arXiv subjects

Elena F. Sheka

Publications and source records attributed to Elena F. Sheka.

At least 19 recordsLinked to original sources

arXiv2024

Digital twins kinetics of virtual free-radical copolymerization of vinyl monomers with stable radicals. 3. N-isopropyl acrylamide

The first experience of virtual free-radical copolymerization (FRCP) of vinyl monomers with stable radicals in the framework of the digital twins (DTs) concept (arXiv:2309.11616 [physics.chem-ph] and arXiv:2311.02752 [cond-mat.mtrl-sci]) is extended to N-isopropylacrylamide (NIPA). The virtualization of the chemical process is based on the concept of a chain reaction that covers a set of elementary reactions (ERs) and is the most suitable for quantum chemical treatment. The calculations were performed using a semi-empirical version of the unrestricted Hartree-Fock approximation. Once input in the chemical reactor, providing free-radical polymerization of NIPA, the fullerene C60 inhibits capturing the formed monomer-radicals and terminates the polymerization. The obtained virtual kinetic data, providing the appearance of an induction period at the initial stage of the NIPA polymerization, are in a full agreement with experimental reality.

cond-mat.mtrl-sci↗

arXiv2024

Digital twins kinetics of virtual free-radical copolymerization of vinyl monomers with stable radicals. 1. Methyl methacrylate

The first experience of virtual free-radical polymerization of a set of vinyl monomers in the framework of the digital twins (DTs) concept (Polymers 2023, 15, 2999) is extended to methyl methacrylate, copolymerized with small additives of stable radicals such as fullerene C60 and TEMPO. The virtualization of chemical processes is based on the assumption of basic chain reactions that lay the foundation of polymerization. In the current study, a set of elementary reactions, covering the initial stage of the polymerization, is considered. The reactions are the most suitable for quantum chemical treatment. The calculations, covering about 30 DTs, were carried out using a semi-empirical version of the unrestricted Hartree-Fock approximation. The main energy and spin-density parameters of the ground state of DTs are determined. The barrier profiles of three DTs were calculated, which led the foundation for determining the activation energy of the studied processes. The decisive role of spins in the formation of transition states of these processes is confirmed. The two stable radicals behave quite differently with respect to the polymerization of the studied monomer. The action of fullerene C60 concerns mainly the capturing of the initiating free radicals, anchoring them to the molecule body and thus slowing the main polarization process. In contrast, TEMPO effectively capture monomer radicals manifesting itself as a killer of the polymerization thus providing the appearance of the induction period in practice. The obtained virtual kinetic data are in a full consent with experimental reality.

physics.chem-ph↗

arXiv2024

Digital twins kinetics of virtual free-radical copolymerization of vinyl monomers with stable radicals. 2. Styrene

The first experience of virtual free-radical copolymerization (FRCP) of methyl methacrylate with stable radicals fullerene C60 and TEMPO in the framework of the digital twins (DTs) concept (arXiv:2309.11616 [physics.chem-ph]) is extended to styrene. The virtualization of the chemical process is based on a conceptual view of this process from the perspective of a chain reaction that covers a set of elementary reactions (ERs). Such an approach is the most suitable for quantum chemical treatment. The calculations, concerning about 50 such ERs as well as associated DTs, were performed using a semi-empirical version of the unrestricted Hartree-Fock approximation. The main energy and spin-density parameters of the DTs ground state are determined. The barrier profiles of the selected DTs were calculated for the activation energy of the studied reactions to be determined. The decisive role of spins in the formation of transition states of these processes was confirmed. The two stable radicals behave quite differently with respect to the standard FRP of styrene. TEMPO effectively captures monomer-radicals of styrene with the acting free radical not depending on which namely radical is in use. Its copolymerizing action manifests itself as a killing of the FRP thus providing the appearance of an induction period (IP) in the total process kinetics. In contrast, the copolymerizing action of fullerene drastically depends on the active free radical. In the case of the alkyl-nitrile one, the fullerene switches to the role of initiator and first provides the styrene polymerization followed with anchoring of the formed polymer chains to the molecule body, after which the standard FRP starts. In the case of benzoyl-peroxide one, fullerene acts an inhibitor thus capturing formed monomer-radicals. The obtained virtual kinetic data are in a full consent with experimental reality.

cond-mat.mtrl-sci↗

arXiv2023

Digitalization of Free-Radical Polymerization

Current paper presents the first attempt of the digitalization of a complex polymerization process representing free-radical polymerization of vinyl monomers in the presence of small additives of stable radicals TEMPO and C60 fullerene. Chain-reaction essence of the chemical process; quantum-chemical concept of elementary reactions; digital-twin presentation of their participants; virtual thermodynamic and kinetic descriptors of the latters; digital polymeryzation passports of individually composed reaction solutions lay the foundation of the digitalization carried out. Polymerization-passport predictions made for a number of solutions with different vinyl monomers and/or different initiating free radicals are fully consistent with chemical reality.

physics.chem-ph↗

arXiv2022

Digital Twins in Graphene Technology

The Digital Twins concept in science has a long history that goes back to the beginnings of now widely accepted modelling. The ever-expanding amount of digital data accompanying modelling could not but cause a qualitative transition from modelling, subordinated to the goal of reproducing a real object, to an equal-right Digital Twins concept that offers an independent view of the real object. This chapter presents the first example of such a conceptual rethinking, using the example of material science of high-tech graphene materials.

cond-mat.mtrl-sci↗

arXiv2022

Digital Twins solve the mystery of Raman spectra of parental and reduced graphene oxides

A still amazing identity of the D-G doublet Raman spectra of parental and reduced graphene oxides is considered from the digital twins viewpoint. About thirty DTs, presenting different aspects of the GO structure and properties, were virtually synthesized using atomic spin-density algorithm, which allowed reliably displaying reasons for this extraordinary spectral feature. In both cases, it was established that the D-G doublets owe their origin to the sp3-sp2 C-C stretchings, respectively. This outwardly similar community of the doublets origin of GO and rGO is thoroughly analyzed to reveal different grounds of the feature in the two cases. Multilayer packing of individual rGO molecules in stacks, in the first case, and spin-influenced prohibition of the 100% oxidative reaction, the termination of which is accompanied with a particular set of highly ordered by length sp3- and sp2 C-C bonds, protecting the carbon carcass from destruction caused by the stress induced sp2-to-sp3 transformation, in the second, are the main reasons. The DT concept has been realized on the basis of virtual vibrational spectrometer HF Spectrodyn.

cond-mat.mtrl-sci↗

arXiv2020

Virtual spectrometer for radicals vibrations. 1. Polyacenes and fullerenes

A virtual vibrational spectrometer, based on efficient software exploring unrestricted Hartree-Fock approximation, has been proposed to perform computational spectroscopy of radicals. The computational device allows obtaining spectra of IR absorption and Raman scattering of large molecules. A number of test cases involving naphthalene and pentacene as well as fullerenes C60 and C70 will be discussed in order to point out strengths, limitations, and ongoing developments of the spectrometer.

physics.chem-ph↗

arXiv2020

Virtual spectrometer for stable radicals vibrations. 2. Graphene molecules

The article presents the result of an extended in silico experiment on computational vibrational spectroscopy of graphene molecules performed using the virtual vibrational spectrometer UHF VVS, previously proposed in the first part of the study [1]. Since in vitro spectroscopy of such individual molecules is practically impossible, the obtained spectra give the first idea of what these spectra could be. A large set of studied molecules allows drawing the first conclusions about the specific features of the vibrational dynamics of the substances, caused by their radical structure and spin features of the ground electronic state.

cond-mat.mtrl-sci↗

arXiv2020

Amorphous state of sp$^2$ solid carbon

Two-mode valence electron configuration of carbon atoms lays the foundation of the unique two-mode amorphous state of the monoatomic carbon solid. From the fundamentals of solid-state physics, sp$^3$ and sp$^2$ amorphous carbons are two different amorphous species characterized by conceptually different short-range orders, namely, groups of tetrahedrally bonded sp3 configured atoms and size-restricted sp$^2$ graphene domains framed with heteroatom necklaces. Molecular character of sp$^2$ amorphics is coherent with the reaction mechanism of the solid amorphicity in due course of the forced fragmentation.

cond-mat.mtrl-sci↗

arXiv2019

Graphene oxyhydride catalysts in view of spin radical chemistry

The article discusses carbocatalysis provided with amorphous carbons. The discussion is conducted from the standpoint of the spin chemistry of graphene molecules, in the framework of which the amorphous carbocatalysts are a conglomerate of graphene-oxynitrothiohydride stable radicals presenting the basic structural units (BSUs) of the species. The chemical activity of the BSUs atoms is reliably determined computationally, which allows mapping the distribution of active sites in these molecular catalysts. The presented maps reliably evidence the BSUs radicalization provided with carbon atoms only, the non-terminated edge part of which presents a set of active cites. Spin mapping of carbocatalysts active cites is suggested as the first step towards the spin carbocatalysis of the species.

cond-mat.mtrl-sci↗

arXiv2019

Spin chemistry of sp2 nanocarbons

Suggested in the paper is the consideration of stable radicals of sp2 nanocarbons from the standpoint of spin-delocalized topochemistry. Characterized in terms of the total and atomically-partitioned number of effectively unpaired electrons as well as of the distribution of the latter over carbon atoms and described by selectively determined barriers of different reactions exhibiting topological essence of intermolecular interaction, sp2 nanocarbons reveal a peculiar topokinetics that lays the foundation of the stability of their radical properties.

cond-mat.mes-hall↗

arXiv2017

Optical Spectra of Open-Shell and Closed-Shell Graphene-Based Molecules

We have computationally investigated absorption spectra of a specifically configured set of graphene-based molecules involving (1) a sp2 bare graphene sheet; (2) framed graphene sheets containing different chemical addends terminating dangling bonds of edge atoms but keeping sp2 configured basal plane; and (3) bulk sp3 graphene sheets resulted from the chemical modification occurred at not only the bare sheet circumference but at its basal plane as well. Framed molecules, open-shell by nature, present different kinds of reduced graphene oxides and present the main building blocks of graphene quantum dots. Closed-shell bulk molecules present models of nanosize graphene oxide. UHF ground states and ZINDO/S excited states of the molecules were analyzed. The UHF-ZINDO/S combination is well coherent in the case of bulk molecules for which UHF and RHF ground state results are identical. In the case of framed molecules, the incoherence of the UHF and close-shell ZINDO/S approaches is revealed in a considerable decreasing of the HOMO-LUMO energy gap, which provides unreal drastic red shift of absorption spectra of open-shell molecules into IR region. The conclusion is justified by a comparative analysis of calculated and experimental data on absorption and fluorescence spectra of graphene quantum dots and graphene oxides.

cond-mat.mtrl-sci↗

arXiv2016

Dirac Material Graphene

The paper presents the author view on spin-rooted properties of graphene supported by numerous experimental and calculation evidences. Dirac fermions of crystalline graphene and local spins of graphene molecules are suggested to meet a strict demand - different orbitals for different spins- which leads to a large spectrum of effects caused by spin polarization of electronic states. The consequent topological non-triviality, making graphene topological insulator, and local spins, imaging graphene chemical activity, are proposed to discuss such peculiar properties of graphene as high temperature ferromagnetism and outstanding chemical behavior. The connection of these new findings with difficulties met at attempting to convert semimetal graphene into semiconductor one is discussed.

cond-mat.mtrl-sci↗

arXiv2016

Reduced Graphene Oxide and Its Natural Counterpart Shungite Carbon

Large variety of structure and chemical-composition of reduced graphene oxide (RGO) is explained from a quantum-chemical standpoint. The related molecular theory of graphene oxide, supported by large experience gained by the modern graphene science, has led the foundation of the concept of a multi-stage graphene oxide reduction. This microscopic approach has found a definite confirmation when analyzing the available empirical data concerning both synthetic and natural RGO products, the latter in view of shungite carbon, suggesting the atomic-microscopic model for its structure.

cond-mat.mtrl-sci↗

arXiv2015

Spin-orbit concept of open-shell systems

The paper presents a first attempt to look at the peculiarities of open-shell systems from the viewpoint of spin-orbit coupling. Contrary to common opinion of a negligible role of the issue for light element molecules, presented in the paper evidences its governing role in such systems peculiarities on example of sp2 nanocarbons. A particular attention is given to the justification of the validity of UHF approach to exhibit the SOC peculiarities of open-shell systems.

physics.chem-ph↗

arXiv2015

Spin-Orbit Coupling Parameters of sp2 Nanocarbons

The paper presents a new approach to the determination of standard spin-orbit coupling parameters, such as the SOC constant a_SO and the Lande parameter g, by using peculiarities of UHF results for open-shell systems. The approach is convincingly approved for stretched ethylene and fullerene C60 as examples, thus justifying at once a high efficacy of the UHF formalism to disclose SOC features.

cond-mat.mes-hall↗

arXiv2014

Molecular theory of graphene chemical modification

The chapter is devoted to chemical uniqueness of graphene. In view of molecular theory, the latter is a consequence of benzenoid packing of carbon atoms, based on three neighbors to each carbon atom leaving the atom fourth valence electron (the odd one) on its own. Due to rather large C-C distances, the odd electrons of graphene molecules are correlated whereupon the molecules are radicalized and molecular chemistry of graphene is the chemistry of dangling bonds. The chapter presents the description of the molecular-theory-dangling-bond view on graphene molecule hydrogenation and oxidation. Specific peculiarities of the graphene molecular chemistry related to size-dependent topochemical features are discussed, as well

cond-mat.mtrl-sci↗

arXiv2014

Photonics of shungite quantum dots

Shungite quantum dots are associated with nanosize fragments of reduced graphene oxide similarly to synthetic graphene quantum dots thus forming a common class of GQDs. Colloidal dispersions of powdered shungite in water, carbon tetrachloride, and toluene form the ground for the GQD photonic peculiarities manifestation. 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.mtrl-sci↗