G. Energy & Fuels 1988,2,668 Farnum, S. A., Messick, D. D. and Farnum, B. W. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1986, 31, 60 Pfeffer, P. E., Gerasimowicz, W. V., Piotrowski, E. G. Anal. Chem. 1984, 56, 834 Wilson, M. A., Vassallo, A. M., Collin, P. J. and Rottendorf, H. Anal. Chem. 1984,56,433

CONCLUDING REMARKS (by Colin E. Snape) All the contributors have highlighted the factors that can affect the quantitative reliability of aromatic and related measurements on coals by CP/MAS 13C n.m.r. The recent work of Botto confirms my earlier suggestion that, in the worst instances, aromaticity values can be underestimated by up to 15 mole “/ This is in general agreement with the scenario outlined by Wilson, in which inertinite carbon is not observed. However, in many instances, the discrimination against aromatic carbon is much less pronounced. The additional problems posed by aromatic sideband suppression in high field measurements have been discussed in detail. While pulse sequences, such as TOSS, are likely to become redundant due to the development of high speed spinners, higher spinning frequencies affect CP dynamics adversely giving rise to less carbon being observed (Axelson, Pruski and Gerstein). Maciel also has strongly advocated the use of low magnetic field strengths. Thus, if research workers are in the fortunate position of having a range of n.m.r. facilities at their disposal, the most accurate results can probably be obtained at low field with relatively slow spinning speeds ( z 4 kHz, just sufficient to eliminate sidebands) in conjunction with the use of radical quenching (SmI, as used by Botto) and single pulse (FID) excitation rather than CP. Admittedly, the latter technique is time consuming, but this is a small price to pay to achieve accurate measurements. Removal of ferromagnetic impurities by demineralization is also likely to improve quantification. Clearly, it is useful to carry out additional experiments

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