Types Of Carbon Atoms Research Articles

Catalysed liquid phase oxidation of acetals by molecular oxygen

Nine different acetals have been oxidized in the presence of Co(OOCCH3)2.4 H2O under isobaric conditions (0.1 - 0.2 MPa O2 while following the uptake of molecular oxygen. The reactivity of acetals was expressed by the rate constants of the autocatalytic model of oxidation. The main products of the oxidation are alcohols, esters and acids. The distribution of products and the total reactivity of acetals are controlled by the structure of both parts of acetal molecule. The dominant effect on the course of the reaction exerts the type of carbon atoms on which radicals are formed. The oxidation is accompanied by consecutive and co-oxidation reactions, by deactivation of the catalyst and by decarbonylation of intermediate products. The effect of oxygen pressure is reported and the more detailed radical mechanism of the oxidation is proposed.

Regularities in the retention of nonsubstituted polynuclear aromatic hydrocarbons in liquid chromatography on polar adsorbents

The retention of polynuclear aromatic hydrocarbons (PAH) is studied in liquid chromatography, using columns packed with silica gel with a hydroxylated surface and the same silica gel with surface-bonded amine and nitrile groups. A linear relationship was found between retention and the number of π-electrons in the molecule, the size of the molecule and the different types of carbon atoms in the molecule. The different carbon atoms in a PAH molecule contribute differently to the retention and this contribution depends on the nature of the polar functional groups on the adsorbent's surface and on the nature and concentration of a polar modifier present in a non-polar eluent.

Secondary isotope multiplet nuclear magnetic resonance spectroscopy of partially labelled entities (SIMPLE): 13C spectra of stachyose and its subunits

13 C N.m.r. measurements have been made, in (C2H3)2SO solutions, of some carbohydrates (sucrose, raffinose, melezitose, and stachyose) in which exchangeable protons have been partially deuteriated (ca.50:50). Signals from a single carbon atom type are observed as a series of multiplets (singlets to octests, at least) resulting from observation of different isotopomers measured under conditions of slow exchange. The multiplets are analysed in terms of the two-bond (β) and three-bond (γ) isotope effects that contribute to the deuterium-induced secondary isotope shift. Magnitudes of β and γ effects vary, giving rise to characteristic multiplets which have been used to check the assignment of the disaccharide sucrose, and to provide the assignment of the 13C n.m.r. spectra of the trisaccharides raffinose and melezitose and the tetrasaccharide stachyose in (C2H3)2SO solutions and in 2H2O solutions by using mixed-solvent studies. The SIMPLE n.m.r. method may be used to provide information on the composition, substitution, and glycosidic linkages of oligosaccharides in solution.

The relation of physical properties of alkanes to connectivity indices: a molecular explanation

Additive schemes for calculating the molar volumes and heats of vaporization of liquid alkanes from increments characteristic of the numbers and different types of carbon atoms (primary, secondary, tertiary, quaternary), of the mole fraction Zg of gauche conformations, and (where necessary) of other steric factors have been developed. It is suggested that schemes for correlating physical properties of alkanes with connectivity indices mχt are successful because these indices are also correlated with numbers and types of carbon atoms and with Zg.

Spectra–structure relationships in carbon‐13 nuclear magnetic resonance spectroscopy. Results from a large data base

AbstractA link between a substructure searching system and a 13C NMR data base has been established and permits the retrieval and examination of the chemical shifts associated with specific substructures. The means by which these searches are accomplished is described and the results from the searches are presented and discussed. The system is interactive, and can be used to locate in the data base the chemical shifts of carbon atoms in precisely defined environments. Alternatively, it may be used to learn the range of the chemical shifts possessed by particular types of carbon atoms, such as N‐methyl or O‐methyl carbons.

Comparison of empirical equations for predicting carbon-13 chemical shifts in hydrocarbons, amines, car☐yluc acids, and amino acids

Published data for the C-13 chemical shifts of hydrocarbons were analyzed with multiple linear regression techniques to obtain predictive equations for these shifts. These equations were based on the sum of parameter terms, and parameter types proposed by various investigators were compared. Published C-13 chemical shift data for amines, car☐ylic acids, and compounds containing both polar groups, at their various ionization states in water, were supplemented with newer data. Computational experiments were done to determine which types of carbon atoms were sufficiently similar to fit the same predictive equation for chemical shifts without significant loss of accuracy. Eight equations were found to be necessary.

Characterization of heavy crude oil fractions by means of the 13C NMR technique

The 13C NMR spectra of samples of heavy crude oil fractions were interpreted. The spectra of all samples were of the same character. In the region of the aliphatic carbon shifts, ten different carbon atom types were discerned; some possible structure types of hydrocarbons were attributed to them and the arrangement of the aromatic rings in a mean molecule of the samples examined was discussed.

Gas chromatographic detection and identification of aromatic and aliphatic hydrocarbons in complex mixtures by coupling photoionization and flame-ionization detectors

A technique has been developed that is based on analyzing the effluent from a gas chromatograph with both photoionization (PID, with 10.2-eV lamp) and flame-ionization (FID) detectors. The relative molar response per mole of carbon (RMR) for the FID is similar for many types of carbon atoms, e. g., aromatic and aliphatic and therefore the FID is used to measure the relative levels of a particular hydrocarbon regardless of the degree of saturation. The RMR for the PID, however, increases with unsaturation. A ten-fold difference is observed between aromatics and alkanes, and the alkene response on the PID is midway between these two classes of compounds. When the PID and FID responses ( i. e., peak areas) are normalized to any alkene ( e. g, n-hexane) and are compared, those compounds with a normalized PID/FID ratio of 2–4 are alkenes, those with a ratio of 5–10 are aromatics and those with similar ratios (<2) are alkanes. Preliminary work on this technique was started with simple hydrocarbon mixtures, then extended to high-molecular-weight heteroatom molecules such as chlorinated and sulfur-based pesticides. Finally, a practical application of this approach was demonstrated with the successful analysis of aromatics in a light hydrocarbon feedstock for a synthetic natural gas plant. This technique should prove useful for analyses (nanogram levels) and for the identification of complex hydrocarbon mixtures.

Carbon-13 NMR spectra of tryptophan, tryptophan peptides and of native and denatured proteins

The assignment of the carbon-13 magnetic resonance spectrum of l-tryptophan in aqueous solution and in trifluoroacetic acid is made by comparison with the spectra of indoles, by double resonance and by deuteration experiments. On the basis of work on small peptides, it is possible to assign all the downfield resonances in denatured proteins to single types of carbon atoms, with the exception of the carbonyl carbons and the histidine Cγ resonance. In the upfield region there are only about eleven resonances which correspond to single types of carbon atoms from eight different amino acid residues. The spectra of a particular protein denatured by heat, 6 M guanidine · HCl or formic acid are all similar. The spectrum of a native protein is much broader than that of the denatured protein due to small differences in chemical shift which result from environmental changes of some carbon atoms, and line broadening due to decreased mobility of carbon atoms.

Natural-abundance carbon-13 Fourier-transform nuclear magnetic resonance spectra and spin lattice relaxation times of unfractionated yeast transfer-FNA.

High-resolution Fourier-transform nuclear magnetic resonance at 15.18 MHz was used to observe the proton-decoupled natural-abundance (13)C spectra of aqueous unfractionated tRNA from baker's yeast in the presence of Mg(+2) (8 ions per tRNA molecule), as a function of temperature in the range of 27-82 degrees C. The spectrum of thermally denatured tRNA at 82 degrees C showed numerous sharp resonances, which were assigned to specific types of carbon atoms by comparison with the (13)C spectra of mononucleotides. Only the resonance of carbon 4' of the ribose rings was appreciably shifted (by about 1.5 ppm upfield) with its average position in the mononucleotides. This effect was also seen in the spectrum of poly(A). In the spectrum of folded tRNA (52 degrees C), carbon 4' was further shifted upfield by about 1.5 ppm, and carbons 2' and 3', which yielded a single resonance at 82 degrees C, now showed two partly resolved peaks. The variation of linewidths with temperature (200 mg/ml of tRNA) was gradual in the range 27-82 degrees C, and did not reflect the expected unfolding behavior of tRNA. Moreover, dilution to 80 mg/ml at 27 degrees C had the same effect as an increase in temperature to about 45 degrees C. The line-width changes below 60 degrees were ascribed to tRNA aggregation. In contrast to the behavior of the linewidths, the (13)C spin-lattice relaxation times (T(1)) of individual ribose carbon atoms, measured by means of partially relaxed Fourier-transform spectra were practically independent of temperature up to about 60 degrees C, and increased rapidly at higher temperatures. The T(1) values indicated that the backbone of thermally denatured tRNA is undergoing rapid segmental motion, with an effective correlation time of (2.6 +/- 0.5) x 10(-10) sec. The T(1) values of folded tRNA yielded no evidence of segmental motion. The correlation time for overall rotational reorientation is about (3 +/- 1) x 10(-8) sec in the range 35-54 degrees C. Within experimental error, the T(1) values of methine carbons of the bases were equal to those of the methine carbons on the backbone at all temperatures. Only an upper limit to the rate of internal rotation of the bases could be established.

Die Strukturen der mittleren Ringverbindungen. X. 1, 6‐trans‐Diaminocyclodecan‐dihydrochlorid, monokline Modifikation

AbstractThe structure of the monoclinic modification of 1,6‐trans‐diaminocyclodecanedihydrochloride has been determined, and refined by full‐matrix least‐squares analysis of three‐dimensional X‐ray data. The cyclodecane skeleton is very similar to that found previously in the triclinic modification, but the substituents are attached to different types of carbon atoms. The two crystal modifications thus correspond to different conformational isomers.

Critical temperatures and pressures of hydrocarbons

AbstractA method has been developed for the calculation of van der Waals' constants both a and b for hydrocarbons usually encountered, including the aliphatic, naphthenic, and aromatic types. With these constants critical temperatures and pressures can be calculated directly.Methods recently made available (12, 13, 14, 15) allow the calculation of these constants through the use of substitution values involving the replacement of hydrogen atoms by methyl groups in a definitely prescribed pattern. In this study van der Waals' constants are directly calculated from a consideration only of the molecular structure of the hydrocarbon. This approach has become possible through the assignment of group contributions to different types of carbon atoms, which can be combined in any manner to produce the van der Waals' constants representative of the molecular structure of the hydrocarbon.