William L. Earl

Resolution in 13C NMR of organic solids using high-power proton decoupling and magic-angle sample spinning

Abstract The 13C NMR linewidths observed in organic solids by means of high-power proton decoupling and magic-angle sample spinning are roughly 10 to 100 times broader than resonances in the liquid phase. This paper investigates important 13C line-broadening mechanisms in organic solids and their dependences on experimental parameters, notably static and rf magnetic field strength. The discussion is limited to glassy (disordered, partially mobile) and crystalline (ordered, rigid) organics at natural isotopic abundance. Excluded are elastomers and systems with a third dipolar coupled nuclear species. Experimental data, primarily at 1.4 T, for glassy and semi-crystalline polymers as well as crystalline materials, illustrate and confirm the linebroadening mechanisms identified. For some specimens, 13C linewidths are compared at 1.4 and 4.7 T. It is found that a substantial linebroadening (0.5 to 6 ppm) corresponding to a dispersion of isotropic chemical shifts can arise from distributions of anisotropic sources of magnetic susceptibility, bond angles, or frozen molecular conformations; in other cases, the resonance lines may be split into many distinct lines by magnetic inequivalences present in the solid but not the liquid phase. For crystalline materials, methods for reducing the broadening from anisotropic bulk susceptibility are discussed. Other broadening mechanisms considered are: insufficient proton-decoupling fields, off-resonance decoupling, imperfections in magic-angle sample spinning, and motional modulation of both the carbon-proton dipolar coupling and the carbon chemical shift anisotropy. On consideration of these mechanisms, it is anticipated (and shown experimentally in limited cases) that no significant gain in resolution will be enjoyed at high magnetic fields, especially when variable-temperature operation is available. In some instances, degradation of resolution may occur at high field if large rf field strengths or high spinning rates cannot be achieved.

Measurement of 13C chemical shifts in solids

Abstract A pulse sequence and sample geometry which allows the measurement of 13 C chemical shifts of solid materials relative to liquid tetramethylsilane (TMS) are described. Using this technique, the chemical shifts of a series of common engineering plastics were measured and reported. A small number of candidate secondary shift reference materials were considered and their chemical shifts measured. Most of these materials proved to be unsuitable for general 13 C shift references for differing reasons. The most promising standard investigated was polydimethylsilane. The measurement of chemical shifts in solid materials is slightly complicated by anisotropic magnetic properties and sensitivity to magic-angle missetting when the material exhibits macroscopic orientation. These complications are discussed in detail and examples of misleading spectra are shown.

Nuclear magnetic resonance studies of ancient buried wood-I. Observations on the origin of coal to the brown coal stage

Various wood fragments buried in sediments under anaerobic conditions for from 450 yr to approximately 8 Myr have been examined by solid-state 13C nuclear magnetic resonance. Cellulose and other carbohydrates, the major components of Holocene wood, have been shown to be gradually hydrolyzed or otherwise lost under the conditions of burial. Lignin structures, however, are preserved relatively unchanged and become concentrated by difference as the carbohydrates disappear. Thus, a fragment of coalified wood isolated from a Miocene brown coal was found to be still composed of approximately 75% lignin and 25% cellulose. On the basis of our observations, we suggest that coalification of woody tissue progresses directly from lignin to coal and that such coalification may be retarded until most of the cellulose disappears.

Use of solid-state 13C NMR in structural studies of humic acids and humin from Holocene sediments

Abstract 13C NMR spectra of solid humic substances in Holocene sediments have been obtained using cross polarization with magic-angle sample spinning techniques. The results demonstrate that this technique holds great promise for structural characterizations of complex macromolecular substances such as humin and humic acids. Quantifiable distinctions can be made between structural features of aquatic and terrestrial humic substances. The aliphatic carbons of the humic substances are dominant components suggestive of input from lipid-like materials. An interesting resemblance is also noted between terrestrial humic acid and humin spectra.

Two approaches to high-resolution high-pressure nuclear magnetic resonance

Abstract High-resolution NMR measurements have been made at elevated pressures. Two separate modifications to a commercial NMR spectrometer are described; one involves a high-pressure NMR tube which rotates in the commercial NMR probe, and the other is a complete probe head containing a nonspinning high-pressure bomb. The relative advantages and disadvantages of both modifications are discussed and it is concluded that for measurements of volumes of activation, the nonspinning probe head is superior. As an example of the utility of this probe head, two determinations of volumes of activation using it are presented.

A cross-polarization—magic-angle sample spinning n.m.r. study of several crystal forms of lactose

Abstract Five different crystalline forms of lactose were investigated by cross-polarization—magic-angle sample spinning (CP—MAS) 13 C-n.m.r. spectroscopy. Both the anhydrous β-lactose and the α-lactose monohydrate structures are known, and the CP—MAS n.m.r. data are in agreement with those structures. The structure of the stable, anhydrous α-lactose has not been reported. The CP—MAS n.m.r. results indicate that the crystal must have two or more lactose molecules per unit cell. The chemical shifts measured for two mixed crystals having α:β ratios of 5:3 and 4:1 are a direct observation of the fact that both materials are real mixed crystals rather than glasses or physical mixtures of crystals of pure α- and β-lactose. The chemical shifts also indicate that the lactose molecules in both mixed crystals are in environments similar to the crystalline environment of the stable, anhydrous α-lactose.

A high-pressure probe for high-resolution nuclear magnetic resonance

Abstract A probe for obtaining high-resolution proton NMR spectra with a Bruker WP-60 spectrometer at elevated pressures is described. The probe has been tested to 3.7 kbar, used between −10 and 80°C, and routinely gives a resolution of about 1 Hz. This probe is easily interchangeable with the commercial probe. We also describe a simple high-pressure sample tube made of inert materials.

Solid-state structures of keto-disaccharides as probed by 13C cross-polarization, “magic-angle” spinning n.m.r. spectroscopy☆

Abstract The 13C cross-polarization, magic-angle spinning (CP-MAS) spectra of maltulose · H2O and anhydrous lactulose were examined at different applied magnetic and proton-decoupling fields B0 and B1, respectively. Whereas the spectrum of lactulose was insensitive to these changes, that of maltulose showed significant responses. From selective-relaxation experiments, a mixture of three “forms” (based on the ratio of the C-2 resonances) of both lactulose and maltulose were shown to exist in the solid state. The 360-MHz 1H spectra of the unmutarotated disaccharides in dimethyl sulfoxide solution were used to establish the tautomeric composition of these crystalline solids. The tautomeric composition of lactulose, as determined by the 1H spectrum in dimethyl sulfoxide, correlated well with the CP-MAS data, but the spectrum of unmutarotated maltulose showed only a single, β-pyranose tautomer to be present. Based on the ratio of the lactulose tautomers determined from the 1H spectrum in dimethyl sulfoxide (referenced to fructose), the C-2 resonances corresponding to each anomeric form of lactulose were assigned in the CP-MAS spectrum. A “crossover” in chemical-shift positions of the anomeric resonances was observed in going from solution to the solid state. Furthermore, a pronounced increase in the proportion of the furanoid anomers was noted for crystalline lactulose relative to its solution.

Mechanism of cellulose smoldering retardance by sulfur

The addition of elemental sulfur is effective in preventing cotton smoldering, a process which leads to a major fraction of fire loss. The objective of this work is to determine the mechanism of the observed retardance. Double thickness samples of cotton terrycloth approximately 18 mm by 100 mm, were smoldered in a pyrex cylinder. The samples contained 0, 5, 8 and 10 percent sulfur by weight. The air velocity over the face of the sample was about 10 cm/s. Ten percent treated samples self-extinguished or smoldered with difficulty. The smoldering velocities and peak temperatures of the other samples were determined using embedded thermocouples; both decreased with increasing sulfur concentration. Calculations and experiments indicate that the sulfur does not simply act as a heat sink as has been suggested for many flame retardants. Electron paramagnetic resonance data showed that a reduction in the number of free radicals in the char by a factor of two resulted from the addition of five percent sulfur. X-ray fluoresence measurements showed that the concentration of bound sulfur in the char was comparable to that of eliminated free radicals. Mass spectrometric studies indicated that the major vapor phase products of sulfur oxidation Mass spectrometric studies indicated that the major vapor phase products of sulfur oxidation were SO 2 and COS, but these were present in much lower concentration than O 2 and CO 2 . This result eliminates the possibility of oxygen depletion by reaction with sulfur as an important contribution to the retardancy. Rather, the sulfur vaporizes and reacts with the radical sites in the char. In the absence of sulfur, these sites would oxidize exothermically and regenerate to continue the smoldering. As a result of some radical sites being poisoned, less heat is released, and the resulting pyrolysis of further fuel is retarded. Thus the critical properties of sulfur are vaporization at the correct temperature, reactivity with the char, and stability of the resulting adduct.