Carbon Hydrogen Atoms Research Articles

Efficiency of the top-down polycyclic aromatic hydrocarbon-to-fullerene conversion in ultraviolet irradiated environments

ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) and fullerenes play a major role in the physics and chemistry of the interstellar medium (ISM). Based on a number of recent experimental and theoretical investigations we developed a model in which PAHs are subject to photo-dissociation (carbon and hydrogen loss) and hydrogenation. We take into account that dehydrogenated PAHs may fold into closed structures – fullerenes. Fullerenes, in their turn, can be also hydrogenated, becoming fulleranes, and photo-dissociated, losing carbon and hydrogen atoms. The carbon loss leads to shrinking of fullerene cages to smaller ones. We calculate the abundance of PAHs and fullerenes of different sizes and hydrogenation level depending on external conditions: the gas temperature, intensity of radiation field, number density of hydrogen atoms, carbon atoms, and electrons. We highlight the conditions, which are favourable for fullerene formation from PAHs, and we conclude that this mechanism works not only in H-poor environment but also at modest values of hydrogen density up to 104 cm−3. We found that fulleranes can be formed in the ISM, although the fraction of carbon atoms locked in them can be maximum around 10−9. We applied our model to two photo-dissociation regions, Orion Bar and NGC 7023. We compare our estimates of the fullerene abundance and synthetic band intensities in these objects with the observations and conclude that our model gives good results for the closest surroundings of ionizing stars. We also demonstrate that additional fullerene formation channels should operate along with ultraviolet (UV)-induced formation to explain abundance of fullerenes far from UV sources.

Microscopic Insight into Water Desalination through Nanoporous Graphene: The Influence of the Dipole Moment.

Nanoporous graphene membranes with controllable pore size and chemical functionality may be one of the most desirable materials for water desalination. Herein, we investigate desalination performance of hydrogen-functionalized nanoporous graphene membranes. The charge values on hydrogen atoms (qH) and carbon atoms at the pore rim are systematically adjusted. For qH > 0, the flow rate decreases as qH increases, whereas for qH < 0, the flow rate tends to increase first and then decrease with increasing qH, yielding a peak at ∼ -0.2 e. Moreover, nanopores with large dipole moments at the rim have little effect on the salt rejection. The calculated oxygen and hydrogen density maps, the potential of mean force for water molecule and salt ion passage through the nanopores, and the coordination number unveil the mechanisms underlying water desalination in nanoporous graphene. This work may inspire the design and improvement of two-dimensional membranes for water desalination.

Effect of hydrogenation of carbon atom on its deposition on graphene

This article discusses the possibility of graphene functionalization by the deposition of carbon atoms with different degrees of hydrogenation and the effect of hydrogenation on the probability of precipitation using computer simulation. According to the results of computer simulation, the binding energy of the single carbon atom, methine, methylene and methyl is more than 1 eV. This corresponds to chemisorption, which means that the chemical bonds are generated due to unpaired valent electrons of the incident particles and defect-free graphene. Methane does not have unpaired valent electrons and in the range of investigated deposition energies its chemisorption on graphene is not observed. The free carbon atom with various degrees of its hydrogenation is considered: methane CH, methylene CH2, methyl CH3 and methane CH4. It is established that if the deposited particles C, CH, CH2 and CH3 form a bound state with graphene, these particles are located near the characteristic points with a different probability of location above graphene at these points. The processes of deposition of hydrogenated and non-hydrogenated carbon atoms with kinetic energies of 1.0, 1.2, 1.5, 2.3 and 3.1 eV on graphene are studied. It is found that the processes of deposition of non-hydrogenated and hydrogenated carbon atoms on defect-free graphene depend on the degree of hydrogenation of the carbon atom: the more hydrogenated the carbon atom is, the less its probability of chemisorption on graphene. Methane, as a fully hydrogenated carbon atom, does not chemisorb on graphene at deposition energies from 1 to 3.1 eV. It is also observed that the maximum probability of chemisorption of C, CH, CH2 and CH3 on graphene took place at deposition energy of about 2.3 eV.

Kinetics of oxygen transfer reactions on the graphene surface. Part II. H2O vs. CO2

This computational chemistry analysis compares the interactions of H2O and CO2 with zigzag graphene edges, and in particular their adsorption and desorption steps. We provide detailed information regarding the geometric and electronic factors that influence both their adsorption and desorption (of H2 and CO) processes. Density functional theory results are compared with experimental information to offer heretofore unavailable insights into key aspects of the rate-limiting steps, inhibition phenomena and nanoscale differences in these two reactions. Thus, for example, the orientation and rotation of the adsorbing H2O molecules are elucidated using intrinsic reaction coordinate calculations and molecular orbital analysis, and these results complement recent pulse field gradient NMR measurements and molecular dynamics calculations. Such mechanistic findings reveal the H2O adsorption process to be a slow rotational phenomenon whereas the low-temperature H2 desorption is geometrically constrained by the presence of contiguous (di)hydrogenated carbon atoms with or without hydroxyl groups. This surface-assisted desorption mechanism is proposed to be responsible for the formation of hexagonal pits during the graphite-H2O reaction. Similar insights were obtained for the graphene-CO2 reaction. Mechanistic schemes are proposed for the desorption products observed in experiments, distinguishing zigzag from armchair sites.

Crystal Structure, Packing, and Analysis of Hirshfeld Surfaces of 3a-(p-tolyl)-3,3a-dihydrobenzo[d]pyrrolo[2,1-b]thiazol-1(2H)-one

Crystal and molecular structures of 3a-(p-tolyl)-3,3a-dihydrobenzo[d]pyrrolo[2,1-b]thiazole-1(2H)-one obtained by condensing 4-oxo-4-(p-tolyl)butane acid with 2-aminothiophenol are studied using XRD spectroscopy. The crystal of compound 1 occurs in the form of two crystallographically independent molecules. The crystal has no hydrogen bonds and stacking interactions, but the analysis of Hirshfeld surfaces shows that the molecules are weakly stabilized by non-covalent interactions between oxygen atoms and hydrogen atoms of methyl groups as well as by the interactions between hydrogen atoms and carbon atoms of the benzothiazole fragment.

Tuning the electronic structure of single-walled carbon nanotube by high-pressure H2 exposure

We report on an electronic structure change of single-walled carbon nanotube (SWNT) on hexagonal boron nitride due to electron doping via high-pressure H2 exposure. The fractional coverage of hydrogenated carbon atom is estimated to be at least θ = 0.163 from the in situ Ids–Vg measurements of the release process. Raman spectroscopy and x-ray photoelectron spectroscopy were carried out to support the in situ electrical measurements. In particular, we used the dissociative Langmuir-type model to yield the desorption coefficient kdes by fitting it to the in situ electrical data. Finally, we applied this hydrogenation method to the SWNT network on the commercial Si/SiO2 substrate to open the possibility of the scalable n-type semiconducting SWNT FETs.

Zigzag graphene nanoribbons separated by hydrogenation

As the integrated circuits continue to miniaturize, the narrower interconnect matched is needed. Using density functional theory combined with non-equilibrium Green's function, we try to adopt a row or a column of hydrogen atoms adsorbed on the zigzag graphene nanoribbons, which are in the middle parallel or perpendicular to the axial direction to limit the movement of π electron. The utilization of the adsorbed hydrogen row blocks the transport of electrons, zigzag graphene nanoribbons are divided into conductive and non-current part. Physically, the nanoribbons are separated laterally or longitudinally to two strips that contain fewer atoms, preserving the structural integrity of zigzag graphene nanoribbons. According to the transmission pathways after hydrogenation, there is almost no current flowing through the intermediate hydrogenated carbon chains when hydrogenated in the direction of the ribbon axis; while perpendicular to the ribbon axis, except for the loop current among the two edge carbon atoms and the hydrogenated carbon atoms, only a small part of the current flows along the zigzag axial direction. In the application of graphene nanoribbons, we usually want to use semiconductor nanoribbons as devices, and metal nanoribbons for interconnects. Our results indicate that zigzag graphene nanoribbons can be qualitatively separated into two narrower strips by regular adsorption of hydrogen to connect smaller devices.

Photochemical Rearrangement of Diarylethenes: Reaction Efficiency and Substituent Effects.

In recent years, great synthetic potential of the photorearrangement of diarylethenes leading to naphthalene derivatives via a cascade process of photocyclization/[1,n]-H shift/cycloreversion has been demonstrated. In this work, first a multifaceted study of the influence of various factors on the efficiency of the photorearrangement of diarylethenes of furanone series containing benzene and oxazole derivatives as aryl residues has been carried out. The efficiency of this phototransformation (quantum yields) and the effect of methoxy substituents in the phenyl moiety have been studied. Despite the multistage process, the quantum yields of the photorearrangement are rather high (0.34-0.49). It has been found that the efficiency of photocyclization of diarylethenes increases with the introduction of electron-donating methoxy groups in the phenyl moiety. Using the DFT calculations, we have been able to estimate in the photoinduced isomer the distance between hydrogen atom and carbon atom to which it migrates in the result of the sigmatropic shift. For all studied diarylethenes, this value was 2.67-2.73 Å, which is less than the sum of van der Waals radii of carbon and hydrogen atoms (2.9 Å).

Effect of collector molecular structure on the wettability of gold for froth flotation

Molecular dynamics simulations were conducted to evaluate the alteration of the hydrophilic state of gold surfaces caused by the adsorption of collectors with different molecular structures, using the contact angle of water droplets as an evaluation parameter. Four collectors were evaluated: SDS (with twelve hydrogenated carbon atoms), PAX (with five hydrogenated carbon atoms), DTP (with two branched aliphatic chains) and MBT (with an aromatic ring). The contact angle was evaluated for coatings of a monolayer (ML) and for surface densities of 2.89μmol/m2 for each collector. For a ML, the hydrophobic effect generated by the aromatic ring of the MBT collector is comparable with the effect of the non-polar short chain of the PAX collector. The increase in hydrophobicity for the gold surfaces achieved by collectors with aliphatic chains is because the water-collector interaction energy is significantly higher (repulsive) than the water-gold interactions (attractive). The lowest increase in hydrophobicity was achieved with the MBT collector, since the carbon-water interaction energy of the aromatic ring is stronger than the interaction with the carbon atoms in the aliphatic chains. The calculated contact angles of the water droplets deviated less than 4% with respect to the experimental values.

Density Functional Theory Calculations, Spectroscopic (FT-IR, FT-RAMAN), Frontier Molecular Orbital, Molecular Electrostatic Potential Analysis of 5-Fluoro-2-Methylbenzaldehyde

Abstract The FT-IR and FT-Raman spectra of 5-fluoro-2-methylbenzaldehyde (5F2MB) have been recorded in the regions 4000–400 cm−1 and 4000–100 cm−1, respectively. Using the observed FT-IR and FT-Raman data, a complete vibrational assignment and analysis of the fundamental modes of vibrations of the compound were carried out. The optimum molecular geometry, vibrational frequencies and Raman scattering activities were calculated by density functional theory with B3LYP/6-311+G** basis sets. A close agreement was achieved between the calculated and observed frequencies by refinement of scale factors. Unambiguously normal modes of vibrational assignments have been made with the help of the GUASSVIEW 5.0 program and the potential energy distribution (PED) calculated by the density functional theory calculations. Natural bonding orbital (NBO) analyses were carried out to discuss the stability of the molecule. The molecular electrostatic potential (MEP) map and electron density map were drawn and analyzed. Using nuclear magnetic resonance (NMR) analysis, the chemical shifts of hydrogen atoms and carbon atoms were calculated. The charge distribution, Mullikan atomic charge values and HOMO–LUMO energy have been calculated to explore the reasons for the change in biological activity. Furthermore, the first hyperpolarizability and the dipole moment of the molecule have also been calculated.

Mechanical properties of hydrogenated electron-irradiated graphene

We report a systematic analysis on the effects of hydrogenation on the mechanical behavior of irradiated single-layer graphene sheets, including irradiation-induced amorphous graphene, based on molecular-dynamics simulations of uniaxial tensile straining tests and using an experimentally validated model of electron-irradiated graphene. We find that hydrogenation has a significant effect on the tensile strength of the irradiated sheets only if it changes the hybridization of the hydrogenated carbon atoms to sp3, causing a reduction in the strength of irradiation-induced amorphous graphene by ∼10 GPa. Hydrogenation also causes a substantial decrease in the failure strain of the defective sheets, regardless of the hybridization of the hydrogenated carbon atoms, and in their fracture toughness, which decreases with increasing hydrogenation for a given irradiation dose. We characterize in detail the fracture mechanisms of the hydrogenated irradiated graphene sheets and elucidate the role of hydrogen and the extent of hydrogenation in the deformation and fracture processes. Our study sets the stage for designing hydrogenation and other chemical functionalization strategies toward tailoring the properties of defect-engineered ductile graphene.

Spectral investigations, DFT based global reactivity descriptors, Inhibition efficiency and analysis of 5-chloro-2-nitroanisole as π-spacer with donor-acceptor variations effect for DSSCs performance

FTIR, FT-Raman, UV, NMR and quantum chemical calculation studies are performed on 5-chloro-2-nitroanisole, in order to gain the insights of its structural, spectroscopic and electronic properties (Fukui indices, HOMO and LUMO energy gap, MESP and Global reactivity descriptors). A complete vibrational analysis of 5-chloro-2-nitroanisole is performed by HF/B3LYP methods using 6-31G(d,p) basis set. To estimate the electronic transitions, the UV spectra of title compound are predicted in gas phase and ethanol. The obtained absorption maxima at 389.94 nm (in ethanol) is predicted possibly due to HOMO→LUMO transition with 85% contribution and assigned as π-π*. The MESP map shows that the negative potential sites are localized on oxygen atom (O10) as well as the positive potential sites are identified around the hydrogen and ring carbon atoms. The analysis of Fukui indices is also carried out to distinguish the nucleophilic and electrophiic centers. The prediction of reactive sites by MESP is well supported by this Fukui indices analysis. The correlations between the statistical thermodynamics and temperature are also obtained. It is seen that the heat capacities, entropies and enthalpies increase with increasing the intensities of the molecular vibrations. Furthermore, the first hyperpolarizability of 5-chloro-2-nitroanisole is calculated and the results are discussed. This result indicates that 5-chloro-2-nitroanisole is a good candidate of nonlinear optical materials.

One Side-Graphene Hydrogenation (Graphone): Substrate Effects

ABSTRACTRecent studies on graphene hydrogenation processes showed that hydrogenation occurs via island growing domains, however how the substrate can affect the hydrogenation dynamics and/or pattern formation has not been yet properly investigated. In this work we have addressed these issues through fully atomistic reactive molecular dynamics simulations. We investigated the structural and dynamical aspects of the hydrogenation of graphene membranes (one-side hydrogenation, the so called graphone structure) on different substrates (graphene, few-layers graphene, graphite and platinum). Our results also show that the observed hydrogenation rates are very sensitive to the substrate type. For all investigated cases, the largest fraction of hydrogenated carbon atoms was for platinum substrates. Our results also show that a significant number of randomly distributed H clusters are formed during the early stages of the hydrogenation process, regardless of the type of substrate. These results suggest that, similarly to graphane formation, large perfect graphone-like domains are unlikely to be formed. These findings are especially important since experiments have showed that cluster formation influences the electronic transport properties in hydrogenated graphene.

Reaction of ethylene molecules with a nickel cluster: &lt;i&gt;ab initio&lt;/i&gt; molecular dynamics study

The reaction of ethylene molecules with a Ni cluster was investigated by ab initio molecular simulations. We found that hydrogen atoms on the Ni cluster hamper dehydrogenation of ethylene molecules, which implies that carbon atoms are not supplied into the Ni cluster when the number of the hydrogen atoms is saturated. However, the hydrogen atoms were removed from the cluster by producing hydrogen molecules and carbon atoms are continuously supplied. This fact suggests that the production of the hydrogen molecules determines the rate of supplying carbon atoms. We investigated activation energies to form the hydrogen molecules on the Ni cluster, and discussed the mechanism of efficient removal of the hydrogen atoms. Key words: ethylene molecule, nickel cluster, carbon nanotube, dehydrogenation, hydrogen molecule

Flash Pyrolysis of Coal with Solid Heat Carrier in a Fluidized Bed

Flash pyrolysis of Shenmu coal with solid heat carrier was carried out in a fluidized bed using semi-coke as the solid heat carrier and nitrogen as the carrier gas. The effects of pyrolysis temperature, reaction time and mass ratio of heat carrier to coal on the yields of products were studied. It is found that the best operating conditions involving pyrolysis temperature 550°C, reaction time 6 min and mass ratio of heat carrier to coal 2. The properties of coal tar from fluidized bed, such as density, viscosity, freezing point, carbon residue and hydrogen carbon atom ratio, are almost higher than that of the above water coal tar and lower than that of the below water coal tar, while the above and below water coal tar obtained from Sanjiang squared retort. The results of simulation distillation show that gasoline and diesel fractions of coal tar from fluidized bed are higher than that of below water coal tar and lower than that of above water coal tar, while the heavy oil fraction is opposed.

Structure and stability of hydrogenated carbon atom vacancies in graphene

Adsorption of hydrogen atoms to a carbon atom vacancy in graphene is investigated by means of periodic first principles calculations, up to the fully hydrogenated state where six H atoms chemically bind to the vacancy. Addition of a single H atom is highly exothermic and barrierless, and binding energies remain substantial for further hydrogenation, with a preference towards structures with the least number of geminal pairs. Thermodynamic analysis shows that defective graphene is extremely sensitive to hydrogenation, with the triply hydrogenated anti-structure prevailing at room temperature and for a wide range of H2 partial pressures, from ∼1 bar down to <10−20bar. This structure has one unpaired electron and provides a spin-half local magnetic moment contribution to graphene paramagnetism. Comparison of our results with recent transmission electron microscopy, scanning tunneling microscopy and muon-spin-resonance experiments suggest that carbon atom vacancies may actually be hydrogenated to various degrees under varying conditions.

Elektronni vlastyvosti funktsionalizovanykh hrafenovykh nanostrichok

Distributions of valence electron density in and the electron energy spectrum of graphene nanoribbons covered with hydrogen, fluorine, or oxygen atoms have been calculated ab initio in the framework of the density functional and pseudopotential theories. The emergence of a forbidden gap for graphene nanoribbons with zigzag edges and 9.23 ˚ A in width and its absence in an unconfined graphene plane are shown. The forbidden gap is demonstrated to decrease, as the graphene nanoribbon width increases. For graphene nanoribbons with hydrogen-decorated edges, the energy gap disappears. The interaction between a hydrogen atom and carbon atoms in the graphene nanoribbon plane that are coordinated in accordance with the sp2-hybridization is shown to induce local changes of the hybridization to the sp3 type.

Hydrogenation enabled scrolling of graphene

Hydrogenation of graphene leads to local bond distortion of each hydrogenated carbon atom. Therefore, programmable hydrogenation of graphene can open up new pathways to controlling the morphology of graphene and therefore enable the exploration of graphene-based unconventional nanomaterials. Using molecular dynamics simulations, we show that single-sided hydrogenation can cause the scrolling of graphene. If a proper size of the graphene is hydrogenated on one side, the graphene can completely scroll up into a carbon nanoscroll (CNS) that remains stable at room temperature. We perform extensive simulations to delineate a diagram in which three types of scrolling behaviour of partially single-sided hydrogenated graphene are identified in the parameter space spanned by the hydrogenation size and the graphene size. Such a diagram can serve as quantitative guidelines that shed important light on a feasible solution to address the challenge of fabricating high-quality CNSs, whose open topology holds promise to enable novel nanodevices.

Theoretical investigations of the H···π and X (X = F, Cl, Br, I)···π complexes between hypohalous acids and benzene

The H···π and X (X = F, Cl, Br, I)···π interactions between hypohalous acids and benzene are investigated at the MP2/6-311++G(2d,2p) level. Four hydrogen-bonded and three halogen-bonded complexes were obtained. Ab initio calculations indicate that the X···π interaction between HOX and C(6)H(6) is mainly electrostatically driven, and there is nearly an equal contribution from both electrostatic and dispersive energies in the case of XOH-C(6)H(6) complexes. Natural bond orbital (NBO) analysis reveals that there exists charge transfer from benzene to hypohalous acids. Atom in molecules (AIM) analysis locates bond critical points (BCP) linking the hydrogen or halogen atom and carbon atom in benzene.

A systematic study of electronic structure from graphene to graphane

While graphene is a semi-metal, a recently synthesized hydrogenated graphene calledgraphane is an insulator. We have probed the transformation of graphene uponhydrogenation to graphane within the framework of density functional theory. By analysingthe electronic structure for 18 different hydrogen concentrations, we bring out some novelfeatures of this transition. Our results show that the hydrogenation favours clusteredconfigurations leading to the formation of compact islands. The analysis of the chargedensity and electron localization function (ELF) indicates that, as hydrogen coverageincreases, the semi-metal turns into a metal, showing a delocalized charge density, thentransforms into an insulator. The metallic phase is spatially inhomogeneous in the sense itcontains islands of insulating regions formed by hydrogenated carbon atoms andmetallic channels formed by contiguous bare carbon atoms. It turns out that it ispossible to pattern the graphene sheet to tune the electronic structure. For example,removal of hydrogen atoms along the diagonal of the unit cell, yielding an armchairpattern at the edge, gives rise to a bandgap of 1.4 eV. We also show that a weakferromagnetic state exists even for a large hydrogen coverage whenever there isa sublattice imbalance in the presence of an odd number of hydrogen atoms.