Sequence distribution of cis-1,4- and trans-1,4-units in polyisoprenes

The ¹³C n.m.r. spectra of chicle polyisoprene and cis-trans isomerized 1, 4-polyisoprenes were studied. The splittings of signals were observed in the C₁, C₂, and C₄ carbon signals of the isomerized polyisoprenes. The newly appearing signals were assigned to the carbon atoms in cis-trans linkages. The fractions of the diad sequences (trans-trans, trans-cis, cis-trans, and cis-cis) were determined by using the four signals of C₁ carbon. It was found that the cis-1, 4- and trans-1, 4-units were randomly distributed in the isomerized polyisoprenes and it was confirmed that the chicle polyisoprene was a mixture of cis-1, 4- and trans-1, 4-polyisoprenes.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

Chemical structure of tar-sand bitumens by 13C and 1H n.m.r. spectroscopic methods

¹H and ¹³C n.m.r. spectra of Athabasca, Cold Lake and Orinoco tar-sand bitumens, their columnchromatographed fractions, their vacuum distillates, their vacuum residues and the columnchromatographed fractions of the vacuum residues were determined to investigate the average molecular structures of these materials. The most probable average structural parameters were calculated without making any assumptions, and average structural models for the Athabasca series are proposed. Compared with corresponding petroleum fractions, tar-sand bitumens contain shorter, paraffinic straight-chains and are abundant in alicyclic carbon atoms. This trend is more marked in the saturate fractions or vacuum distillates. The vacuum residues of tar-sand bitumens contain relatively longer, paraffinic straight-chains than the original bitumens.     

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.     

Potential of proton-enhanced 13C n.m.r. for the classification of kerogens

High resolution n.m.r. spectroscopy, involving the technique of cross-polarization, along with magicangle spinning, was used in the structural characterization of eight kerogens of different origins, selected to represent the three types of kerogens (types I–III evolution paths). The influence of cross-polarization dynamics on the sensitivity of the method and the ratio of individual fractions in the spectrum was studied in more detail. It is suggested that an analysis of the influence of the mixing time is necessary prior to definitive characterization of any sample. Good separation of signals in aliphatic, aromatic, and carboxylic regions was achieved. The general correlation between the ¹³C n.m.r. structural characteristics and the classification based on ultimate analysis of the kerogens (types I–III, van Krevelen atomic H/C vs. O/C diagrams) was found to be satisfactory. The structural features of kerogens derived from ¹³C n.m.r. analysis agreed quite well with characteristics constituting the above mentioned classification. The ¹³C n.m.r. method used in this paper may be considered promising in the classification of kerogens.     

13C n.m.r. spectra of some poly(N-acyliminoalkenes)

¹³C n.m.r. spectra are reported for six poly(N-acyliminoalkenes), N(COR) (CH)₂)_n (n = 2, R = H, CH₃, C₆H₅; n = 3, R = H, C₆H₅), and N(COC₆H₅)CH(CH₃)CH₂ (isotactic and atactic), and compared with that for poly(N-formyliminopropylene). These linear polymers were made by the isomerization polymerization of the appropriate oxazoline or oxazine derivatives. For the polymers with N-formyl or N-acetyl side groups the spectra of the main chain carbons show fine structure resulting from restricted rotation about the two nearest N-COR bonds. This fine structure shows a small temperature dependence and in some cases there are indications of the influence of a third N-COR group at the lower temperatures. For the polymers with N-benzoyl side groups the rotation of these groups is less restricted than for N-formyl and N-acetyl and at 120°C no fine structure is observed except that resulting from tacticity in atactic poly(N-benzoyliminopropylene), where the carbonyl and main-chain carbons exhibit partly resolved triad structure. At lower temperatures some of the lines broaden because of restricted rotation. In the model compound N-ethyl-N-methylbenzamide all eight types of carbon atom give two lines at ?30°C (in CDCl₃), the two conformers being present in the approximate ratio 58:42. The pairs of lines coalesce in turn as the temperature is raised to 50°C. Spin-lattice relaxation times were measured for the carbon nuclei in poly(N-formyliminoethylene), in N-ethyl-N-methylformamide and in N-isopropyl-N-n-propylformamide. T₁ values increase by two orders of magnitude in going from the polymer to dimethylformamide. ¹³C chemical shifts are recorded for seven oxazoline and two oxazine derivatives as well as for six model compounds of the polymers.     

Study of structural parameters on some petroleum aromatic fractions by 1H n.m.r./i.r. and 13C, 1H n.m.r. spectroscopy

¹H n.m.r. and i.r. spectroscopy were used to derive molecular parameters of petroleum fractions. Relative amounts are estimated of methylene and methyl groups in substituted alkyl side chains bonded to the aromatic ring system. The resolution of equation combinations leads to estimation of $\text{H}{}_{\mathrm{s}}\text{C}{}_{\mathrm{s}}\text{(=x)}$, which is an important parameter for structural analysis. Several petroleum fractions were characterized in terms of hypothetical average molecular structures using ¹H n.m.r./i.r. procedures, ¹³C coupled proton n.m.r. and Brown—Ladner methods. It is proposed that the ¹H n.m.r./i.r. method gives more precise average molecular parameters than the Brown-Ladner method with the most precise analytical procedure, up to date, being ¹³C coupled proton n.m.r. analysis. The Brown-Ladner method is especially suitable for structural analysis of low aromaticity molecular structures with long straight-chain alkyl substituents.     

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.     

13C n.m.r. determination of units formed by irregular addition in polyisoprene

¹³C n.m.r. spectroscopic data obtained for model compounds imitating regular and irregular addition of monomer units in linear polyisoprene are compared with the chemical shifts calculated using the empirical regularities found for the branched alkanes and alkenes and a good correlation is established. The validity of the results obtained was confirmed by investigation of the carbon spectra of hydrogenated and unhydrogenated polyisoprenes which contain chain fragments with irregular addition of units. Samples of hydrogenated polyisoprene are shown to give resonance lines that correspond to the methylene carbons of head-to-head and tail-to-tail addition and show chemical shifts at 34.62 ppm and 27.61 ppm, respectively. For the unhydrogenated polyisoprenes, the methylene carbons of trans- and cis-units in head-to-head addition were found to absorb at 38.6 ppm and 31.4 ppm, respectively, with those in the tail-to-tail addition of both isomers absorbing at 28.4–28.8 ppm. The latter findings offer a practical means of characterizing irregularities in polyisoprenes.     

Short chain branching in high density polyethylene: 13C n.m.r. study

The character and amount of branching has been determined by ¹³C. n.m.r. spectroscopy, in a series of samples of high density polyethylene (HDPE) and in some ethylene copolymers of high density. HDPE samples prepared by different methods were studied and only methyl branches were found; however, the number of branches differs considerably in various polymers. The quantitative determination of the branches is in good agreement with the results of infra-red spectroscopy. The relation between the type of branches and the crystallinity of polyethylene is discussed.     

Quantitative FT 13C n.m.r. of solvent-refined coals using sym-triazine as solvent

Aromaticities (f_a) determined for several solvent-refined coals (SRC) by ¹³C n.m.r. in sym-triazine solution and by the Brown and Ladner ¹H n.m.r. technique have been found to be the same within experimental error (0.01). Use of sym-triazine to determine the f_a of the soluble and insoluble fractions of a SRC in several solvents showed that the soluble fraction reflected reasonably accurately the properties of the whole sample for dioxan, carbon disulphide, chloroform and benzene extracts. The properties of the insoluble fraction varied only slightly. A more detailed analysis of the aromatic region of the ¹³C n.m.r. spectrum has led to some useful structural relations.     

Structure analysis of coals by resolution enhanced solid state 13C n.m.r. spectroscopy

The application of experimental n.m.r. and chemical resolution enhancement techniques in cross-polarization/magic angle spinning (CP/MAS) ¹³C-n.m.r. spectroscopy yields spectra of coals and coalderived solids which contain structural information within the hybridization resonance envelopes. The sp²- and sp³-carbon resonance manifolds are partitioned into components arising from carbon centres bonded directly to oxygen, hydrogen and only other carbon atoms. The unique, observable chemical shift bands in the spectrum are increased from three (the relative areas of the sp²- and sp³-carbon resonance envelopes and a separate carbonyl band) to nine. This resolution permits the principal structural changes in chemically-modified coals to be mapped in unprecedented detail. The reductive alkylation of a typical bituminous coal has been examined by this method.     

13C n.m.r. study of optically active polymers: poly(4-methyl-1-hexene)

Isotactic polymers of optically active and racemic 4-methyl-1-hexene, obtained by polymerization with Ziegler—Natta catalysts, were studied by ¹³C n.m.r. Polymer fractions with different stereoregularity and molecular weight, derived from the monomer with high optical purity, show in their ¹³C n.m.r. spectra differences which are tentatively associated with possible conformation effects. Stereoselectivity is observed in polymers obtained from the racemic monomer.     

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.     

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.     

Gas hydration of trans-1,2-dimethylcyclohexane with cooperative assistance of methane and cis-1,2-dimethylcyclohexane

It has been regarded that the limit of the largest cage occupancy for the structure-H hydrate is between the 1,2-dimethylcyclohexane stereo-isomers, because the cis-isomer is able to generate the structure-H hydrate in the presence of methane while the trans-isomer is not. In the present study, gas hydration of trans-1,2-dimethylcyclohexane in the presence of methane and cis-1,2-dimethylcyclohexane is found from stability boundaries for the structure-H hydrate system.     

Determination of aromatic hydrocarbon fraction in oil shale by 13C n.m.r. with magic-angle spinning

A method for estimation of aromatic content in oil shales is demonstrated. Magic-angle spinning at 2 kHz is shown to remove chemical shift anisotropy to a sufficient degree to resolve aromatic and aliphatic ¹³C n.m.r. spectral regions for a lithic oil shale specimen. The proton and carbon n.m.r. relaxation parameters are such as to allow room-temperature use of this proton-enhanced ¹³C n.m.r. technique as a quantitative analytical tool. Cross polarization times of a millisecond or less and experiment repetition periods of 0.5 s or less are optimum. The specimen examined is represented by an aromatic carbon fraction 0.264 ± 0.007; this determination is quite insensitive to the proton-carbon cross polarization time. Spectra for kerogen, shale oil, and dawsonite are also presented. Dawsonite may interfere in the determination of the aromatic fraction.     

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.