Nikolaus M. Szeverenyi

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.

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.

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

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).

13C spin diffusion of adamantane

Two‐dimensional exchange spectroscopy of natural abundance 13C–13C spin diffusion in solid adamantane illustrates the influence that 13C–1H dipole–dipole coupling exerts on 13C spin diffusion by determining spectral overlap in the 13C system. 2D 13C spectra were obtained for several values of mixing time τm and compared with spectra calculated in the limit of nearest‐neighbor coupling. Good agreement is obtained for short τm, during which the equilibration of neighboring spins dominates. For longer τm, slower spin diffusion that is not acounted for by the simple model is seen; after nearest‐neighbor spins equilibrate, communication over larger distances produces further mixing. It is possible to modify spin diffusion rates by altering experimental conditions, e.g., magic‐angle spinning, low‐power 1H decoupling, or spin locking 13C in the rotating frame during τm.

Chemical shift anisotropy in powdered solids studied by 2D FT CP/MAS NMR

Abstract A set of new experiments for obtaining the conventional anisotropy powder patterns utilizes a series of π or 2π pulses synchronized with the rotation of the magic-angle spinner. The experiment is most conveniently performed in a two-dimensional fashion. The new pulse sequences are rather insensitive to imperfections of the pulses. Experimental results are shown for hexamethylbenzene and paradimethoxybenzene.

Characterization of the residual carbon in retorted oil shale by solid-state 13C n.m.r.

Abstract Cross-polarization and magic-angle spinning suggest that the aromatic carbons in oil shales are largely inert to thermal processes and instead are responsible for the carbonaceous residue obtained during retorting. These results are based on 13 C n.m.r. measurements of the organic carbon distribution of oil shales, before and after Fischer assaying, and for oil shales of different grades, geographic location, geologic ages and formations. The n.m.r. measurements suggest further that measurements of the organic carbon distribution of oil shales heated to various temperatures have practical relevance, and that this information can be of value in discriminating between unconverted kerogen and residual carbon in heated oil shales.

Chemical-shift-anisotropy powder patterns by the two-dimensional angle-flipping approach. Effects of crystallite packing

Abstract A recently reported 2D FT experiment in which the orientation of the sample spinning axis is different during the evolution and detection periods is very promising for obtaining CSA powder patterns. However, care must be taken in this and analogous experiments to ensure that the orientation of crystallites in a powder is truly random, if one wishes to obtain the expected type of CSA powder patterns. Examples of the effects of partial crystallite ordering are given, and CSA results are presented for 1-menthol, cholic acid, and thymol.

Magic-angle hopping as an alternative to magic-angle spinning for solid state NMR☆

Abstract A new variation is presented on the recently reported two-dimensional magic-angle hopping technique. In this new experiment, the frequency domain that provides chemical-shift anisotropy information has been “sacrificed” in favor of improved sensitivity in the isotropic chemical-shift spectrum. This is accomplished by repeatedly refocusing the initial magnetization in the t2 (data acquisition) domain and measuring the resulting string of echoes. Although the sensitivity is still inferior to that of a standard magic-angle spinning (MAS) experiment, it far surpasses that achieved in the original 2-D approach, and may render the hopping approach an attractive alternative to MAS in some kinds of applications.

Pulsed polarization transfer for 13C NMR in solids

Abstract A new pulsed polarization transfer experiment method is described for the polarization of 13 C spins in a solid by magnetization transfer from protons. The method is directly analogous to the INEPT sequence for liquids introduced by Freeman and Morris. As polarization is transferred in PPT between individual 1 H 13 C pairs, rather than between spin reservoirs, different opportunities exist for structurally selective experiments. Results on p -diethoxybenzene and coronene are presented.