arXiv subjects

E. F. Sheka

Publications and source records attributed to E. F. Sheka.

At least 19 recordsLinked to original sources

arXiv2025

Spin covalent chemistry of carbon

This review presents the covalent chemistry of carbon within the spin-radical concept of electron interaction. Using the language of valence bond trimodality, the regions of classical spinless covalence and its spin counterpart are defined. Carbon is the only element exhibiting spin covalent chemistry. Classical covalent chemistry of carbon concerns molecular substances whose valence bond structure includes segregate or chained single sp3C-C bonds. Substances with double sp2C-C and triple sp1C-C bonds are the subject of spin covalent chemistry of carbon. The mathematical apparatus of spin covalence forms the basis of algorithms governing the chemical modification of carbon substances, polymerization processes, and catalysis involving them, making it possible to supplement the empirical spin covalent chemistry of carbon with its virtual analog.

physics.chem-ph↗

arXiv2020

Graphene Domain Signature of Raman Spectra of sp2 Amorphous Carbons

The paper presents a joint consideration of Raman spectra of sp2 amorphous carbons alongside with the nature and type of their amorphicity. The latter was attributed to the enforced fragmentation. The fragments, presented with size-restricted graphene domains with heteroatom necklaces in the circumference, are the basic structural units (BSUs) of the solids, determining them as amorphics with molecular structure. The standard G-D-2D pattern of Raman spectra of polycyclic aromatic hydrocarbons, sp2 amorphous carbons, graphene and/or graphite crystal is attributed to BSUs graphene domains. The molecular approximation allows connecting the G-D spectra image of one-phonon spectra with a considerable dispersion of the C=C bond lengths within graphene domains, governed by size, heteroatom necklace of BSUs as well as BSUs packing. The interpretation of 2D two-phonon spectra reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization in graphene domains.

cond-mat.mtrl-sci↗

arXiv2020

Raman Scattering by sp$^2$ Amorphous Carbons

The paper presents a cooperative consideration of Raman spectra of $^2$ amorphous carbons as well as the nature and type of their amorphicity. The latter was attributed to the amorphization of a new type named as enforced fragmentation. The fragments are stable graphenous molecules, which are the basic structural units (BSUs) of the solids, determining them as amorphics with molecular structure. Due to weak intermolecular interaction, BSUs, once aggregated, are the main defendants for IR absorption and Raman scattering of the solids, just justifying the consideration of them at molecular level. The standard G-D-2D pattern of Raman spectra of polycyclic aromatic hydrocarbons, $^2$ amorphous carbons, graphene and/or graphite crystal is attributed to extended honeycomb composition of carbon atoms and are suggested as manifestation of molecule-crystal dualism of graphenous materials. The molecular approximation, applied to the analysis of one-phonon spectra of the studied $^2$ ACs, makes it possible to trace a direct connection of the G-D spectra image as well as their broadband structure with a considerable dispersion of the C=C bond lengths within BSUs honeycomb structure, caused by the influence of chemical action, deformation, etc. This approximation, applied to the interpretation of two-phonon spectrum of graphenous molecules for the first time, reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization. A size-stimulated transition from molecular to quasi-particle phonon consideration of Raman spectra was experimentally traced, which allowed evaluation of a free path of optical phonons in graphene crystal.

cond-mat.mtrl-sci↗

arXiv2019

XPS Supported INS and DRIFT Spectroscopy of sp2 Amorphous Carbons

We carried out a joint analysis of the INS, DRIFT, and XPS spectra of a set of sp2 amorphous carbons of the highest carbonization rank representing natural substances (shungite carbon, anthraxolite, and anthracite), technical graphenes (laboratory reduced graphene oxides), and industrial products (carbon blacks). It was determined, that the DRIFT spectra of the studied substances consist of two components determined by hydrogen and oxygen compositions in the circumference of graphene molecules, which represent the basic structural units of amorphic compounds. Methine groups typify the hydrogen component of natural amorphics while hydroxymethyls and methyls do the same job for the studied technical graphenes and hydroxyfurans for carbon blacks. A particular specificity of the methine-based hydrogen compositions to enhance electrooptic characteristics of the DRIFT spectrum of carbon atoms has been established. A comparable analysis of DRIFT and XPS spectra has allowed a reliable personification of the oxygen functional groups compositions of the studied amorphics resulting in a set of dependable molecular models of their basic structural units.

cond-mat.mtrl-sci↗

arXiv2019

Computationally supported neutron scattering study of natural shungite, anthraxolite, and synthetic carbon black

A set of sp2 amorphous carbons involving natural mineral shungite carbon and antraxolite as well as two synthetic carbon blacks were investigated by using neutron powder diffraction and inelastic neutron scattering at low temperature. NDP revealed nanographite-like structure of all the samples, stacks of which are formed by basic structure units representing framed graphene molecules of ~2.5 nm in lateral dimension. INS study showed the presence of hydrogen atoms in the BSU framing area as well as of adsorbed water in the samples pores. Simulated INS spectra of adsorbed water showed its mono-layer disposition within the pores of the studied amorphics. Due to BSUs radical character, their INS spectra were simulated in the framework of both spin-nondependent (DFT) and spin-dependent (UHF) molecular dynamics. The obtained results allowed suggesting a specific INS classification of sp2 amorphous carbons with respect to their hydrogeneousness based on H-standard INS spectra, on the one side, and riding-stimulated spectrum of heavy atoms, on the other.

cond-mat.mtrl-sci↗

arXiv2019

Amorphous carbons in view of multianalytical consideration: normal, expected, and new

The approach based on selected set of samples and selected set of analytical tools occurred quite efficient when applying to amorphous carbons thus leading to a transformation of the current representation of the issue based on particulars into that one based on a limited set of fixed commonalities. The approach first part implies a set of different origin solid samples. The second part concerns different analytical tools. The combining part means the application of each tool to the whole set of samples. In the current study two natural amorphous carbons shungite carbon and antraxolite, as well as two engineered products carbon blacks CB632 and CB624, all of the four belonging to the elitist highest-carbon-content species, were subjected to analytical study by using modern structural and compositional analytical techniques. The approach has allowed disclosing steady points that are common to the whole class of this carbon allotrope and that may lay the foundation of a consolidate representation of what are amorphous carbons.

cond-mat.mtrl-sci↗

arXiv2013

Fractals of graphene quantum dots in photoluminescence of shungite

Photoluminescence of graphene quantum dots (GQDs) of shungite, attributed to individual fragments of reduced graphene oxide (rGO), has been studied for the frozen rGO colloidal dispersions in water, carbon tetrachloride, and toluene. Morphological study shows a steady trend of GQDs to form fractals and a drastic change in the colloids fractal structure caused by solvent was reliably established. Spectral study reveals a dual character of emitting centers: individual GQDs are responsible for the spectra position while fractal structure of GQD colloids provides high broadening of the spectra due to structural inhomogeneity of the colloidal dispersions and a peculiar dependence on excitation wavelength. For the first time, photoluminescence spectra of individual GQDs were observed in frozen toluene dispersions which pave the way for a theoretical treatment of GQD photonics.

cond-mat.mes-hall↗

arXiv2013

Molecular theory of graphene

Odd electrons of benzenoid units and correlation of these electrons having different spins are the main concepts of the molecular theory of graphene. In contrast to the theory of aromaticity, the molecular theory is based on the fact that odd electrons with different spins occupy different places in the space so that the configuration interaction becomes the central point of the theory. Consequently, a multi-determinant presentation of the wave function of the system of weakly interacting odd electrons is absolutely mandatory on the way of the theory realization at the computational level. However, the efficacy of the available CI computational techniques is quite restricted in regards large polyatomic systems, which does not allow performing extensive computational experiments. Facing the problem, computationists have addressed to standard single-determinant ones albeit not often being aware of how correct are the obtained results. The current chapter presents the molecular theory of graphene in terms of single-determinant computational schemes and discloses how reliable information about electron-correlated system can be obtained by using either UHF or UDFT computational schemes.

cond-mat.mtrl-sci↗

arXiv2012

Computational Strategy for Graphene: Insight from Odd Electrons Correlation

The correlation of odd electrons in graphene turns out to be significant so that the species should be attributed to correlated ones. This finding profoundly influences the computational strategy addressing it to multireference computational schemes. Owing to serious problems related to the schemes realization, a compromise can be suggested by using single-determinant approaches based on either Hartree-Fock or Density-Functional theory in the form of unrestricted open-shell presentation. Both computational schemes enable to fix the electron correlation, while only the Hartree-Fock theory suggests a set of quantities to be calculated that can quantitatively characterize the electron correlation and be used for a quantitative description of such graphene properties as magnetism, chemical reactivity, and mechanical response. The paper presents concepts and algorithms of the unrestricted Hartree-Fock theory applied for the consideration of magnetic properties of nanographenes, their chemical modification by the example of stepwise hydrogenation, as well as a possible governing the electron correlation by the carbon skeleton deformation.

cond-mat.mtrl-sci↗

arXiv2012

Hydrogenation of graphene in view of odd electrons correlation

The paper presents evidence of a rather strong correlation of odd electrons in the singlet state of graphene. Due to the correlation, the chemical modification of graphene can be considered following a certain algorithmic computational procedure. Originated due to the correlation and distributed over the carbon atoms of graphene membrane with fraction numbers NDA, effectively unpaired electrons lay the algorithm foundation. The highest NDA value points to the target atom that enters a chemical reaction at the considered step. Following the pointers, a stepwise design of polyderivatives can be performed. Applied to the hydrogenation, the algorithmic design has exhibited that graphene hydrogenation should be attributed to a highly complicated event, whose final hydride products depend on a number of factors such as: 1) the manner of the graphene membrane fixation; 2) the accessibility of the membrane both sides to hydrogen; 3) the composition (molecular or atomic) of the hydrogen. In general, the hydride formation is multimode in regards composition and structure. Thus, the formation of 100% hydride with regular chairlike hexagonal packing of CH units which can be attributed to graphane is possible if only the graphene membrane is fixed over perimeter while its basal plane is accessible to hydrogen atoms from both sides.

cond-mat.mtrl-sci↗

arXiv2011

Mechanochemical reaction in graphane under uniaxial tension

The quantum-mechanochemical-reaction-coordinate simulations have been performed to investigate the mechanical properties of hydrogen functionalized graphene. The simulations disclosed atomically matched peculiarities that accompany the deformation-failure-rupture process occurred in the body. A comparative study of the deformation peculiarities related to equi-carbon-core (5,5) nanographene and nanographane sheets exhibited a high stiffness of both bodies that is provided by the related hexagon units, namely benzenoid and cyclohexanoid, respectively. The two units are characterized by anisotropy in the microscopic behavior under elongation along mechanochemical internal coordinates when the later are oriented either along (zg) or normally (ach) to the C-C bonds chain. The unit feature in combination with different configuration of their packing with respect to the body C-C bond chains forms the ground for the structure-sensitive mechanical behavior that is different for zg and ach deformation modes. Hydrogenation of graphene drastically influences behavior and numerical characteristics of the body making tricotage-like pattern of the graphene failure less pronounced and inverting it from the zg to ach mode as well as providing less mechanical resistance of graphane it total.

cond-mat.mtrl-sci↗

arXiv2011

How Graphene is Transformed into Regular Graphane Structure

The paper presents the first computational experiment on the transformation of a graphene sheet (graphene molecule Cn) into graphane (CH)n of regular chairlike structure. The transformation is considered as stepwise hydrogenation of the pristine molecule governed with a particular algorithm. A spatial distribution of the number of effectively unpaired electrons NDA over the carbon carcass lays the algorithm foundation. The atomically mapped high rank NDA values are taken as pointers of target atoms at each reaction step. A complete hydrogenation followed by the formation of regular chairlike graphane structure (CH)n is possible if only all the edge carbon atoms at the perimeter of pristine sheet are fixed thus simulating a fixed membrane, while the sheet is accessible for hydrogen atoms from both side. The calculations were performed within the framework of unrestricted broken symmetry Hartree-Fock approach by using semiempirical AM1 technique.

cond-mat.mtrl-sci↗

arXiv2011

C60-Based Composites in View of Topochemical Reactions I. C60 Dimers and Oligomers

The current paper opens a series of papers that are aimed at the determination of barriers that govern the covalent coupling between partners of C60-based composites consisting of two or more fullerenes C60 (C60 dimer and oligomers) (Part 1), C60 and single-walled carbon nanotube ([C60+(4,4)] carbon nanobud) (Part 2), and C60 and graphene ([C60+(5,5)] and [C60+(9,8)] graphene nanobuds) (Part 3). C60 dimers and oligomers are considered in the current paper. The formation of composites is considered from the basic points related to the regioselective chemical reactivity of the fullerene molecule atoms. The dissonance between the predicted trimer and tetramer structures and experimental observations is suggested to evidence the topological nature of the C60 oligomerization. The barrier that governs the oligomer formation is determined in terms of the coupling energy and is expanded over two contributions that present the total energy of deformation of the composites' components and the energy of covalent coupling . The computations were performed by using the AM1 semiempirical version of unrestricted broken symmetry Hartree-Fock approach.

cond-mat.mtrl-sci↗

arXiv2011

C60-Based Composites in view of Topochemical Reactions. II. C60 + Carbon Nanobuds

The current paper presents the second part of the study devoted to composites formed by fullerene C60 and single-walled carbon nanotube ([C60-(4,4)] carbon nanobud). The formation of composites is considered from the basic points related to the atomic chemical reactivity of the fullerene molecule and carbon nanotube. The barrier that governs the composites formation is determined in terms of the coupling energy and is expanded over two contributions that present the total energy of deformation of the composites' components and the energy of covalent coupling . The computations were performed by using the AM1 semiempirical version of unrestricted broken symmetry Hartree-Fock approach.

cond-mat.mtrl-sci↗

arXiv2011

C60-Based Composites in view of Topochemical Reactions. III. C60 + Graphene Nanobuds

The current paper presents the third part of the study devoted to composites formed by fullerene C60 and two graphene nanosheets ([C60+(5,5)] and ([C60+(9, 8)] graphene nanobuds). The formation of composites is considered from the basic points related to the atomic chemical reactivity of the fullerene molecule and nanographene. The barrier that governs the composites formation is determined in terms of the coupling energy and is expanded over two contributions that present the total energy of deformation of the composites' components and the energy of covalent coupling. In view of these energetic parameters and in contrast to expectations, seemingly identical reactions that are responsible for the formation of intermolecular contacts in (C60)2 dimer, [C60+(4,4)] carbon nanobuds, and [C60+(5,5)] and [C60+(9,8)] graphene nanobuds result in different final products. The peculiarity is suggested to result from a topochemical character of the covalent coupling between two members of the sp2 nanocarbons' family. The computations were performed by using the AM1 semiempirical version of unrestricted broken symmetry Hartree-Fock approach.

cond-mat.mtrl-sci↗

arXiv2010

Synergistic Nanophotonics of Fullerene

A deep similarity of photo-stimulated effects occurring in physical and biological objects involving fullerene forces to raise the question: what is meant under nanophotonics of fullerene and if should we not imply under this conventional term, usually restricted to optical events, something more general? Discussed in the paper, makes it possible to suggest that the formation of positive-negative fullerene ion pair at each photon absorption act is common for photo stimulated events in chemistry, medicine, and optics thus providing their common origin.

physics.chem-ph↗

arXiv2010

Nanochemistry of fullerene c60. cyano- and azo-polyderivatives

Cyanation C60 to C60(CN)18 and aziridination from C60 to C60(NH)9 have been studied by unrestricted broken spin symmetry Hartree-Fock approach implemented in semiempirical codes based on AM1 technique. The calculations were focused on successive addition of CN and NH moieties to the fullerene cage following the indication of the cage target atoms by the highest atomic chemical susceptibility calculated at each step. The obtained results are analyzed from the viewpoint of criteria on parallelism between these derivatives as well as C60 fluorides and hydrides. The difference of the first stage C60 chlorination from other sterically free processes is discussed.

cond-mat.mes-hall↗

arXiv2010

Continuous symmetry of C60 fullerene and its derivatives

Conventionally, the Ih symmetry of fullerene C60 is accepted which is supported by numerous calculations. However, this conclusion results from the consideration of the molecule electron system, of its odd electrons in particular, in a close-shell approximation without taking the electron spin into account. Passing to the open-shell approximation has lead to both the energy and the symmetry lowering up to Ci. Seemingly contradicting to a high-symmetry pattern of experimental recording, particularly concerning the molecule electronic spectra, the finding is considered in the current paper from the continuous symmetry viewpoint. Exploiting both continuous symmetry measure and continuous symmetry content, was shown that formal Ci symmetry of the molecule is by 99.99% Ih. A similar continuous symmetry analysis of the fullerene monoderivatives gives a reasonable explanation of a large variety of their optical spectra patterns within the framework of the same C1 formal symmetry exhibiting a strong stability of the C60 skeleton.

cond-mat.mes-hall↗