B. C. Gerstein

The concept of a mobile or molecular phase within the macromolecular network of coals: A debate

Abstract The question addressed here is whether coals contain a substantial fraction of relatively small molecules clathrated within the macromolecular network and requiring long time or extra thermal energy for release. The debate primarily hinges on the interpretation of 1 H n.m.r. data, but some other relevant findings reported in the literature are recapitulated. Cases are made out by the original authors for two different interpretations of the n.m.r. data (it should be emphasized that there is no dispute about the actual data, only about its interpretation). It appears that there are two (or possibly three) populations of protons in coals, which exhibit appreciably different free induction decay times and hence have quite different levels of rotational mobility. This is a fact of considerable interest in its own right, but the possession of high mobility does not necessarily mean that the mobile protons must be in relatively small molecules which are free to tumble in cages within the macromolecular network. Taking into account evidence from extractability and other experiments, it is possible to draw the conclusion that the content of relatively small molecules held within coal is larger than has been supposed and is essentially equal to the content of substances containing highly mobile protons, though a substantial proportion of these substances can probably be extracted only under various special conditions. However, it should be stated that there is disagreement between the authors on the admissibility of evidence from solvent extraction experiments. There is certainly evidence that the very high extractability of lignites with ethylenediamine is due to chemical reaction, and it is argued that this may be true also of higher-rank coals.

An experimental study of resolution of proton chemical shifts in solids: Combined multiple pulse NMR and magic‐angle spinning

High‐resolution nuclear magnetic resonance spectra of protons in rigid, randomly oriented solids have been measured using combined homonuclear dipolar decoupling (via multiple pulse techniques) and attenuation of chemical shift anisotropies (via magic‐angle sample spinning). Under those conditions, isotropic proton chemical shifts were recorded for a variety of chemical species, with individual linewidths varying from about 55 to 110 Hz (1–2 ppm). Residual line broadening was due predominately to (i) magnetic‐field instability and inhomogeneity, (ii) unresolved proton–proton spin couplings, (iii) chemical shift dispersion, (iv) residual dipolar broadening, and (v) lifetime broadening under the multiple pulse sequences used. The magnitudes of those effects and the current limits of resolution for this experiment in our spectrometer have been investigated. The compounds studied included organic solids (4, 4′‐dimethylbenzophenone, 2, 6‐dimethylbenzoic acid, and aspirin), polymers (polystyrene and polymethylmethacrylate), and the vitrain portion of a bituminous coal.High‐resolution nuclear magnetic resonance spectra of protons in rigid, randomly oriented solids have been measured using combined homonuclear dipolar decoupling (via multiple pulse techniques) and attenuation of chemical shift anisotropies (via magic‐angle sample spinning). Under those conditions, isotropic proton chemical shifts were recorded for a variety of chemical species, with individual linewidths varying from about 55 to 110 Hz (1–2 ppm). Residual line broadening was due predominately to (i) magnetic‐field instability and inhomogeneity, (ii) unresolved proton–proton spin couplings, (iii) chemical shift dispersion, (iv) residual dipolar broadening, and (v) lifetime broadening under the multiple pulse sequences used. The magnitudes of those effects and the current limits of resolution for this experiment in our spectrometer have been investigated. The compounds studied included organic solids (4, 4′‐dimethylbenzophenone, 2, 6‐dimethylbenzoic acid, and aspirin), polymers (polystyrene and polymethylm...

Determination of the apparent ratio of quaternary to tertiary aromatic carbon atoms in an anthracite coal by 13C-1H dipolar dephasing n.m.r.

Abstract 13 C- 1 H heteronuclear dipolar dephasing n.m.r. techniques allow discrimination between different chemical species contributing to the 13 C n.m.r. spectra of complex hydrocarbons. Model compound studies show significantly different effective transverse relaxation constants for carboxyl and quaternary carbon atoms (≈200 μs), secondary and tertiary (≈20 μs), and primary carbon atoms (≈80 μs). Use of these effective relaxation data, together with appropriately timed windows in the continuous wave decoupling applied in standard cross-polarization-magic-angle spinning experiments on anthracite coal allow discrimination between aromatic tertiary and aromatic quaternary ring carbon atoms in this coal. Within the accuracy of experimental error, and of the structural modelling experiments herein reported, the use of the dipolar dephasing technique together with results of X-ray diffraction on coals allows a reasonable estimate to be made of the average number of condensed polynuclear rings in an ‘average molecule’ in the anthracite studied. Based on a model of pericondensed aromatic rings, this number lies between 33 and 45.

Residual linewidths of NMR spectra of spin-12 systems under magic-angle spinning

Abstract The residual linewidths of NMR spectra of spin - 1 2 systems in solids under magic-angle spinning conditions are calculated in the rigid lattice limit for the general case where both homo- and heteronuclear dipolar interactions as well as shielding anisotropy may be present. The results are used to quantitatively interpret 1H MAS NMR spectra of crystalline water in gypsum and of hydroxyl groups in zeolites.

Proton NMR in a hydrogen molybdenum bronze

Solid state nuclear magnetic resonance (NMR) has been used to probe the nature of hydrogen in the hydrogen bronze H1.64MoO3. The line shape of the proton NMR at room temperature is found to be associated only with shielding anisotropy. Below 230 K, dipolar interactions become important in broadening the line. The response of the protons under a magic angle spinning experiment indicates that there is predominantly only one type of hydrogen in this sample. The shielding anisotropy indicates that at room temperature this species is not a hydroxyl proton. The longitudinal relaxation time of protons in this material exhibits a minimum between 190–320 K indicating that dipolar interactions modulated by motion are responsible for T1 relaxation. Comparison of longitudinal relaxation times of partially deuterated and nondeuterated samples indicates that there exists a contribution to T1 relaxation not dependent upon interproton distance. There does not appear to be a conduction electron contribution to the proton T1.

Shielding anisotropies of 13C in poly(ethylene terephthalate)

The shielding anisotropies of 13C in poly(ethylene terephthalate) are reported at room temperature. Off-magic-angle spinning (OMAS) is used in conjunction with the static spectrum, and the isotropic spectrum to infer anisotropies. A statistical analysis of the use of these data to infer anisotropies is presented.

Nuclear magnetic resonance studies of 1H in zirconium halide hydrides: Shielding tensors

The magnetic shielding tensors and T*2’s of 1H have been determined at room temperature in ZrClH0.5, ZrBrH1.0, and ZrBrH0.5 utilizing multiple pulse NMR. In addition, stoichiometries have been determined by spin counting. Each proton shielding tensor exhibits a large anisotropy compared to those found in hydrocarbons and salts, and is similar to that found in ZrClH1.0. Site occupations of the protons in these ’’sandwichlike’’ compounds has been inferred from isotropic shifts and from anisotropies.

NMR of petroleum cokes II: Studies by high resolution solid state NMR of 1H and 13C

Chemical functionalities of hydrogen and carbon in a series of cokes obtained from heavy crude oils in a Mobile continuous flow laboratory coker pilot unit are probed with high-resolution solid state NMR. The fractions of aromatic hydrogen and carbon, as determined from 1H combined rotation and multiple-pulse spectroscopy (CRAMPS), and 13C magic angle spinning (MAS) experiment with and without cross polarization (CP), varied only slightly between 0.49 and 0.65 and between 0.88 and 0.92, respectively, for the samples studied. A comparison with the results of direct excitation (13C MAS) NMR showed that CP/MAS NMR spectra taken with a contact time of 1 ms well represented relative carbon intensities. The high-resolution spectra, in combination with previously reported wideline 1H NMR data and the results of elemental analysis, are used to derive several structural parameters, including aromatic and aliphatic hydrogen to carbon ratios and the average formula per 100 carbon atoms. Finally, the “average” structures for studied cokes are proposed and discussed. Most cokes are concluded to consist of molecules having approximately 10 aromatic rings bearing only few substitutions.

Small variations in chemical shifts for atoms in similar environments

Abstract It is shown that, at least for the particular case of selected proton chemical shifts in benzenoid conjugated hydrocarbons, minor variations in chemical shifts for atoms in similar environments have definite structural origin. The analysis is based on the construction of a partial order (in the algebraic sense) for structures induced by an ordered list of a particular atomic property. Specifically, selected CH environments for catacondensed benzenoid hydrocarbons are discussed. The minor variations, i.e., the gradual decrease of chemical shifts, show a parallelism with the similar decrease in the degree of local aromaticity for the rings at which the corresponding protons are attached. The differences in chemical shifts to which significance is now attached fall in the range of 0.1 to less than 0.01 ppm.

NMR of petroleum cokes I: Relaxation studies and quantitative analysis of hydrogen by magnetic resonance

Abstract A set of 15 petroleum cokes produced by delayed coking in a Mobil laboratory coker has been studied using solid state 1H NMR. From solid echo and free induction measurements, the concentrations of hydrogen as well as inter- and intramolecular second moments due to dipolar proton-proton interactions were measured. Electron spin resonance (ESR) was used to determine the concentrations and nature of paramagnetic sites. The longitudinal relaxation times in the laboratory and rotating frames were measured and the relaxation mechanisms discussed. Relaxation parameters were found to be derived from two-exponential fits of experimental data. These parameters were not related to structural parameters of cokes obtained in subsequent studies by high resolution NMR of 1H and 13C.