Cation sitting in aromatic cages: ab initio computational studies on tetramethylammonium–(benzene)n (n=3–4) complexes

Quantum chemistry study was performed on interaction between tetramethylammonium (TMA) and aromatic cages by means of the MP2 method to show how TMA sits in an aromatic cage that is composed of benzenes. The MP2 calculations on TMA–(benzene)_n complexes demonstrate that the more the benzene molecules in the aromatic cage, the stronger the binding strength between the cage and TMA. In details, the structure of TMA–(benzene)_n (n = 1–4) complexes can be easily constructed by superimposing n TMA‐benzene complexes via TMA, and the binding energies of the TMA–(benzene)_n complexes are the sum of the n corresponding TMA‐benzene systems. For instance, the distances between the N of TMA and the plane of the benzene ring are 4.238, 4.252, 4.264 ,and 4.276 Å, respectively, for TMA–(benzene)_n (n = 1–4) complexes, and the BSSE corrected binding energies at MP2/6‐311++G** level are ?8.8, ?17.3, ?25.8 and ?34.3 kcal/mol, respectively, for TMA– (benzene)_n (n = 1–4) complexes. Thus, this study provides us useful information on how a cation interacts with an aromatic cage in terms of complex geometry and binding strength. Copyright © 2007 John Wiley & Sons, Ltd.     

Control of morphology for energy dissipation in carbon nanotube forests

This study focuses on the effect of carbon precursor on the carbon nanotube (CNT) morphology and energy dissipation. Benzene, toluene, and m-xylene were used as carbon precursors for the synthesis of CNT forests following a chemical vapor deposition process. The results indicate that substituents on the benzene ring increase entanglement in the CNT forests. The absorbed energy was slightly greater for CNT forests synthesized using m-xylene than for toluene, but was much smaller for benzene. When compressed to a strain of 0.67, the toluene CNTs absorbed more energy than the m-xylene CNTs. The restitution was much higher for the forests synthesized with m-xylene than toluene while it further decreased for the forests made with benzene. A strong correlation is also observed between the average diameter of the CNTs and the number of methyl substituents on the benzene ring. The control of the entanglement of the CNT forests can potentially be used to design high energy absorbing composites for blast energy dissipation.     

Structural,electronic and magnetic properties of hcp Fe,Co and Ni nanowires encapsulated in zigzag carbon nanotubes

The structural, electronic and magnetic properties of hcp transition metal (TM = Fe, Co or Ni) nanowires TM₄ encapsulated inside zigzag nanotubes C(m, 0) (m = 7, 8, 9, 10, 11 or 12), along with TM _n (n = 4, 10 or 13) encapsulated inside C(12, 0), have been systematically investigated using the first-principle calculations. The results show that the TM nanowires can be inserted inside a variety of zigzag carbon nanotubes (CNTs) exothermically, except from the systems TM₄@(7, 0) and TM₁₃@(12, 0) which are endothermic. The charge is transferred from TM nanowires to CNTs, and the transferred charge increases with decreasing CNT diameter or increasing nanowire thickness. The magnetic moments of hybrid systems are smaller than those of the freestanding TM nanowires, especially for the atoms on the outermost shell of the nanowires. The magnetic moment per TM atom of TM/CNT system increases with increasing CNT diameter or decreasing nanowire thickness. Both the density of states and spin charge density analysis show that the spin polarization and the magnetic moments of all hybrid systems mainly originate from the TM nanowires, implying these systems can be applied in magnetic data storage devices.     

A cytosine-assisted carbon nanotubes junction: DFT studies

Since nucleobase-functionalized carbon nanotubes (CNTs) are important in the biological applications; the junction of a pair of CNTs through a bridging cytosine linkage is investigated based on density functional theory (DFT) calculations. In the exact model of study, the CNTs are bound to N₁ and C₅ atomic sites of cytosine to make possible the CNT–cytosine–CNT model. To systematically investigate the purpose, the models of original CNT, original cytosine, and primary models of cytosine–CNT in which one CNT is only bound to N₁ or C₅ atomic site of cytosine are also considered. The results of dipole moments and binding energies indicated that the CNT–cytosine–CNT model is the most stable one among all three possible models cytosine-functionalized CNT. The values of energy gaps indicated that the conducting properties of primary cytosine–CNT models are not changed referring to the original CNT but better conductivity could be observed for the CNT–cytosine–CNT model. The values of evaluated quadrupole coupling constants indicated that the electronic densities of nitrogen and oxygen atoms of cytosine detect notable affects during the functionalization processes by the zigzag CNTs and the oxygen atom of CNT–cytosine–CNT model could be proposed as the most proper interacting site of cytosine among other functionalized zigzag models and also the original cytosine. However, the changes of quadrupole coupling constants for the atoms of cytosine are almost negligible during the functionalization processes by the armchair CNTs.     

Functionalization of (<Emphasis Type="Italic">n</Emphasis>, 0) CNTs (<Emphasis Type="Italic">n</Emphasis> = 3–16) by uracil: DFT studies

Density functional theory (DFT) calculations were performed to investigate stabilities and properties for uracil (U)-functionalized carbon nanotubes (CNTs). To this aim, the optimized molecular properties were evaluated for (n, 0) models of CNTs (n = 3–16) in the original and U-functionalized forms. The results indicated that the dipole moments and energy gaps were independent of tubular diameters whereas the binding energies showed that the U-functionalization could be better achieved for n = 8–11 curvatures of (n, 0) CNTs. Further studies based on the evaluated atomic-scale properties, including quadrupole coupling constants (C _Q ), indicated that the electronic properties of atoms could detect the effects of diameters variations of (n, 0) CNTs, in which the effects were very much significant for the atoms around the U-functionalization regions. Finally, the achieved results of singular U, original CNTs, and CNT-U hybrids were compared to each other to demonstrate the stabilities and properties for the U-functionalized (n, 0) CNTs.     

Enhanced interface interaction between modified carbon nanotubes and magnesium matrix

Carbon nanotube (CNT)/metal interface interaction is critical to the mechanical properties of CNT-reinforced metal matrix composites (MMCs). In this paper, in order to realize the chemical modification of the interface interaction between CNTs and Mg matrix, different types of defects (monovacancy, carbon and oxygen adatoms, as well as p-type boron and n-type nitrogen substitution) are introduced in CNTs to investigate the effect of the defects on the interface interaction (E_ib) between CNT and Mg (0 0 0 1) surface. Moreover, two models (adsorption model and interface model) are compared and validated to investigate the interface interaction. It is revealed that the CNT with the carbon adatom has the highest E_ib with the Mg (0 0 0 1), and the effect of boron doping on E_ib is superior to the intermediate oxygen which has already been proved experimentally in the enhancement of the interface interaction in MMCs. In terms of the electronic structure analysis, we reveal the micro-mechanism of the increase of E_ib under the action of different types of defects, and propose that the presence of holes (boron dopant) and the unsaturated electrons in CNTs can generate the chemical interaction between CNT and Mg matrix effectively. Our results are of great scientific importance to the realization of robust interfacial bonding between CNTs and Mg matrix via the reinforcement modification, so as to enhance the mechanical properties of CNTs reinforced Mg matrix composites.     

Crystal structure of ethyl (2Z)-2-(3-methoxy-4-acetyloxy benzylidene)-7-methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate

The title compound, C₂₆H₂₄N₂O₆S, (I), crystallizes in the monoclinic space group, P2₁/c, with cell parameters a = 16.248(1), b = 7.927(1), c = 19.371(4) ?, β = 105.295(2)°, Z = 4. The central pyrimidine ring in the compound (I) is significantly puckered, assuming a screw-boat conformation. The C11–C16 benzene ring stands vertical while thiazole and C18–C23 benzene rings are coplanar to the mean plane of pyrimidine ring having dihedral angles of 87.48(12), 3.63(11) and 0.94(12)°, respectively. In the absence of potential hydrogen bonding interaction, the crystal packing is influenced by intramolecular C-H…S interaction and intermolecular C-H…π interactions.     

Structural,electronic and magnetic properties of gold cluster doped with calcium: Au n Ca (n = 1–8)

The geometrical structures, relative stabilities, electronic and magnetic properties of calcium-doped gold clusters Au _n Ca (n?=?1–8) have been systematically investigated by employing density functional method at the BP86 level. The optimised geometries show that the ground-state structures are planar structures for Au _n Ca (n?=?3–8) clusters. Ca-substituted Au _{n +1} clusters, as well as Au-capped Au _{n ?1}Ca clusters, are dominant growth patterns for the Au _n Ca clusters. The relative stabilities of Au _n Ca clusters for the ground-state structures are analysed based on the averaged binding energies, fragmentation energies and second-order difference of energies. The calculated results reveal that the Au₂Ca isomer is the most stable structure for small size Au _n Ca (n?=?1–8) clusters. The HOMO-LUMO energy gaps as a function of the cluster size exhibit a pronounced even–odd alternation phenomenon. Subsequently, charge transfers and magnetic moment of Au _n Ca (n?=?1–8) clusters have been analysed further.     

A density functional theory study of the electronic structures and magnetic properties of Fe（1-x）Cox alloy nanowires encapsulated in （10,0） carbon nanotubes

Under the generalized gradient approximation, the electronic structures and magnetic properties of Fe_(1-x)Co_x alloy nanowires encapsulated inside zigzag (10,0) carbon nanotubes (CNTs) are investigated systematically using firstprinciple density functional theory calculations. For the fully relaxed Fe_(1-x)Co_x/CNT structures, all the C atoms relax outwards, and thus the diameters of the CNTs are slightly increased. Formation energy analysis shows that the combining processes of all Fe_(1-x)Co_x/CNT systems are exothermic, and therefore the Fe_(1-x)Co_xalloy nanowires can be encapsulated into semiconducting zigzag (10,0) CNTs and form stable hybrid structures. The charges are transferred from the Fe_(1-x)Co_xnanowires to the more electronegative CNTs, and the Fe-C/Co-C bonds formed have polar covalent bond characteristics. Both the spin polarization and total magnetic moment of the Fe_(1-x)Co_x/CNT system are smaller than those of the corresponding freestanding Fe_(1-x)Co_xnanowire, and the magnetic moment of the Fe_(1-x)Co_x/CNT system decreases monotonously with increasing Co concentration, but the Fe_(1-x)Co_x/CNT systems still have a large magnetic moment, implying that they can be utilized in high-density magnetic recording devices.     

Theoretical studies on the geometrical and electronic structures of supramolecule bis(2,2′-bipyridine)-5-amino-1,10-phenanthroline ruthenium(II)/functionalized SWCNT dyads

Functionalization of carbon nanotube (CNT) with multiple redox and photo active entities is one of the extensive processes due to its importance in building molecular or supramolecular electronic devices, solar energy storage and conversion systems. Thus, to have better understanding about structural aspects and correct electronic structure of these large systems, the quantum studies have gained increased popularity. In present study, we have investigated the structural and electronic properties of functionalized CNTs (fCNTs) with [Ru(bpy)₂(5-NH₂-1,10-phen)]⁺², (Ru-bpy-phen)⁺², supramolecule based on DFT calculations. Main attention has been applied to obtain stable configuration, binding energies and effect of functionalization on electronic behavior of the selected supramolcule. We also evaluate the effect of nanotube’s diameter and chirality on electronic properties of considered supramolecule. Calculated binding energies show that interaction between the (Ru-bpy-phen)⁺² and the host CNTs depends on the tube diameter while the chirality doesn’t affect significantly on the binding nature of respected complex. We have also investigated the influence of non-local dispersion interactions (vdW) and temperature on the stability and electronic structure of the considered system. Results obtained from the ab initio MD simulations showed that increasing the temperature can affect the distance between C and N atoms in the linkage position. The charge analysis indicates the existence of remarkable charge transfer between (Ru-bpy-phen)⁺² as donor and fCNTs as acceptor moieties in the isolated molecule. In the presence of vdW forces and at higher temperature, the charge transfer was decreased but the direction of transferred charge remains unchanged.     

Simulations on the mechanism of CNT bundle growth upon smooth and nanostructured Ni as well as <Emphasis Type="Italic">θ</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

In the current study, we have performed ab initio DFT calculations on the gradually growing 2D periodic models of capped single-wall carbon nanotubes (SW CNTs) upon their perpendicular junctions with the Ni(111) substrate, in order to understand the peculiarities of the initial stage of their growth on either smooth or nanostructured catalytic particles. Appearance of the adsorbed carbon atoms upon the substrate follows from the dissociation of CVD hydrocarbon molecules, e.g., CH₄: (CH₄)_ads → (CH)_ads+3H_ads and (CH)_ads → C_ads+H_ads. (Since the effective growth of CNTs upon Ni nanoparticles occur inside the nanopores of amorphous alumina, we have also simulated analogous surface reactions upon the θ-Al₂O₃(010) slabs). Association of the adsorbed carbon atoms upon the catalyst surface precedes further swelling of the (C_n)_ads islands after appearance of pentagonal defects within a honeycomb sheet which are more probable upon the catalyst surface containing either defects or nanoclusters (as in the case of the nanostructured substrate). The gradual growth of the capped CNTs is considerably more effective upon the nanostructured Ni(111) substrate compared to a smooth nickel substrate (cf. values of CNT adhesion energy per boundary C atom for chiralities of either armchair-type, 4.04 vs. 2.51 eV, or zigzag-type, 4.61 vs. 2.14 eV, respectively). The electronic charge transfer from the Ni catalyst towards the CNTs has been calculated for both chiralities (> 1 e per C atom), i.e., quite strong chemical bonds are formed within the CNT/Ni(111) interconnects.     

Spin-polarised DFT study of cobalt-doped gallium clusters

The structural, electronic and magnetic properties of small gallium clusters doped with Cobalt have been studied using spin-polarised density functional theory. The binding energy per atom, second-order differences of total energies and fragmentation energies of equilibrium geometries of the host Ga_n+1 and doped Ga_nCo (n = 1–12) clusters are computed. Doped clusters are found to be more stable than pure Ga clusters; Ga₃Co, Ga₅Co and Ga₈Co clusters are exceptionally stable. Doping with Co changes the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap, and also affects the magnetic moments of clusters.     

Additivity and non-additivity of dissociation energies in intermolecular interactions. Theoretical studies on (H2)n,n = 2-8, (CO2)n,n = 2-6 and (HF)n,n = 2-8

CCSD(T) and MP2 results using the aug-cc-pV5Z basis set are reported for chain, cyclic and other structures of the clusters (H₂)_n, n?=?2-8, (CO₂)_n, n?=?2-6 and (HF)_n, n?=?2-8. In chain-like structures of (H₂)_n and (CO₂)_n, with the bonding type of the dimer maintained, the dissociation energy D_e of the dimer doubles for the trimer, triples for the tetramer, and so on. Due to these systems being dominated by short-range forces, interactions are essentially restricted to neighbouring monomers. For other types of (H₂)_n and (CO₂)_n structures, the multipliers relative to the dimerisation energy can be much higher. Dissociation energies for the hexamers in S₆ symmetry of both H₂ (379?cm^?1) and CO₂ (4925?cm^?1) are over ten times the respective dimerisation energies. For the chain-like trimer of HF, however, D_e is in excess of double the dimer value. Mainly due to longer-range dipolar forces, the interactions reach beyond the neighbouring monomers. The interaction energy between HF monomers in chains follows an approximate R^?2 (R being the F–F distance) relationship, The calculated dissociation energies of the HF octamer are 15,985?cm^?1 (factor of 10.4) for the chain, and 21,003?cm^?1 (factor of 13.7) for the C_6h cyclic structure.     

Geometries, stabilities, and electronic properties of Be-doped gold clusters: a density functional theory study

We have systematically investigated the geometrical structures, relative stabilities and electronic properties of small bimetallic AunBe (n = 1, 2, . . . , 8) clusters using a density functional method at BP86 level. The optimized geometries reveal that the impurity beryllium atom dramatically affects the structures of the Aun clusters. The averaged binding energies, fragmentation energies, second-order difference of energies, the highest occupied-lowest unoccupied molecular orbital energy gaps and chemical hardness are investigated. All of them exhibit a pronounced odd-even alternation, manifesting that the clusters with even number of gold atoms possess relatively higher stabilities. Especially, the linear Au2Be cluster is magic cluster with the most stable chemical stability. According to the natural population analysis, it is found that charge-transferring direction between Au atom and Be atom changes at the size of n = 4.     

Effect of carbon nanotubes on the isotropic to nematic and the nematic to smectic- A phase transitions in liquid crystal and carbon nanotubes composites

A high-resolution ac-calorimetric study on the weakly first-order isotropic to nematic (I -N and the continuous nematic to smectic-A (N -SmA phase transitions of the liquid crystal octyl-cyanobiphenyl (8CB) doped with well-dispersed multiwall carbon nanotubes (CNTs) as a function of CNT concentrations is reported. Thermal scans were performed for all samples having CNT weight fraction from f_w \phi_{{w}}^{} = 0.0005 to 0.0060 over a wide temperature range well above and below the two transitions in pure 8CB. Both the I -N and the N -SmA transitions evolve in character and have their transition temperatures qualitatively offset by ∼ 1.10 K lower as compared to that in pure 8CB for all 8CB+CNTs samples. The enthalpy change associated with each phase transition is essentially the same as that of pure 8CB and remains unchanged with increasing f_w \phi_{{w}}^{} . However, there is an evidence that the thermal transport properties of the composites differ from the pure LC upon cooling below a f_w \phi_{{w}}^{} -dependent temperature within the nematic phase. In addition, a new C_p feature is resolved for intermediate f_w \phi_{{w}}^{} samples that appears to be correlated to this onset temperature.     

Ab initio calculation of the geometric,electronic and magnetic properties of neutral and anionic Au n Pd (n = 1–9) clusters

The ab initio method based on density functional theory at the B3PW91 level has been applied to study the geometric, electronic, and magnetic properties of neutral and anionic Au _n Pd (n?=?1–9) clusters. The results show that the most stable geometric structures adopt a three-dimensional structure for neutral Au₇Pd and Au₈Pd clusters, but for anionic clusters, no three-dimensional lowest-energy structures were obtained. The relative stabilities of neutral and anionic Au _n Pd clusters were analysed by means of the dependent relationships between the binding energies per atom, the dissociation energies, the second-order difference of energies, the HOMO–LUMO energy gaps and the cluster size n, and a local odd–even alternation phenomenon was found. Natural population analysis indicates the sequential transfer from the Pd atom to the Au _n frame in Au_1,2,3,5Pd and Au_2,3Pd^? clusters, and from the Au _n frame to the Pd atom in other clusters. Much to our surprise, irrespective of whether it is the total magnetic moment or the local magnetic moment, the magnetic moment presents an odd–even alternation phenomenon as a function of the cluster size n. The magnetic effects are mainly localized on the various atoms (Au or Pd) for different cluster size n.     

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.     

A new force field for the adsorption of H2, O2, N2, CO,H2O,and H2S gases on alkali doped carbon nanotubes

The main goal of this work is the generation of a new force field data set to the interaction of several gases such as H₂, O₂, N₂, CO, H₂O, and H₂S with alkali cation-doped carbon nanotubes (CNTs) using ab initio calculations at the MP2(full)/6-311++G(d,p) level of theory. Different alkali cations including Li⁺, Na⁺_, K⁺ and Cs⁺ were used to dope in the CNT. The calculated potential energy curve for the interaction of each gas molecule with each alkali cation-doped CNTs was fitted to an analytical potential function to obtain the parameters of the potential function. A modified Morse potential function was selected for the fitting in which the electrostatic interactions has been accounted by adding the β/r term to the Morse potential. The accuracy of the calculated force field was checked via Grand Canonical Monte Carlo (GCMC) simulation of the H₂ adsorption on Li-doped graphite and Li-doped CNT. The results of these simulations were compared with the experimental measurements and the closeness of the simulation results with the experimental data indicated the accuracy of the proposed force field. The main merit of this work is the derivation of a specific force field for interaction of each of six gases with four alkali cation-doped CNT, which can be used in molecular simulation of these 24 of systems. The simulation results showed the increase of the H₂ adsorption capacity of nanotube and graphite up to 50% and 10%, respectively, due to the insertion of Li ions.     

Structural and electronic properties of endohedral doped SWCNTs: A DFT study

The structural and electronic properties of semiconductors (Si and Ge) and metal (Au and Tl) atoms doped armchair (n, n) and zigzag (n, 0); n=4–6, single wall carbon nanotubes (SWCNTs) have been studied using an ab-initio method. We have considered a linear chain of dopant atoms inside CNTs of different diameters but of same length. We have studied variation of B.E./atom, ionization potential, electron affinity and HOMO–LUMO gap of doped armchair and zigzag CNTs with diameter and dopant type. For armchair undoped CNTs, the B.E./atom increases with the increase in diameter of the tubes. For Si, Ge and Tl doped CNTs, B.E./atom is maximum for (6, 6) CNT whereas for Au doped CNTs, it is maximum for (5, 5) CNTs. For pure CNTs, IP decreases slightly with increasing diameter whereas EA increases with diameter. The study of HOMO–LUMO gap shows that on doping metallic character of the armchair CNTs increases whereas for zigzag CNTs semiconducting character increases. In case of zigzag tubes only Si doped (5, 0), (6, 0) and Ge doped (6, 0) CNTs are stable. The IP and EA for doped zigzag CNTs remain almost independent of tube diameter and dopant type whereas for doped armchair CNTs, maximum IP and EA are observed for (5, 5) tube for all dopants.     

Quantum chemical studies on (ZnO)n/(NiO)n heterostructured nanoclusters

The structural stability and electronic properties of (ZnO)_n, (NiO)_n, (ZnO)_n/(NiO)_n for n = (1 to 4) and 3D structures were studied using density functional theory. The geometrical optimisation of clusters implies that when the atoms in the cluster increase it leads to an increase in its stability. The stability drastically increases for the heterostructure of (ZnO)_n/(NiO)_n. The dipole moment of the clusters depends on the geometry of the cluster and it is found to be minimum for heterostructures representing more neutralised clusters. HOMO-LUMO energies, ionisation potential, electron affinity, chemical hardness, binding energies and vibrational analysis of different clusters are calculated and reported. The adsorption of CO on the different sites of nanoclusters are studied and discussed.