Characterization of polyurethane elastomers by 13C n.m.r. spectroscopy

A ¹³C n.m.r. study of a series of polyurethanes and selected model compounds is reported. Comparison of the observed chemical shifts with values predicted by the Grant and Paul additive theory shows reasonable agreement. The latter approach, therefore, is a useful method for the assignment of carbon resonances in these systems. The positions of carbonyl peaks are more difficult to predict, but model compound studies allow their unambiguous assignment. These peak positions are very useful for qualitative and quantitative measurements. Nuclear Overhauser Enhancement (NOE) factors have been measured for typical copolymers, and it is possible to obtain quantitative estimates of the compositions of unknown polymers and prepolymers.     

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.     

Spectroscopic investigations of solvent-refined coal fractions

The chemical characteristics of Amax solvent-refined coal are investigated on a molecular size and component basis. Gel permeation chromatography (g.p.c.) is used to characterize the tetrahydrofuran-soluble portion of the SRC and to obtain the molecular size distributions, and is also used as a preanalysis step, in which fractions are obtained according to elution time. The THF-soluble portion of the SRC has elution times comparable to asphaltene plus oil. The resultant g.p.c. eluent is divided into six fractions, three of which have molecular sizes and elution times comparable to asphaltene alone, two to oil, and one to asphaltene plus oil. These observations are confirmed by chemical ionization and electron impact mass spectrometry. Various analytical techniques are used to establish further the composition of the fractions, including: infrared spectrometry, elemental analysis, flourescence excitation and emission spectrometry, high-pressure liquid chromatography and gas chromatography.     

Two-dimensional n.m.r. analysis of aromatic fractions from a coal liquefaction solvent

Two-dimensional n.m.r. experiments correlating heteronuclear ¹H-¹³C and homonuclear ¹H-¹H spin-spin coupling have enabled various aliphatic groups to be identified unambiguously in complex mono- and diaromatic fractions from a coal liquefaction solvent. These groups include those that are generally difficult to identify by g.c.-m.s. such as five membered rings and CH₃ substituted in cyclic structures.     

The characterization of coal tar pitches used in electrode binder manufacture by n.m.r. spectroscopy

Coal tar pitches dissolved in a mixed solvent system comprising S₂Cl₂ and SO₂Cl₂ were analysed by ¹H and ¹³C n.m.r. spectroscopy. The extensive chlorination of the pitch by this solvent unfortunately leads to serious underestimations in aromatic hydrogen concentrations when these solutions are examined by ¹H n.m.r. spectroscopy. However, quantitative data can be obtained using ¹³C n.m.r. spectroscopy where the carbon skeleton is unaffected by the extent of chlorination of the pitch. The ¹H n.m.r. spectra of individual aromatic compounds, similar to those found in coal tars have been recorded. The ¹H n.m.r. spectra confirmed that extensive chlorination of the aromatic nuclei had occurred which results in the non-quantitative estimation of hydrogen concentrations.     

Characterization of coal liquids by 13C n.m.r. and FT-i.r. spectroscopy — fractions of oils of SRC-I and asphaltenes and preasphaltenes of SRC-I and SRC-II

Coal-derived products of SRC-I liquefaction of Blacksville coal and vacuum tower bottoms (VTB) of SRC-II liquefaction of Powhatan mine coal (both bituminous and from the Pittsburgh Seam) were separated into fractions by solvent extraction. The SRC-I was first extracted with ethylacetate, and solubles were subsequently separated into oils and a mixture of asphaltenes and preasphaltenes (APA). The VTB was Soxhlet extracted with pentane to remove any residual oils, followed by tetrahydrofuran to recover APA. The APA portions were then separated by sequential elution solvent chromatography (SESC) into fractions differing in chemical functionality, and then examined by ¹³C n.m.r. and FT-i.r. spectroscopic techniques. APA are intermediates and end products of coal liquefaction. SRC-II APA are of higher molecular weight than the APA of SRC-I. The lower numbered fractions of APA of SRC-I in SESC separation have the same functional groups as the corresponding fractions of middle and heavy distillates. However, the higher numbered fractions are rich in oxygen, which is mainly in carbonyl groups. Part of the carbonyl groups are in esters which cross-link aromatic clusters. Therefore, APA and the coal itself are ‘oligomeric’ in structure, with aromatic clusters linked by carbon bridges with different functional groups. The nature of carbonyl groups in APA has been analysed in detail.     

Evaluation of solvents for carbon-13 n.m.r. analysis of solvent-refined coals. sym-Triazine a new carbon-13 n.m.r. solvent

Solvent-refined lignite (SRL) and bituminous coals (SRC) are only partially soluble in good standard carbon-13 n.m.r. solvents. A survey of other possible solvents for SRLs and SRCs showed that sym-triazine at 86 °C solvated SRL as well as pyridine, but did not absorb in the regions of interest (5–60 and 90–160 ppm relative to TMS). Model hydrocarbons, phenols and an alcohol in sym-triazine gave aromaticities (f_a) accurate within the standard deviation (0.01 to 0.001) when proper FT carbon-13 techniques were used. An alkene and an aliphatic amine reacted with sym-triazine and gave inaccurate values of f_a.     

Co-carbonization of fractions of solvent-refined coal with a non-coking coal

Co-carbonizations of solubility fractions of a solvent-refined coal with a high-volatile, inertinite-rich non-coking coal are reported. Both chloroform-soluble and chloroform-insoluble fractions give rise to improved fusion in the cokes derived from the blends, but only the coke derived from the blend containing the chloroform-insoluble fraction contains areas of anisotropic microtexture.     

Study of Ti in solvent-refined coal by X-ray absorption spectroscopy

The chemical environment of titanium in solvent-refined coal (SRC-I and SRC-II) has been studied using X-ray absorption spectroscopy. Evidence that organic complexes of Ti are produced in the liquefaction process from mineral forms of Ti present in the feed coal is presented, based on the pre-edge structure of the X-ray absorption spectra.     

Conformational studies of N-acetyl-N-methyl-l-alanine dimethylamide-d6 by n.m.r. spectroscopy

Nuclear magnetic resonance of $\text{N-}\text{acetyl}\text{-N-}\text{methyl-}\text{l}\text{-alanine dimethylamide-d}{}_6$ in methylene $\text{chloride}\text{-d}{}_2\text{dichloroacetic acid}$ was investigated. In methylene chloride-d₂ the amide assumed nearly all-trans conformation. Small signals corresponding to trace amounts of cis confirmation were observed. Dichloroacetic acid induced the shift of the resonance signals immediately after the addition. On leaving the solution containing dichloroacetic acid, the amide nitrogen was protonated and the isomerization around the amide bond took place. After a sufficient time lapse a complex spectrum resulted, which corresponded to at least five different conformations. On heating the solution several signals disappeared without coalescence and the resonance signals corresponding to three different conformations remained at 120°C. These conformations should include non-planar conformations as well as planar cis and trans conformations.     

Quantitative solid-state C n.m.r. measurements on cokes, chars and coal tar pitch fractions

Bloch decay or single pulse excitation (SPE) ¹³C n.m.r., generally recognized as the best approach to obtain quantitatively reliable aromaticity values and other skeletal parameters for coals, was applied to partly carbonized coal samples, a biomass char and the toluene-insolubles from a coal tar and a corresponding pitch. As found previously for coals, the aromaticities and non-protonated carbon concentrations were generally higher than those estimated by cross-polarization (CP). Furthermore, in terms of accumulation times, the shorter ¹³C T₁'s of low-temperature chars makes SPE a more efficient technique than for coals. The higher concentrations of paramagnetic centres responsible for the shorter ¹³C T₁'s still result in observability of 75% of the carbon in the chars by the SPE technique. The ratios derived from the SPE measurements agree well with those obtained from elemental analysis.     

Structural investigation of coal-tar pitches and coal extracts by 13C n.m.r. spectroscopy

The structural composition of hard and brown coal tar pitches and coal extracts is analysed by ¹³C n.m.r. spectroscopy on the basis of the chemical-shift data for selected model compounds. For hard-coal-tar pitch, the high aromaticity is confirmed, the predominance of alternant hydrocarbon building blocks established, and the minute aliphatic resonances assigned to definite types of methyl groups and methylene linkages. In brown-coal-tar pitch, long straight-chain alkanes are identified as the major constituents. For three coal extracts obtained by different technical processes, the similar ¹³C spectral patterns demonstrate their analogous composition. The most striking feature in these spectra is an aliphatic resonance at 30 ppm which is assigned to a structural principle of the dihydrophenanthrene type, in accordance with a modified Given model for the structure of coal.     

Conformational studies of poly(N-methyl-l-alanine) by n.m.r. spectroscopy

Nuclear magnetic resonance (n.m.r.) and circular dichroism of $\text{poly}\text{(N-}\text{methyl-}\text{l}\text{-alanine}\text{)}$ in methylene $\text{chloride}\text{-d}{}_2\text{dichloroacetic acid}$ were investigated. In methylene chloride-d₂$\text{poly}\text{(N-}\text{methyl-}\text{l}\text{-alanine}\text{)}$ was found to consist of nearly all-trans amide bonds and assume a stable secondary structure. Trace amounts of cis amide bonds were also present. The addition of dichloroacetic acid destroyed the secondary structure and induced a drastic change of the n.m.r. spectrum, which was similar to that observed with the monomeric amide, $\text{N-}\text{acetyl}\text{-N-}\text{methyl-}\text{l}\text{-alanine}$ dimethylamide. In comparison of the polymer with the monomeric amide, it was concluded that the transition of $\text{poly}\text{(N-}\text{methyl-}\text{l}\text{-alanine}\text{)}$ was caused by the isomerization of amide bonds. The complex n.m.r. spectrum was interpreted in terms of the distribution along the polymer chain of various non-planar amide links as well as planar cis and trans amide links. The difference of the mechanism of conformational transition between poly(amino acid) and poly(imino acid) is also discussed.     

Comparison of fractions of pyridine extract and solvent-refined coal from Illinois No. 6 coal

A pyridine extract of Illinois No. 6 coal and a solvent-refined coal from the same coal have been fractionated. Parallels between the two sets of fractions in composition and molecular weight and some experiments on phenol-pyridine interactions lead to the conclusion that solvent refining cleaves ether links and removes much of the organic 0 and S, without much change in acidic and basic groups. Gel-permeation chromatograms of the fractions indicate that they have moderately narrow molecular weight distributions. A plot of number-average molecular weights against retention times for both sets of fractions gives a smooth curve that is consistent with relations for polystyrene and poly(ethylene oxide).     

Proton and carbon n.m.r. spectra of butylated coal

The proton and carbon n.m.r. spectra of butylated Illinois No. 6 coal which has been fractionated by gel-permeation chromatography (GPC) are reported. The chemical shifts and spin-lattice relaxation times of model compounds provide the bases for the assignment of the spectroscopic results. The spectra of the various molecular weight fractions are clearly different. There are significant variations in the degree of aromaticity, the presence of linear alkanes, the ratio of C-butylation to O-butylation, the extent of butylation on aliphatic and aromatic carbon atoms, and the amount of carbonyl and vinyl derivatives. The results suggest that electron transfer, proton abstraction, ether cleavage, and elimination reactions are important under the conditions of the Sternberg process and that both nucleophilic and free-radical substitution reactions occur. The results also strongly suggest that the distribution of functional groups in coal is not uniform.     

Dynamic nuclear polarization 13C n.m.r. of coal

A novel n.m.r. method for determining the aromaticity of coal has been evaluated. Polarization transfer from the paramagnetic electrons is used to enhance the intensity of the ¹³C n.m.r. spectrum. Analysis of five samples of Gondwana bituminous steam coals shows that the aromaticity of inertinite is far greater than that of vitrinite and that variation of maceral composition is a major factor determining coal aromaticity. This has been confirmed by the study of maceral concentrates.     

Chromatographic and n.m.r. analysis of coal liquefaction products

An efficient Chromatographic separation scheme has been developed to characterize the SYNTHOIL hydrodesulphurization coal liquefaction product. Chromatographic fractions were analysed by nuclear magnetic resonance to determine carbon and hydrogen aromatic/aliphatic distribution ratios of major molecular types. The SYNTHOIL product and its coal-derived asphaltenes were found to contain intermolecularly hydrogen-bonded complexes that may affect the physical properties of the oil.     

An investigation of coal by means of e.s.r., 1H n.m.r., 13C n.m.r. and dynamic nuclear polarization

Sixty coal samples of different rank and origin have been investigated by means of e.s.r., ¹H n.m.r. and ¹³C n.m.r., the last two in combination with dynamic nuclear polarization (DNP). The following parameters have been determined: the number of free radicals, the e.s.r. linewidth, the ¹H Zeeman relaxation rate, the ¹H relaxation rate in the rotating frame, the ¹H DNP enhancement, the ¹³C DNP enhancement, the ¹³C Zeeman relaxation rate and the ¹³C aromaticity, observed via ¹H¹³C cross-polarization (CP), both with and without magic-angle spinning (MAS). The relations between these parameters and coal rank have been investigated. Moreover, with DNP special experiments have been performed which provide information about the localization and the mobility of the unpaired electrons present in these coals. Finally, DNP has been used to investigate various features of the quantitative analysis of coal via ¹³C n.m.r. MAS was found to reduce the measured ¹³C aromaticity, and for three coals it was shown that even without MAS only ≈ 50% of the aromatic ¹³C nuclei are detected by the CP technique.     

Characterization of coal-derived liquids by C n.m.r. and FT-i.r. spectroscopy: Fractions of middle and heavy distillates of SRC-II

Coal-derived products of the SRC-II liquefaction of Powhatan Mine (Pittsburgh Seam) bituminous coal were separated into various fractions either by solvent extraction or by distillation. Subsequently, the middle and heavy distillates were separated by sequential elution solvent chromatography into fractions differing in chemical functionality. These fractions were examined by ¹³C n.m.r. and FT-i.r. spectroscopic techniques. In addition to developing the techniques, the work was undertaken to relate the product composition to the possible reactions occurring during the solvent-induced pyrolytic fragmentation of coal. The bulk of the SRC-II generated middle distillate is composed of two-ring nonpolar aromatic compounds, tetralins, one-ring phenols, indoles, and some alkanes. The heavy distillate contains three-and four-ring systems.