Pentagonal Rings Research Articles

NBN-embedded Polycyclic Aromatic Hydrocarbons Containing Pentagonal and Heptagonal Rings.

Two novel nonhexagonal ring fused NBN-dibenzophenalenes (NBN-penta and NBN-hepta) were designed and synthesized. X-ray analysis reveals the planar structure of NBN-penta, while NBN-hepta displays a double-helical structure. Both compounds possess higher oxidation potential compared to the NBN-type zigzag-edged polycyclic aromatic hydrocarbons. The resultant NBN-penta and NBN-hepta exhibit global aromatic character based on X-ray analysis and DFT calculations. Furthermore, single crystal transistors of NBN-penta and NBN-hepta were fabricated, manifesting their promising potential in organic electronics.

Theoretical studies in the stability of vacancies in zeolite templated carbon for hydrogen storage

In this work, we report DFT calculations of the energy formation and stability of multi-vacancies in a unit of Zeolite Template Carbon (C39H9). We label as Vn the respective vacancy where n carbon atoms have been removed from the pristine C39H9 structure. The results show that V2, V4, V6 and V9 are the most stable vacancies on the ZTC structure. This result agrees with many other studies. Besides, the most stable vacancy of ZTC structure is when nine carbon atoms are removed (V9) from the ZTC structure. The formation of pentagon rings in the reconstruction of the ZTC vacancy give drastic effect on the energetics stability. Therefore, the formation of pentagon rings eliminates the dangling bonds thus lowering the energy formation. It is also carried out the decoration of ZTC vacancy with Lithium and Calcium atoms, this is the way to use de ZTC vacancy decorated as a medium for hydrogen storage. The results show that the ZTC vacancy decorated with 3 Lithium atoms can adsorb a maximum of nine hydrogen molecules (3 hydrogen molecules per Lithium atom). This gives a gravimetric storage capacity of 4.44 wt percent (wt. %), which is not enough for meeting DOE gravimetric target. On the other hand, to reach DOE gravimetric target, the study of ZTC vacancy decorated with 3 Calcium atoms is carried out, which can adsorb maximum of fifteen hydrogen molecules (5 hydrogen molecules per Calcium atom), this gives gravimetric storage capacity of 5.81 wt %, which meet DOE gravimetric targets, besides the binding energy of hydrogen molecules on ZTC vacancy decorated with 3 Calcium is calculated. These energies are in the range 0.2453–0.2053 eV/H2, which are desirable energies for hydrogen adsorption. This is demonstrated by building isotherm adsorption path. The results show that forming vacancies on ZTC structure decorated with three Calcium atoms (3CaC30H9) is good candidate as medium for hydrogen storage.

How does acetonitrile modulate single-walled carbon nanotube diameter during CVD growth?

There is a commercial demand for single-walled carbon nanotubes (SWCNTs) with uniform diameters and (n, m) chiralities. However, controlling these structural parameters in practice remains a challenge. Recent studies have shown that acetonitrile reversibly modulates SWCNT diameter during chemical vapour deposition (CVD) growth. Here we propose a mechanism to explain this phenomenon using non-equilibrium quantum chemical molecular dynamics simulations. We reveal that acetonitrile-derived radicals actively abstract hydrogen from surface hydrocarbon species as the SWCNT nucleates. This forms hydrogen (iso)-cyanide as a principal chemical product, and decreases the overall surface carbon density during nucleation. By liberating hydrogen, the number of dangling bonds present at the interface of the nucleating carbon structure is increased, which in turn accelerates SWCNT nucleation kinetics. Critically, the number of pentagon rings formed in the SWCNT precursor cap structure increases. Because the nucleation kinetics are much faster than the kinetics of ring defect healing, the pentagons become ‘trapped’ in the growing SWCNT cap structure, and this leads to more highly-curved SWCNT caps. These more highly-curved caps, combined with the lower surface carbon density and the faster kinetics of nucleation and growth, will ultimately yield narrower-diameter SWCNTs in the presence of acetonitrile.

Tetrahydrofuran (THF)-Mediated Structure of THF·(H2O)n=1–10: A Computational Study on the Formation of the THF Hydrate

Tetrahydrofuran (THF) is well known as a former and a promoter of clathrate hydrates, but the molecular mechanism for the formation of these compounds is not yet well understood. We performed ab initio calculations and ab initio molecular dynamics simulations to investigate the formation, structure, and stability of THF·(H2O)n=1–10 and its significance to the formation of the THF hydrate. Weak hydrogen bonds were found between THF and water molecules, and THF could promote water molecules from the planar pentagonal or hexagonal ring. As a promoter, THF could increase the binding ability of the CH4, CO2, or H2 molecule onto a water face, but could also enhance the adsorption of other THF molecules, causing an enrichment effect.

A theoretical investigation of elastic scattering of H atom by C60 and Kr@C60

We report an extensive study of cross section, phase shift and time delay for H–Kr@C60 elastic scattering. To demonstrate the effect of encapsulation, we extend this study to H–C60 elastic scattering also. Interaction potential is calculated using density functional theory for the scattering through both hexagonal and pentagonal rings of bare C60 and Kr@C60. The interaction potential for H–C60 is a double-humped barrier. In H–Kr@C60 case, this interaction has an additional huge core repulsive region near the center of C60, which is due to presence of the encapsulated atom. The scattering cross section exhibits glory oscillations and resonances, which are qualitatively different for the scattering through pentagonal and hexagonal rings of C60. The observed resonances arise due to combined effect of centrifugal and C60 potential barrier. We notice bunching of phase shifts for even–even and odd–odd ℓ partial waves in H–C60 scattering, consequently this happens in all measured scattering observables. The presence of the confined atom eliminates this phase bunching; therefore, H–Kr@C60 scattering rather gives well-organized and sequential placement of resonances in order of ℓ values.

C-57 carbon: A two-dimensional metallic carbon allotrope with pentagonal and heptagonal rings

By means of the first-principles calculations, we have theoretically investigated the structural stability and electronic properties of a two-dimensional planar metallic carbon allotrope named C-57 carbon which possesses the P6¯2m (D3h3) symmetry. This carbon allotrope is an all-sp2 hybridized bonding network consisting of 5–7 rings of carbon atoms. The stability of C-57 carbon is confirmed through phonon-mode analysis, total energy and elastic constants calculations, as well as first-principles molecular dynamics simulations. We conceived that the metallicity of C-57 carbon is attributed to the large states across Fermi-level contributed by py orbital due to the bond distortion, which is much different from that of graphite. This new carbon sheet can also serve as a precursor for stable one-dimensional nanotubes with metallic character. These results broaden our understanding of two-dimensional carbon allotropes and will attract more researchers to focus the research on the field of two-dimensional carbon materials. Besides, the C-57 carbon may be useful for designing of nano-electronic devices.

Carbon fragments as highly active metal-free catalysts for the oxygen reduction reaction: a mechanistic study.

In metal-free carbon-fullerene-based or defective graphene-based electrocatalysts, pentagon rings are known to play a key role in boosting catalytic activities for the oxygen reduction reaction (ORR). However, the fundamental chemical mechanism underlying the remarkable catalytic effect of the pentagon rings towards the ORR is still not fully understood. Herein, we perform a comprehensive computational study of the catalytic activities of various carbon fullerenes and fullerene fragment species, all containing pentagon rings, by using the density functional theory (DFT) and computational hydrogen electrode (CHE) methods. We find that more active sites on carbon are associated with more neighbouring pentagon rings and stronger adsorption of the key intermediates of O*, OH* and OOH* for the ORR. Importantly, two C60-based fragments, namely, C60-frag1 and C60-frag2l, show a very high activity towards the ORR, as both yield overpotentials as low as 0.389 and 0.407 V, and entail suitable adsorption free energy of OH* and OOH* species. These desirable chemical properties of fullerene fragments can be attributed to the high-energy HOMO orbitals, induced by the low-symmetry fullerene-fragment structures. Both the number of neighbouring pentagon rings and the degree of overall symmetry of the fragment appear to be the two important factors that can be adjusted for the design of optimal metal-free carbon electrocatalysts towards high ORR activities.

Penta-MX2 (M = Ni, Pd and Pt; X = P and As) monolayers: direct band-gap semiconductors with high carrier mobility

Two-dimensional binary MX2 (M = Ni, Pd and Pt; X = P and As) exhibiting a beautiful pentagonal ring network is discussed through first principles calculations.

NMR Theoretical Studies of Trapped Cations inside Fullerenes

A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings [1].

Planar net-τ: A new high-performance metallic carbon anode material for lithium-ion batteries

Based on first-principles calculations, we propose a new two-dimensional (2D) carbon allotrope (net-τ), which consists of quadrilateral, pentagonal, hexagonal, octagonal and decagonal rings. Our results demonstrate that the net-τ is a dynamically stable phase with intrinsic metallicity. Remarkably, it is found that the maximum theoretical capacity of the net-τ reaches 558 mAh g−1 on one side, which is 1.6 times that of commercial graphite and recently predicted ψ-graphene (372 mAh g−1). Meanwhile, the net-τ has a relatively low diffusion energy barrier (0.296 eV) and open-circuit potential (0.039 V), which shows its good potential as an anode material for lithium-ion batteries. Equally important, the net-τ may be synthesized by a bottom-up scheme through multiple intermediate reactions with benzocyclobutene and fulvalene. These interesting findings not only increase the family of two-dimensional metallic carbon allotropes, but also provide a novel strategy to realize high-performance anode materials by designing a 2D structure with high-order carbon rings.

Robust metal‐pentagon interactions in the Th‐based endohedral metallofullerenes revealed by DFT calculations

Endohedral metallofullerenes (EMFs) all feature obvious charge transfer from the metallic core to the carbon shell with the donated electrons largely accepted by the cage pentagons. In this work, a series of Th@C2n (2n = 64‐88) were thoroughly investigated by means of density functional theory calculations. Interestingly, we found that the tetravalent thorium atom mainly coordinates to three pentagonal rings with the metal–pentagon interactions independent on the distribution and distance among these pentagons. This coordination pattern is not only in sharp contrast to that of common organometallic complexes, where four pentagons are indispensable for stabilizing Th(IV), but also different from that of Ti‐containing fullerenes, whose valence state highly depends on the pentagon distribution. The specificity of Th‐based EMFs was rationalized by the synergetic effect of small metal ion size, low electronegativity, strong metal‐cage electrostatic attractions and effective orbital overlap between the metal and cage orbitals. Our work highlights the role of cage pentagons in the Th‐cage interactions, and points out the fundamental difference between EMFs and common organometallic complexes.

Electron Propagator Methods for Vertical Electron Detachment Energies of Anions: Benchmarks and Case Studies.

Ab initio electron propagator methods are efficient and accurate means of calculating vertical electron detachment energies of closed-shell, molecular anions with nuclei from the first three periods. Basis set extrapolations enable definitive comparisons between electron propagator results and benchmarks defined by total energy differences obtained with coupled-cluster, single, double, plus perturbative triple substitution theory. The best compromises of accuracy and efficiency are provided by the renormalized, partial third-order, diagonal (P3+) self-energy and by the nondiagonal, renormalized, second-order (NR2) approximation. The outer-valence Green function, the two-particle-one-hole Tamm-Dancoff approximation, the third-order algebraic diagrammatic construction, and the renormalized third-order methods also are examined. A detailed analysis of errors for small anions is performed. Case studies include F-(H2O) and Cl-(H2O) complexes, C5H5-, two P2N3- pentagonal rings, and a superhalide, Al(BO2)4-, whose electron detachment energy is more than double those of the halide anions. These applications illustrate the versatility of electron propagator methods, their utility for interpreting negative-ion photoelectron spectra, and their promise in the discovery of unusual properties and patterns of chemical bonding. Composite methods, which combine basis set effects calculated at the relatively efficient diagonal, second-order level and higher correlation effects calculated with small basis sets, provide excellent estimates of basis set-extrapolated P3+ or NR2 results and facilitate applications to large molecules. In the P3+ and NR2 methods, a judicious choice of low-order couplings between hole operators that correspond to the assumptions of Koopmans's theorem and operators that describe final-state relaxation and polarization and initial-state correlation leads to predictive accuracy, computational efficiency, and interpretive lucidity.

Chemically Functionalized Penta-stanene Monolayers for Light Harvesting with High Carrier Mobility

Two pentagonal tin-based two-dimensional materials, penta-stanane (p-SnH) and fluorinated penta-stanane (p-SnHF), are theoretically predicted on the basis of the density functional theory calculations. Both the proposed monolayers are formed by virtue of some pentagonal Sn rings in which all of Sn atoms are sp3 hybridized to the four nearest adjacent Sn atoms. Interestingly, they have lower formation energy than the free-standing stanene that has been grown on a Bi2Te3(111) substrate. Moreover, they have significant prospects for high-efficiency light harvesting with optical band gaps (1.88 eV for p-SnHF and 2.02 eV for p-SnH) that closely match the solar light spectrum. In particular, the band edge of p-SnHF is properly aligned that enables the band gap to cover the redox potential of H+ and O2– ions for water splitting, whereas the band edge of p-SnH can be moved down to the desired level when a small tensile strain (<5%) is applied. Meanwhile, the high carrier mobility (up to 769 and 2520 cm2 V–1 s–1 f...

Mechanisms of Covalent Coupling Reaction of Dibromofluoranthene on Au(111)

The reaction mechanism of on-surface coupling of 7,10-dibromofluoranthene (Br2FL) on Au(111) was studied on the basis of density functional theory calculations, as a possible route for fabricating graphene nanoribbons (GNRs) including pentagonal rings. The reaction pathways and energy barriers of debromination, radical coupling, and diffusion processes were investigated. The results indicate that the reaction mechanism is substantially different for Br2FL compared to that for the phenyl radical, which has been extensively studied as a model system. The rate-limiting step was radical coupling associated with the formation of a gold-metallic intermediate, which is rarely observed on Au(111), because of steric repulsion between the two radicals and that between the radicals and the substrate. The energy barriers were comparable with those for cyclohexa-m-phenylene on Ag(111), and the reaction rate estimated using transition state theory was consistent with the experimental results. These results suggest that Br2FL is preferred as a precursor of the coupling reaction to fabricate fluoranthene polymers. Well-defined GNRs including pentagonal rings would be formed by further cyclodehydrogenation.

New Nanostructure in a Metastable Ice Phase

Hydrogen bonds between water molecules can form a polygonal structure, which determine the physical properties of different ice phases. Pure pentagonal rings are not stable in crystal ice. In this work, we discover a new nanostructure of ice in heterogeneous nucleation on metal surfaces by molecular dynamic simulation. In this nanostructure, pentagonal rings are connected by dodecahedrons and staggered hydrogen bonds. The presence of dodecahedrons could help the stability of ice structure with pentagons. This nanostructure serves as a skeleton unit in ice. Our results provide a new explanation for the existence of pentagonal rings in ice, which are helpful to understand the complex ice phase behaviors.

Polar curved polycyclic aromatic hydrocarbons in soot formation

In this paper, we consider the impact of polar curved polycyclic aromatic hydrocarbons (cPAH) on the process of soot formation by employing electronic structure calculations to determine the earliest onset of curvature integration and the binding energy of curved homodimers. The earliest (smallest size) onset of curvature integration was found to be a six ring PAH with at least one pentagonal ring. The σ bonding in the presence of pentagons led to curvature, however, the π bonding strongly favored a planar geometry delaying the onset of curvature and therefore the induction of a flexoelectric dipole moment. The binding energies of cPAH dimers were found to be of similar magnitude to flat PAH containing one or two pentagons, with an alignment of the dipole moments vectors. For the more curved structures, steric effects reduced the dispersion interactions to significantly reduce the interaction energy compared with flat PAH. Homogeneous nucleation of cPAH at flame temperatures then appears unlikely, however, significant interactions are expected between chemi-ions and polar cPAH molecules suggesting heterogeneous nucleation should be explored.

A Triple-Frequency, Vertex-Fed Antenna for WLAN\/WiMAX Applications [Antenna Applications Corner

A triple-band antenna is essential in modern wireless applications to minimize interference from other communication systems by reducing the number of antennas installed on wireless devices. The proposed antenna can be utilized for Worldwide Interoperability for Microwave Access (WiMAX) (3.2-3.6 GHz), wireless local area network (WLAN) (5.15-5.35 GHz), and 5.8 GHz WLAN (5.725-5.825 GHz) applications. The antenna resonates at 3.24, 5.17, and 5.85 GHz and meets the desired requirement of voltage standing-wave ratio (VSWR) lower than 2. The pentagonal ring slot antenna is analyzed with and without an E-strip. With the E-strip, gain improvements of 1.41, 1.91, and 1.75 dB are achieved at 3.24, 5.17, and 5.85 GHz, respectively, and the measured results agree well with simulated values. The proposed antenna provides increased gain when compared with the existing triple-band designs reported in the literature.

The interaction of hydrogen with Li-coated C70 fullerene: A DFT study

We have applied density functional calculations to study the structure, stability and hydrogen storage properties of Li-coated C70 fullerenes. Our results show that among different possibilities for the geometry of Li-coated C70 fullerenes, Li atoms prefer to occupy exohedral and endohedral positions on top of the pentagons. Among isomers of Li2C70 and Li6C70, those in which one of the Li atoms occupy the endohedral position of one of the polar pentagonal rings and the other ones situate on the top of the pentagons around the opposite polar pentagon, have higher binding energies. Charge transfer is occurred from Li to C70, putting extra electrons in the vicinity of the Li atom. This results in the binding of hydrogen in atomic form to the on-top C site that is nearest to the Li atom in the preferred configuration of hydrogenated Li-coated C70 fullerene. Therefore, there are two types of hydrogen binding in Li6C70. One group of hydrogen bind to Li atoms in quasi-molecular form, which will desorb at a lower temperature, and the other group of H atoms bind to C in atomic form, which will desorb at higher temperatures.

Polyelectrolyte-Assisted Oxygen Vacancies: A New Route to Defect Engineering in Molybdenum Oxide.

The presence of oxygen vacancy sites fundamentally affects physical and chemical properties of materials. In this study, a dipole-containing interaction between poly(diallyldimethylammonium chloride) PDDA and α-MoO3 is found to enable high-concentrations of surface oxygen vacancies. Thermal annealing under Ar resulted in negligible reduction of MoO3 to MoO3- x with x = 0.03 at 600 °C. In contrast, we show that the thermochemical reaction with PDDA polyelectrolyte under Ar can significantly reduce MoO3 to MoO3- x with x = 0.36 (MoO2.64) at 600 °C. Thermal annealing under H2 gas enhanced the substoichiometry of MoO3- x from x = 0.62 to 0.98 by using PDDA at the same conditions. Density functional theory calculations, supported by experimental analysis, suggest that the vacancy sites are created through absorption of terminal site oxygen (Ot) upon decomposition of the N-C bond in the pentagonal ring of PDDA during the thermal treatment. Ot atoms are absorbed as ionic O- and neutral O2-, creating Mo5+-vO· and Mo4+-vO·· vacancy bipolarons and polarons, respectively. X-ray photoemission spectroscopy peak analysis indicates the ratio of charged to neutral molybdenum ions in the PDDA-processed samples increased from Mo4+/Mo6+ = 1.0 and Mo5+/Mo6+ = 3.3 when reduced at 400 °C to Mo4+/Mo6+ = 3.7 and Mo5+/Mo6+ = 2.6 when reduced at 600 °C. This is consistent with our ab initio calculation where the Mo4+-vO·· formation energy is 0.22 eV higher than that for Mo5+-vO· in the bulk of the material and 0.02 eV higher on the surface. This study reveals a new paradigm for effective enhancement of surface oxygen vacancy concentrations essential for a variety of technologies including advanced energy conversion applications such as electrochemical energy storage, catalysis, and low-temperature thermochemical water splitting.

Low-Energy Structures and Electronic Properties of Large-Sized SiN Clusters (N = 60, 80, 100, 120, 150, 170)

Global search based on semiempirical and first-principles calculations has been performed to determine the lowest-energy structures of large-sized silicon (Si) clusters: SiN (N = 60, 80, 100, 120, 150, 170). We found that the stuffed-cage structural motif is dominant for these large clusters, where the ratio of the number of surface atoms to core atoms agrees well with a previously proposed space-filling model up to N = 120 and the structures of outer cages contain not only the majority of the pentagonal and hexagonal rings but also a few seven-membered or eight-membered rings. Triple-layered stuffed-cage structures are found for Si150 and Si170. Based on the lowest-energy structures, the physical properties of Si clusters (including binding energy, ionization potential, adiabatic detachment energy, electronic gap, and photoelectron spectrum) are computed and compared with the experimental data. For the first time, our theoretical study provides a fundamental picture for large Si clusters up to 170 atoms.