Gary E. Maciel

Observation of spin exchange by two-dimensional fourier transform 13C cross polarization-magic-angle spinning☆

The solid-state cross polarization and magic-angle spinning analog of an earlier 2-D FT experiment on liquids is introduced. This technique permits the observation of spin exchange processes. 13C applications include examples of chemical exchange, spin diffusion, the effect of 14N relaxation, and intrinsic T2 effects.

Setting the magic angle using a quadrupolar nuclide

Quadrupolar nuclei yield Fourier transform magic-angle spinning NMR spectra exhibiting many spinning sidebands. The widths and the intensities of the sidebands are very sensitive functions of the angle the spinning axis makes with the magnetic field. This dependence can be used in adjusting the spinning angle at least to within ± 0.1° of the magic angle. In particular, the 79Br signal from powdered KBr is especially convenient for monitoring the angle in 13C MAS experiments. Large (approximately 3 mm) crystals of KBr offer greater sensitivity to angle, but are less convenient to use than powder. NaBr, KBrO3, and K55MnO4 were also tested and found to be less suitable as angle markers than is KBr.

Quantitative reliability of aromaticity and related measurements on coals by 13C n.m.r. A debate

While solid state 13C n.m.r. has made a major contribution to the characterization of coal and other insoluble carbonaceous materials over the past decade, there has been considerable uncertainty concerning the quantitative reliability of the technique. This debate addresses this important topic and comprises six contributions from authors who are recognized experts in n.m.r. characterization of solid fuels. The principal issue discussed is the accuracy of aromaticity measurements on coals by cross-polarization — magic-angle spinning (CP/MAS) 13C n.m.r., together with additional problems posed by high field measurements and spectral editing, and with some discussion of emerging techniques. There is a consensus that significant errors can arise in CP/MAS 13C n.m.r. measurements of aromaticity due to the unfortunate spin-dynamics of coals, which typically result in only ≈50% of the carbon being observed for bituminous coals. There is clear discrimination against aromatic carbon, but differences of opinion exist over the magnitude of the errors (from 2 to 15 mole carbon %) and whether high field (⩾ 50 MHz) measurements are as accurate as those of low field (< 25 MHz) because either sideband suppression or extremely high speed MAS has to be employed to eliminate sidebands. From the evidence presented, it is suggested that a combination of low field, single pulse excitation with long relaxation delays and the use of a suitable reagent to quench paramagnetic centres is the most satisfactory, albeit time consuming, recipe for obtaining reasonably reliable results on unknown samples. An inter-laboratory exercise is being organized by Argonne National Laboratory to check the precision and to further investigate quantitative reliability of 13C n.m.r. measurements on coals from their Premium Coal Sample Bank.

Nuclear magnetic resonance studies of ancient buried wood—II. Observations on the origin of coal from lignite to bituminous coal

Coalified logs ranging in age from Late Pennsylvania to Miocene and in rank from lignite B to bituminous coal were analyzed by 13C nuclear magnetic resonance (NMR) utilizing the cross-polarization, magic-angle spinning technique, as well as by infrared spectroscopy. The results of this study indicate that at least three major stages of coalification can be observed as wood gradually undergoes transformation to bituminous coal. The first stage involves hydrolysis and loss of cellulose from wood with retention and differential concentration of the resistant lignin. The second stage involves conversion of the lignin residues directly to coalified wood of lignitic rank, during which the oxygen content of intermediate diagenetic products remains constant as the hydrogen content and the carbon content increases. These changes are thought to involve loss of methoxyl groups, water, and C3 side chains from the lignin. In the third major stage of coalification, the coalified wood increases in rank to subbituminous and bituminous coal; during this stage the oxygen content decreases, hydrogen remains constant, and the carbon content increases. These changes are thought to result from loss of soluble humic acids that are rich in oxygen and that are mobilized during compaction and dewatering. Relatively resistant resinous substances are differentially concentrated in the coal during this stage. The hypothesis that humic acids are formed as mobile by-products of the coalification of lignin and function only as vehicles for removal of oxygen represents a dramatic departure from commonly accepted views that they are relatively low-molecular-weight intermediates formed during the degradation of lignin that then condense to form high-molecular-weight coal structures.

Characterization of organic material in coal by proton-decoupled 13C nuclear magnetic resonance with magic-angle spinning

13C n.m.r. spectra have been obtained on ten solid coal samples of various types and on three coal-derived materials, using high-power proton decoupling, cross polarization and magic-angle spinning, and provide valuable information on the carbon distribution between aromatic and non-aromatic structures in the sample. Apparent carbon aromaticities, fa′, have been determined and have been correlated with H/C ratios and as one factor in fuel values. Both solvent refining and reverse combustion (see introduction) are found to increase the aromatic fraction. These techniques should be very useful in characterizing and optimizing coal-conversion processes.

Correlation of isotropic shifts and chemical shift anisotropies by two-dimensional fourier-transform magic-angle hopping nmr spectroscopy

During the 1960s Andrew and others examined the rapid spinning of a sample about an axis that makes an angle of 54” 44’ with the direction of the static magnetic field (I-LJ in order to remove broadening effects in the NMR spectra of solids (l-3). It was much later when Schaefer and Stejskal (4) applied this approach, magic-angle spinning (MAS), to remove broadening due to chemical shift anisotropy (CSA) in 13C NMR, combining this approach with high-power ‘H decoupling and cross polarization (CP). The resulting levels of resolution and sensitivity obtained with this combination have made the 13C CP-MAS experiment the most widely applied solid state NMR experiment in recent years. As powerful, versatile, and popular as the 13C CP-MAS experiment has become, there remain some characteristics that limit its usefulness in certain types of applications. Technological problems persist in techniques for spinning the sample rapidly, problems that are intensified by the scaling of CSA with increasing magnitude of the static field (H,,), although recent advances show great promise for alleviating these problems (5, 6). Another limitation of the usual CP-MAS 13C experiment is that it eliminates the potentially useful information embodied in the CSA pattern, i.e., independent values of the three principal elements of the shielding tensor, ul, , uz2, and g33. Only the trace, actually (a, I + ~2~ + a&/3, of the shielding tensor survives under MAS. Techniques have been proposed for retrieving CSA information from a MAS experiment (7-1 I); although each of these techniques has merits, each suffers from disadvantages. Introduced here is a two-dimensional (2-D) Fourier transform (FT) technique which presents the isotropic average chemical shift, q = (a, 1 + 622 + ~~~)/3, in one frequency dimension (F,) and the static CSA powder pattern along the other frequency axis (F2). The experiment is carried out using discrete “hops” between evolution segments, rather than continuous sample spinning, and no spinning sidebands are produced. As the detection occurs on a static sample, the signal decays more rapidly than in a normal MAS experiment, and sensitivity suffers correspondingly. Nevertheless, the experiment shows considerable promise, not only for the CSA results it

Prospects for carbon-13 nuclear magnetic resonance analysis of solid fossil-fuel materials

Abstract Because of excessive line broadening in solids due to magnetic dipole—dipole interactions and chemical shift anisotropies, and because of long spin-lattice relaxation times, standard continuous wave (CW) or pulse Fourier transform techniques do not normally yield structurally informative 13 C n.m.r. spectra from solid samples. However, the techniques of dipolar decoupling, cross polarization and magic-angle spinning show great promise for routine 13 C n.m.r. studies of solids. Applications of these techniques to analysis of anthracite, lignite and algal coal samples, and to oil shale and kerogen samples, are discussed. It is shown that cross-polarization spectra obtained with dipolar decoupling display chemical shift anisotropy which interferes with attempts to distinguish the resonances of aromatic and aliphatic carbons. However, with magic-angle spinning the distinction can be made. Prospects and potential difficulties with applications of these techniques are discussed.

Chemical Shift Anisotropy in Powdered Solids Studied by 2D FT NMR with Flipping of the Spinning Axis

We propose a new two-dimensional approach for obtaining the anisotropy information. In this new experiment, the spinning axis of the sample is flipped from 90 to 54.7” between the evolution and detection periods. The experiment appears to be widely applicable and has great promise for the study of complex samples. The experimental scheme is set out in Fig. 1. Cross polarization of, in our case, 13C nuclei is performed while the sample is spun about an axis that makes an angle of 90” with the static magnetic field. It can be shown (IO) that the powder anisotropy pattern that obtains under these conditions is reversed and collapsed to half the width of the static nonspinning case, but keeps the same shape. At the end of the evolution period (t,), the x component of the transverse 13C magnetization is stored along the z axis, parallel to the static magnetic field, by means of a 90,” 13C pulse. The orientation of the spinning axis of the sample is then changed to the magic angle. The sample is spun fast compared with the width of the anisotropy patterns, so that spinning sidebands have negligible intensities. A final 90 o 13C pulse rotates the z-stored 13C magnetization back into the transverse plane, where it precesses in the time domain, tZ, with the corresponding isotropic chemical shift frequencies. Cycling of the phase of the first 90 o 13C pulse alternately along +y and -y, together with adding and subtracting of the acquired data, is used to eliminate spurious signals. The detected isotropic spectrum, S(tl , F2) obtained by Fourier transformation with respect to t2, is modulated in amplitude with the frequencies existing during the evolution period, t,. Hence, the powder anisotropy information and the isotropic chemical shifts will appear in the F, dimension. Because of the amplitude modulation, a pure 2D absorption spectrum can be obtained by calculating the cosine Fourier transform, P(F,, FJ (II, 12).

Aliphatic structure of humic acids; a clue to their origin

Abstract Nuclear magnetic resonance spectra (both 1 H and 13 C) of humic acids from diverse depositional environments indicate the presence of aromatic chemical structures, most likely derived from lignin of vascular plants, and complex, paraffinic structures, most likely derived from algal or microbial sources. The latter components account for a major fraction of humic acid structures in both terrestrial and aquatic environments, suggesting that algae or microbes play a large role in humification of organic remains from both systems.

Cross-polarization magic-angle spinning 13C NMR spectra of coals of varying rank

Cross-polarization, magic-angle spinning 13C nuclear magnetic resonance spectra have been obtained on a series of coals that span the ASTM rank classification. Aromaticities thus obtained follow the classification reasonably well, increasing as rank increases NMR parameters were extensively varied to determine optimum conditions for the cross polarization contact time and pulse repetition rate. Good correlations were obtained between both the fixed carbon and the volatile matter obtained from the proximate analysis, and the aromatic carbon obtained from NMR. Generally the NMR spectra of lignites and low rank coals exhibit varying degrees of fine structure.