13C Nuclear magnetic resonance characterization of coal macerals by magic-angle spinning

¹³C cross-polarization and magic-angle spinning (CP/MAS) nuclear magnetic resonance (n.m.r.) experiments are reported on two fusinites, three vitrinites, one micrinite, one sample of attrite, one exinite, one resinite and one lignite. With one exception, the maceral spectra show no more fine structure than the CP/MAS n.m.r. results for whole coals. The maceral spectra are discussed in relation to each other and to the corresponding spectra of whole coals.     

Examination of some melamine-formaldehyde adducts by 13C nuclear magnetic resonance

The ¹³C n.m.r. spectra of mixtures of melamine-formaldehyde adducts exhibit several azine carbon peaks. These peaks have been assigned to six of the nine possible methylolmelamines. The areas of the peaks may be used to estimate the relative proportions of mono-, di- and trimethylolmelamines in the mixtures. Melamine-formaldehyde resins show an additional characteristic resonance.     

Characterization of high-boiling petroleum distillate fractions by proton and 13C nuclear magnetic resonance spectrometry

High-boiling (535–675 °C) distillate fractions of Wilmington (Calif. USA) and Gach Saran (Iran) crude oils were separated into saturate, monoaromatic, diaromatic and polyaromatic-polar fractions by passage through a silica-gel—alumina dual packed chromatography column. These fractions were further separated on the basis of molecular volume by gel-permeation chromatography (g.p.c.). Select g.p.c. fractions were then analyzed by ¹H and ¹³C n.m.r. spectroscopy. The fraction of aromatic carbons (Ar-C) of the total carbon content of a given series of g.p.c. fractions, obtained directly from the ¹³C n.m.r. spectra, showed a significant range of values (e.g. 13.2% to 29.2%) within each series. Furthermore, the values of %Ar-C within a series of g.p.c. fractions showed a maximum in each case. No such maximum was observed in the ¹H n.m.r. spectra for the fraction of aromatic proton (%Ar-H) content compared to the total proton content of any of the g.p.c. fraction series. Signals observed in the ¹³C n.m.r. spectra confirmed the presence of aliphatic chains attached to aromatic structures in the monoaromatic, diaromatic and polyaromatic-polar g.p.c. series of each distillate fraction. Well-defined signals, not attributable to straight-chain aliphatic material, were also observed in the ¹³C n.m.r. spectra. Comparison of the chemical shifts of these ¹³C n.m.r. signals with those spectra of model compounds obtained experimentally, and with appropriate systems in the literature, strongly suggested the presence of saturated terpenoid-like structures as well as the presence of methyl groups in sterically hindered positions on an aromatic ring system. Gated decoupling techniques and the use of a relaxation agent were used to overcome the deleterious effects of slow relaxation times and Nuclear Overhauser Enhancement (NOE) on the analytical quality of the ¹³C n.m.r. spectra.     

Characterization of vitrinite concentrates. 2. Magic-angle 13C nuclear magnetic resonance studies

The fraction aromaticity determined by ¹³C n.m.r. with cross-polarization and magic-angle spinning of 19 vitrinite concentrates obtained from the Lower Kittaning seam shows a range of values from ≈ 0.65 for the samples of lowest rank (83 wt% C (dmmf) to about 0.83 for those of highest rank (91 wt%C (dmmf)). It was determined that the wt% aromatic carbon correlates to the wt% fixed carbon and is in good agreement with the results reported by other authors. The combination of the ¹³C n.m.r. results with FTIR measurements allows a number of coal parameters to be estimated. The atomic ratios of aliphatic hydrogen to carbon were demonstrated to vary from 1.8–2.0 to between 2.4–2.6 and previous assumptions that a single value can be used in calculating structural parameters for coal of any rank are not strictly valid. The calculation of the aromatic $\text{H}\text{C}$ ratio indicates that in mean structural units there is approximately one aromatic hydrogen atom for every six carbons in vitrinites of carbon content 83 wt%C (dmmf) and that this ratio changes progressively with rank to a value of about one aromatic hydrogen for every four carbons for vitrinites of carbon content 91 wt%C (dmmf).     

Characterization of coals and coal oxidation by magic-angle 13C n.m.r. spectroscopy

Magic-angle ¹³C n.m.r. spectra have been obtained for a series of vitrinite concentrates. Proper modification of the cross-polarization pulse sequence allows separation of protonated and nonprotonated carbon resonances. This technique is used to determine the relative fraction of nonprotonated aromatic carbons for each of the vitrinites, a parameter observed to decrease with increasing rank. Another parameter, related to the aromatic hydrogen content, is also calculated from these data and the results correlate with those from Fourier transform i.r. spectroscopy. The methods used for analysis of the vitrinite concentrates were then applied to the low-temperature oxidation of coal. The fractional aromaticity as determined by n.m.r. increases with longer oxidation times, indicating preferential attack on aliphatic structures. Here the FT-i.r. results are in quantitative agreement with those from n.m.r. Finally, the advantages of using various pulse sequences to extend the range of magic-angle n.m.r. and of combining FT-i.r. and n.m.r. results are discussed in the context of their potential for coal science.     

Application of quantitative 13C nuclear magnetic resonance spectroscopy to coal-derived materials

The difficulties associated with obtaining quantitative results from ¹³C nuclear magnetic resonance spectroscopy are discussed. By careful choice of the experimental conditions quantitative results have been obtained for a mixture of model compounds, containing most of the types of carbon linkages present in coal-derived materials, and for an aromatic fraction of a coal extract.     

Correlation between oil yields of oil shales and 13C nuclear magnetic resonance spectra

Using the cross polarization/dipolar decoupling method, ¹³C n.m.r. spectra have been obtained on twenty oil shales and kerogens from around the world. A correlation is found between the integrated signal intensity of the aliphatic region of the spectra and the oil yields of the shales.     

Determination of chain branching in low density polyethylene by 13C nuclear magnetic resonance and infra-red spectroscopy

Pulsed Fourier transform ¹³C n.m.r. studies clearly demonstrate the presence of ethyl, butyl and longer chain branches in commercial low density polyethylene. The most suitable conditions for the measurement of the relative intensities of the resonance in the ¹³C spectrum, were determined using a standard linear long chain hydrocarbon. The ¹³C spectra of polyethylenes obtained using these parameters, in conjunction with a total methyl content by infra-red spectroscopy allow a quantitative measurement of the number of each branch type to be made.     

Structural investigation on column-chromatographed vacuum residues of various petroleum crudes by 13C nuclear magnetic resonance spectroscopy

Proton and carbon-13 nuclear magnetic resonance spectroscopy of various vacuum residues and their fractionated samples were carried out to investigate average molecular structures. The structural parameters derived from carbon distribution agreed with those derived from p.m.r. within experimental error. In the aliphatic region of the c.m.r. spectra, characteristic peaks assignable to $\text{(CH}{}_2\text{)}{}_{\mathrm{n}}\text{? n ≧ 6 (29.7}\text{ppm}\text{)}$ and CH₃CH₂CH₂CH₂ (14.1 ppm) etc. can be observed. The relative intensities of these peaks suggest that the methylene chain contained in vacuum residues is much longer ($\text{(CH}{}_2\text{)}{}_{\mathrm{n}}\text{? n ≧ 12}$) than has been considered previously. Using the information about the aliphatic region of the c.m.r. spectrum and the structural parameters, an average structural model was deduced.     

13C cross-polarization and magic-angle spinning nuclear magnetic resonance of polymorphic forms of three triacylglycerols

The cross-polarization and magic-angle spinning nuclear magnetic resonance (CP/MAS-NMR) technique has been used to analyze the polymorphic forms of three triacylglycerols, 1,3-dipalmitoyl-2-oleoyl glycerol (POP), 1, 3-racpalmitoyl-stearoyl-2-oleoyl glycerol (POS), and 1,3-distearoyl-2-oleoyl glycerol (SOS). Specific attention has been paid to glycerol, carbonyl, olefinic, and methyl end carbon resonances. Many distinct differences were observed in each polymorphic form of SOS. In the α form, the saturated and unsaturated acyl chains exhibit liquid state-like conformations. However, olefinic conformations of the γ and β′ forms were asymmetric with respect to thecis double bond. Spectral difference between β₂ and β₁ was observed only for the methylene carbon, and not in the other regions. Spectra of corresponding polymorphic forms of POP and POS were almost identical to those of SOS. However, some spectral differences were observed in the glycerol and methyl regions of γ and β′. From the chemical shifts of the methylene carbons, the crystal structures of the polymorphic form have been discussed, particularly in terms of the subcell structures.     

Estimation of coal aromaticities by proton-decoupled carbon-13 magnetic resonance spectra of whole coals

A recently developed nuclear magnetic resonance (n.m.r.) cross-polarization technique has been applied to obtain high-resolution carbon-13 spectra of vitrains from two coals. The method, which overcomes the problem of dipolar line broadening by protons, permits estimates of the carbon aromaticities of solid coals. Preliminary results support the classical views that coals are highly aromatic materials and that the aromaticity of coal increases with increasing rank. Limitations of the technique, the accuracy of representation of the distribution of carbons in the sample, and consequently the reliability of the resulting f_a values are discussed.     

Molecular motion in polyisobutylene and poly(propylene oxide) studied by 13C nuclear magnetic relaxation

The ¹³C spin-lattice relaxation times of low molecular weight (up to 2500) samples of polyisobutylene and poly(propylene oxide) have been measured as a function of molecular weight, temperature and concentration in chloroform solution. For both polymers there is little dependence on molecular weight indicating a flexible conformation in the liquid state, but the relaxation time increases with increasing dilution in CHCl₃. The motion of the polymer backbone therefore depends on the microviscosity of the solution, rather than the bulk viscosity. In polyisobutylene the methyl re-orientation rate increases in parallel with the backbone re-orientation rate, showing that the two motions are interlinked. In poly(propylene oxide) the methyl re-orientation rate is independent of the backbone motion.     

Chemical characterization of solid coals through magic-angle 13C nuclear magnetic resonance

Recently developed ¹³C n.m.r. techniques for high resolution in solids are applied to native coals. The quantity and distribution of hydroxyl groups in the samples are determined by combining chemical modification through acetylation with the n.m.r. measurements. Spectral subtraction isolates the reaction product resonances. Spectroscopic separation of protonated and non-protonated carbons through pulse programming techniques is also demonstrated. Finally, the susceptibility of the aliphatic carbons in coals to low-temperature oxidation is shown clearly by subtraction of spectra of the oxidized and unoxidized materials.     

Aromaticity of coal extract by 1H and 13C pulsed n.m.r. methods

High-resolution gated decoupling ¹³C spectra of soluble materials such as coal extracts have been analysed to evaluate the carbon aromaticity (f_a). A correlation was found between f_a and the reciprocal of the square of the shorter component in spin—spin relaxation time (T₂), which was obtained by pulsed ¹H n.m.r. in the solid state at low temperature. Values of carbon aromaticity for several coals as received were estimated by using the above correlation and compared with those by van Krevelen's densimetric method.     

The diffusion of 14C in nuclear graphites

The high temperature exchange of ¹⁴C between ¹⁴CO and ¹²C-graphite has been used to label a large number of samples of various nuclear graphites. The technique has made it possible to obtain data on the self-diffusion of carbon in graphite. Three nuclear graphites are used, Pile Grade A and two isotropic varieties. Diffusion constants at 2473 K lie in the range 0·64 × 10^?19to 2·97 × 10^?19m²/sec. D₀ values vary from 1 ·20 × 10^?12to 1·06 × 10^?6m²/sec and activation energies, E_a, from 337 to 621 kj/mol. The D values found in this work are lower than those found by other workers. Possible reason for this are examined. Activation energies are also compared with values found by other workers and discussed in relation to theoretical calculations on the activation energy for direct interchange, vacancy and interstitial mechanisms for diffusion. It is concluded that direct interchange is the predominant mechanism for the diffusion of ¹⁴C in the highly graphitised samples used in this work.     

Reductive alkylation of illinois No. 6 coal. 1H and 13C n.m.r. spectra of the 13C-enriched alkylation products

The reductive alkylation of Illinois No. 6 coal has been carried out using potassium and naphthalene in tetrahydrofuran and methyl-¹³C and butyl-1 -¹³C iodides to alkylate the resultant polyanion. The soluble products of the reductive alkylation reaction were isolated by extraction and chromatography. Proton and carbon n.m.r. spectra were recorded. The intense resonance signal at δ3.95 which appears in the proton n.m.r. spectra of Illinois No. 6 coal butylated with unenriched butyl iodide is split into a doublet by the ¹³C nuclei. Similar results were obtained for the methylation products. The chemical shift and coupling interaction establish that aryl ethers are a very important constituent of the alkylated coal. The carbon n.m.r. spectra of the coal alkylated with ¹³C-enriched alkyl iodides are intense. The resonances of the C-alkylation products appear in a single broad band with a maximum intensity in spectral regions compatible with the formation of the reductive alkylation products of certain polynuclear aromatic hydrocarbons or the base-catalysed alkylation of certain benzylic carbon atoms. The resonances of the N -alkylation products appear in two distinct bands. These resonances are tentatively assigned to amines produced as a result of reductive alkylation of heterocyclic compounds. The resonances of the 0-alkylation products appear in three distinct bands which can be assigned to alkyl aryl ethers, alkyl aryl ethers with substituents at the adjacent positions, and to alkyl carboxylates. The ratio of ethers to carboxylates in the soluble alkylation products was determined to be 7.8 for butylation and 8.0 for methylation. The Chromatographic fractions contain different amounts of C-, N-, and 0-alkylation products. This finding suggests that the coal structure is not highly uniform.     

13C nuclear spin-lattice relaxation study of anisotropic solvent rotational diffusion in the polystyrene/trans-decalin system

The rotational diffusion tensor for the solvent trans-decalin has been determined as a function of temperature (21°–55°C) and polystyrene concentration (0–13% w/w) from ¹³C spin-lattice relaxation data. The sensitivity of rotational diffusion behaviour of the trans-decalin molecule to changes in solution conditions was found to be essentially proportional to the axial ellipticities of the respective principal rotational axes. The degree of anisotropic motion was found to increase strongly upon increasing the polystyrene concentration and decreasing the temperature. Apparent activation energies for rotation about each of the principal axes of inertia have been calculated for the series of polystyrene concentrations.     

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

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 ¹³C 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.     

A study of solvent motion in acetone-PMMA solutions using 13C and 1H spin-lattice relaxation measurements

The ¹³C and ¹H spin-lattice relaxation times of acetone in solutions containing from 0 to 20% poly(methyl methacrylate) have been measured from ?55°C to 120°C. The ¹³C measurements show that except at low temperature and the higher polymer concentrations, solvent rotation is little affected by the presence of polymer molecules. By assuming that the ¹³C relaxation time is entirely intramolecular in origin, and by progressive deuteration of the acetone, the ¹H relaxation time has been separated into intramolecular, acetone-acetone intermolecular and acetone-polymer intramolecular contributions. These results show that up to 10% of polymer or so does not affect solvent diffusion, but for 20% polymer, solvent diffusion is perceptibly slowed down, and the activation energy increased by 50%. The data give no indication of polymer conformational transitions in this temperature range, as suggested by others elsewhere, but this may be because of the very short-range sensitivity of n.m.r. measurements.