Polyconjugated Systems Research Articles

Characterization of Humic Substances from Taiga and Tundra Soils by EPR Spectroscopy

Paramagnetic activity is a fundamental property of humic substances (HSs). The agricultural use of soils does not only affect the qualitative and quantitative compositions of HSs, but also the content of free radicals (FRs) in their structure. Changes in the composition of incoming plant residues and hydrothermal conditions have an effect on soil humification rates and the paramagnetic properties of humic (HAs) and fulvic acids (FAs). Data on the influence of various factors on the content of FRs in HAs and FAs are not enough. Therefore, the purpose of this study was to examine the influence of the degree of hydromorphism and agricultural use on the paramagnetic properties of HA and FA samples obtained from taiga and tundra soils. Studies have shown that the increased hydromorphism in taiga soils leads to the growing concentration of FRs in the HA molecular structure. HAs in virgin tundra soils exhibit a lower content of unpaired electrons when shifting from automorphic soils to hydromorphic ones. Going from the south to the north, the paramagnetic activity of both HAs and FAs tends to decrease due to the overall reduction of the number of polyconjugated systems in the tundra soil HSs. The comparative analysis of the paramagnetic properties in HAs and FAs of virgin and arable soils revealed that their agricultural use reduces the FR concentration in the structure of HSs, in other words it leads to the accumulation of biothermodynamically stable and more humified compounds in the arable horizons. This contributes to the stabilization of SOM in arable soils.

Biochar characteristics produced via hydrothermal carbonization and torrefaction of peat and sawdust

Biochars made by hydrothermal carbonization and torrefaction of sawdust and peat in a wide temperature range have been studied. A comparative evaluation of the properties of the resulting materials was carried out by the following methods: elemental analysis, FTIR, Raman Spectroscopy, TG/TGA, SEM, Fluorescence analysis, BET. It has been established that hydrothermal carbonization has a greater effect on the degree of carbonification of the initial biomass than torrefaction. Biochars made from peat by this method have a more stable structure and contain asphaltene-like fragments and a developed system of poly-couplings. The combination of these properties makes it possible to use these materials not only as solid commercial carbon dioxide neutral fuel, but also as a promising analogue of fossil coals for the refining process. • Peat hydrochar obtained at 230 °C obsessed asphaltene-like structures. • HTC enables directed evolution of biomass. • Thermal effect on peat is stronger than on sawdust. • HTC reduces more ash in peat then in sawdust. • Polyconjugation systems were found in hydrochars.

Quantitative Estimate of the Resonance Effects in Some Unsaturated, Monocyclic, and Aromatic Hydrocarbons Based on the Renewed Optical Exaltations

: The present review discusses a new viewpoint on refractometry as the oldest experimental physical method, whose scientific potential in the estimation of structural effects in organic chemistry has been missed so far. The author demonstrates that upon certain adjustment and redesign of refractometry, this potential can be tapped and successfully used to determine a type of π-electron interaction, delocalization degree of π-electrons in organic compounds, and to perform quantitative estimates of resonance effects in unsaturated, (polycyclic) aromatic, and other polyconjugated systems (e.g., fullerenes). The method for accurate separation of molar refraction into additive and constitutive components was suggested; the method is based on the specially developed additive scheme. It was revealed that the negative deviations from additivity for cycloalkanes depend linearly on the number of carbon atoms in the ring. Excellent linear correlations between renewed optical exaltations, the number of π-electrons in a conjugated system, and experimentally found resonance energy (determined from hydrogenation heat values) were demonstrated. Angular coefficients of the correlation series (ρ-constants) are considered as a criterion of classification, which characterizes the degree of mobility of π-electrons in the conjugated system of a given type. It is emphasized that the development of methods for precise measurement of the constitutive components of molar refraction may become a useful additional source of information about resonance and other effects in organic and polymer chemistry.

Biomaterials based on nanosized forms of polyconjugated systems

Biomaterials based on nanosized forms of polyconjugated systems

Synthesis of New Polyconjugated Systems Containing Iminodihydrofuran and Benzene Rings

Synthesis of New Polyconjugated Systems Containing Iminodihydrofuran and Benzene Rings

Application of polyaniline and polypyrrole in electronics

Aspects of application of nitrogen-containing polyconjugated systems in electronics are considered. It is shown that the use of polyaniline and polypyrrole in the manufacture of structural elements of current sources and supercapacitors has significant prospects for increasing the efficiency of these devices. The fundamental possibility of using polyaniline in the creation of electrochromic displays and photovoltaic cells is shown.

Electronic and Crystal Structures of Nitrogen-Containing Electroconductive and Electroactive Polymers

Current ideas about the electronic and crystal structures of nitrogen-containing conductive polymers are considered. A comparative critical analysis of literature data on electronic, vibrational, and NMR spectra for different nuclei, and X-ray diffraction data for various forms of polyaniline, polyphenylenediamine, and polypyrrole is presented. Particular attention is paid to the relationship between the electronic and crystal structure of nitrogen-containing polyconjugated systems and their electrical conductivity.

Paramagnetic Properties of Humic Substances in Taiga and Tundra Soils of the European Northeast of Russia

The study of paramagnetic activity of humic substances in taiga and tundra soils of the Komi Republic and the assessment of the influence of soil hydromorphism on concentrations of free radicals in the structure of humic acids (HAs) and fulvic acids (FAs) have been performed. The concentration of free radicals in HA specimens was up to 11 times higher than that in FA specimens due to a higher content of aromatic and other condensed structures in HA molecules. This fact attests to the high capacity of HAs to polymerization and complexation reactions with participation of radicals. The average value of g-factor is higher for FA specimens than for HA specimens, which attests to a greater electron density shift of unpaired electron to oxygen atom in the structure of FAs because of its spin-orbital interaction with oxygen-containing functional groups, the concentrations of which are significantly higher in FAs than in HAs. An increase in the concentration of free radicals in the molecular structure of HAs is observed in taiga soils with an increase in the degree of their hydromorphism (from automorphic to semihydromorphic soils), which is related to the biohydrothermal conditions of humus formation in bog-podzolic soils with retarded biochemical processes and low degree of plant litter humification. As a result, HAs with the high content of free radicals in their structure are formed. An opposite situation is observed for HAs in tundra soils with a decrease in the content of unpaired electrons under conditions of the increased hydromorphism. The difference in the character of changes in the paramagnetic activity of HAs in taiga and tundra soils with different degrees of hydromorphism may be related to different natures of plant residues participating in humification processes. A tendency for a decrease in the paramagnetic activity in both HAs and FAs from the south to the north is observed, which may be related to a general decrease in the content of poly-conjugated systems in the structure of humic substances in tundra soils.

Interaction of humic compounds with constant and variable valence metals in acidic solutions

The mechanism of the interaction of the humic substances (HSs) of peat and brown coal with the ions of constant (Zn and Pb) and variable valence (Cu, Cr, Co, and Ni) metals in acidic solutions (pH 2) was studied. It was established that the constant valence metals weakly interacted with HSs by an ion-exchange mechanism. However, the variable valence metal ions in an aqueous medium also interacted with the polyconjugation systems of HSs to create prerequisites for developing sorbents based on the HSs of caustobioliths for variable valence metals in acidic solutions.

Interaction between the bituminous and humic components of peat

It was established that an increase in the bituminosity of peat with the degree of decomposition can be explained by an exchange interaction between the π electrons of the molecular polyconjugation systems of humic acids and the quasi-polyconjugation systems of bitumens (the presence of the latter is caused by a synergistic effect of the interaction of separated aromatic rings and the hydrogen bonds of functional groups). This interaction leads to the formation of unique associates, and this fact explains an increase in the concentration of humic substances in a composition with bitumens with increasing the degree of decomposition. The concept of associates is confirmed by a considerable increase in the EPR signal observed upon the removal of a low bitumen content, which was accompanied by changes in its parameters in a number of cases. The formation of these associates occurs under different geoclimatic conditions of peat deposit formation. The lower bituminosity of lowland peats is caused by steric hindrances created by the Ca2+ ions bound to the functional groups of humic substances for the access of hydrocarbon molecules.

Controlling Asymmetric Remote and Cascade 1,3-Dipolar Cycloaddition Reactions by Organocatalysis.

The regio- and stereoselective control of cycloaddition reactions to polyconjugated systems has been demonstrated by applying asymmetric organocatalysis. Reaction of 2,4-dienals with nitrones allows for a highly regio- and stereoselective 1,3-dipolar cycloaddition in the presence of an aminocatalyst. The first cycloaddition on the remote olefin can be followed either by a cascade reaction or by other selective reactions of the remaining olefin. The chiral products are obtained in good to high yields and excellent diastereo- and enantioselectivities. The remote selective concept has been extended to 2,4,6-trienals by means of a novel enantioselective triple cascade 1,3-dipolar cycloaddition reaction. The formation of chiral poly 1,3-amino alcohols is also demonstrated.

ChemInform Abstract: Polyaromatic N‐Heterocyclic Carbene Ligands and π‐Stacking. Catalytic Consequences

AbstractReview: [40 refs.

The simulation of carbon material structure based on polyacrylonitrile obtained under IR heating

Since recently an economically efficient method including IR heating has been used for the synthesis of polymer based functional carbon nanocrystalline materials. One of the most widely used polymers having a broadest range of physicochemical properties and various applications is polyacrylonitrile (PAN). Under IR heating PAN undergoes chemical and structural transformations and a polyconjugated system forms with an increase in heating intensity. However, there are still no data on the quantitative and qualitative effects of carbon, nitrogen, hydrogen or metal atoms on the structural stability and configuration of the PAN based carbon material.We have for the first time determined using the MNDO semiempirical quantum chemical model for a carbon material (CM) structure based on heat treated polyacrylonitrile (PAN) that an increase in the N content from 14 to 18 atoms in CM monoatomic layers C46N14H10, C44N16H12, and C42N18H14 and in the H content from 12 to 22 atoms in CM monoatomic layers C44N16H12 and C44N16H22 leads to a decrease in the binding energy (Eb) from 7.40 and 7.12 to 6.88 and 6.25eV, respectively, and to an increase in the difference between the maximum and minimum bond length (Δl), between the maximum and minimum valence angle (Δθ), and between the maximum and minimum local charge (Δq) from 0.176 Е, 12.0°, and 0.487 to 0.238 Е, 20.8°, and 0.613, respectively, and promotes curving of the CM structure. Quantum chemical simulation results are confirmed by the element analysis of the CM specimens and FeNi3/C nanocomposite. When the IR heating temperature was increased from 30 to 500°С, the concentrations of N (СN) and H (СH) in the CM and in the FeNi3/C nanocomposite decreased from 27 to 18 and 10wt% and from 6 to 1 and 0.5wt%, respectively.

Changes in the composition of humic substances depending on the depth of peat occurrence

The fractional group composition of humic substances and the chemical structure of humic acids isolated from peats of different genesis from different depths of occurrence in peat deposits with the use of two extractants (0.1 N NaOH and 0.1 M Na4P2O7 at pH 7) were studied. Peat from the upper aerated layer of the peat deposits contained more humic substances than peat from the deep layer, which occurred under anaerobic conditions, due to the pyrophosphate fraction of humic acids and fulvic acids. The humic acids of peat isolated from the deep layers of deposits had smaller molecular weights, and they were characterized by lower concentrations of carbohydrate and saturated hydrocarbon fragments and an increased contribution of aromatic polyconjugation systems, as compared with the humic acids of a surface layer.

Effect of aluminum chloride on formation of a polyconjugated bond system in the initial stage of polyvinyl alcohol thermal decomposition

Fine-structure bands have been located in electronic absorption spectra of polyvinyl alcohol (PVA) films decomposed thermally in air at 80–150°C. Doping of the PVA film with aluminum chloride (thermolysis catalyst) was found both to enhance the degree of thermal decomposition and to reduce the starting temperature for formation of poly-π-conjugated chains although it had no effect on the spectral positions of the polyene-unit absorption bands. Values of the band gap of heat-treated PVA films were determined to be in the range 1.65–1.78 eV based on an analysis of the electronic spectra long-wavelength edge.

Transformation of the polyconjugation systems of humic acids in the course of the metamorphism of caustobioliths

As found with the use of EPR and IR spectroscopy, the formation of aromatic polyconjugation systems by the destruction of aliphatic molecular fragments is the main transformation mechanism of the molecular structure of the humic acids (HAs) of caustobioliths in the course of their metamorphism. In this case, the final stage of the formation of the polyconjugation systems is the polymerization of monoaromatic fragments. At the stage of the genesis of brown coal, when a sharp decrease in the concentration of monoaromatic fragments is observed and the concentration of the polyconjugation systems increases, the sizes of planar graphite-like structures also increase. It is shown that the method of colloid chemical fractionation [1] makes it possible to obtain HA preparations with regulated concentrations of condensed aromatic structures and aliphatic fragments.

Solvent effect in the formation of polyconjugated system during pyrolysis of polyacrylonitrile

IR spectroscopy and quantum chemistry were used in the studies of structural transformations taking place in polyacrylonitrile films, formed from dimethylformamide and dimethyl sulfoxide upon their IR pyrolysis and convection annealing. Residual solvent was found to affect conditions of the formation of polyconjugated bonds in polyacrylonitrile films.

Controlled synthesis of polyacrylonitrile via reversible addition-fragmentation chain-transfer pseudoliving radical polymerization and its thermal behavior

The polymerization of acrylonitrile mediated by various trithiocarbonates as reversible addition-fragmentation chain-transfer agents is studied. It is shown that, when polymerization is performed in DMSO, a narrowly dispersed PAN with a controlled molecular mass can be prepared. The pseudoliving radical polymerization of acrylonitrile is conducted for the first time via the reversible addition-fragmentation chaintransfer mechanism in carbon dioxide at an increased pressure. The structure of the polymers is investigated via NMR and IR spectroscopy. As shown by DSC and IR pyrolysis, the thermal behavior of PAN is determined by its molecular mass, the width of the molecular-mass distribution, and the conditions of synthesis. The incorporation of functional groups of the reversible addition-fragmentation chain-transfer agent into a macromolecule changes the structure of the polyconjugated system and makes it possible to control the conditions of its formation.

Transformation of polyconjugation systems as a factor of enhancing the sorption activity of peat upon its modification with phosphoric acid and organic acids

The treatment of peat with the solutions of phosphoric, citric, and oxalic acids in concentrations of 10−4 and 10−2 mol/l at solid phase to liquid ratios of 1: 2.5 and 1: 5 increased the absorption of ammonia by 6.4–39.7%. The absorption of ammonia was higher than the concentration of doping ion-exchange groups by a factor of 5–2000. With the use of EPR and IR spectroscopy, it was found that this phenomenon was caused by the transformation of polyconjugation systems as a result of the interaction of acids with the organic matrix of peat by a macrocoordination mechanism, which also improved the technological characteristics of the resulting sorbents. The absence of the destruction of organic matter with the use of low concentrations of weak acids makes it possible to use these sorbents after operation in gas-purifying equipment as organic fertilizers enriched in nitrogen.

ChemInform Abstract: Organometallic Chemistry Directed Towards the Synthesis of Electroactive Materials: Stereoselective Routes to Extended Polyconjugated Systems

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