Studying (n, 0) and (m,m) GaP nanotubes (n = 3–10 and m = 2–6) through DFT calculations of Ga-69 quadrupole coupling constants

We investigated properties of representative zigzag and armchair gallium phosphide (GaP) nanotubes by performing density functional theory (DFT) calculations. To achieve our purpose, eight models of (n,0) zigzag GaP nanotubes with n = 3–10 and five models of (m,m) armchair GaP nanotubes m = 2–6 were considered. Each model was firstly optimized and quadrupole coupling constants (C_Q) were subsequently calculated for gallium-69 atoms of the optimized structures. The results indicated that the optimized properties including dipole moments, energy gaps, binding energies, and bond lengths could be mainly dependent on the diameters of GaP nanotubes, which are directly determined by n or m indices. Moreover, comparing the values of C_Q parameters indicated that the narrower GaP nanotubes could be considered as more reactive materials than the wider nanotubes, in which the reactivities are very important in determining the applications of nanotubes. And finally, the atoms at the sidewalls of nanotubes could be divided into atomic layers based on the similarities of properties for atoms of each layer, in which the properties of Ga atoms at the edges of nanotubes are significantly different from other layers only for wider nanotubes.     

Probing the Chemical Functionalization of Single‐Walled Carbon Nanotubes with Multiple Carbon Ad‐Dimer Defects

Drying‐tube‐shaped single‐walled carbon nanotubes (SWCNTs) with multiple carbon ad‐dimer (CD) defects are obtained from armchair (n,n,m) SWCNTs (n=4, 5, 6, 7, 8; m=7, 13). According to the isolated‐pentagon rule (IPR) the drying‐tube‐shaped SWCNTs are unstable non‐IPR species, and their hydrogenated, fluorinated, and chlorinated derivatives are investigated. Interestingly, chemisorptions of hydrogen, fluorine, and chlorine atoms on the drying tube‐shaped SWCNTs are exothermic processes. Compared to the reaction energies for binding of H, F, and Cl atoms to perfect and Stone–Wales‐defective armchair (5,5) nanotubes, binding of F with the multiply CD defective SWCNTs is stronger than with perfect and Stone–Wales‐defective nanotubes. The reaction energy for per F₂ addition is between 85 and 88 kcal mol^?1 more negative than that per H₂ addition. Electronic structure analysis of their energy gaps shows that the CD defects have a tendency to decrease the energy gap from 1.98–2.52 to 0.80–1.17 eV. After hydrogenation, fluorination, and chlorination, the energy gaps of the drying‐tube‐shaped SWCNTs with multiple CD defects are substantially increased to 1.65–3.85 eV. Furthermore, analyses of thermodynamic stability and nucleus‐independent chemical shifts (NICS) are performed to analyze the stability of these molecules.     

On the influence of diameter and length on the properties of armchair single-walled carbon nanotubes: A theoretical chemistry approach

Open-ended fragments of armchair single-walled carbon nanotubes (n, n) with n = 3, 4, 5, and 6, have been modeled, with increasing lengths from 0.6 to 3 nm long. The geometries of all the studied fragments have been fully optimized. The influence of diameter and length on different electronic properties has been analyzed. These properties are electronegativity, ionization potential, electron affinities, and hardness, and all of them have been expressed as functions of the frontier orbitals. The binding energies per C atom have been calculated, using an expression that improves the previously reported ones. Their absolute values were found to steadily increase with tubes length and diameter, which allows extrapolations to obtain BE_/C atom for tubes of infinite length. The extrapolated values are 8.45, 8.65, 8.74, and 8.79 eV for armchair nanotubes with n = 3, 4, 5, and 6, respectively.     

Theoretical study of C60(OH)20 and C60(OH)18 fullerenols and B12(OH) 12 2? , Si20O30(OH)20, and Ti20O30(OH)20 polyhydroxyl clusters and their Li-substituted derivatives

The equilibrium geometric parameters and the energetic characteristics of fullerenol molecules C₆₀(OH)₂₀ and C₆₀(OH)₁₈ and fullerenol-like inorganic clusters B₁₂(OH) ₁₂ ^2? , Si₂₀O₃₀(OH)₂₀, and Ti₂₀O₃₀(OH)₂₀ and their derivatives C₆₀(OH)_{20 ? n} (OLi) _n , C₆₀(OH)_{18 ? n} (OLi) _n , B₁₂(OH)_{12 ? n} (OLi) _n ^2? , Si₂₀O₃₀(OH)_{20 ? n} (OLi) _n , and Ti₂₀O₃₀(OH)_{20 ? n} (OLi) _n , in which the H atoms of all or one-half of hydroxyl groups are replaced by Li atoms, have been calculated by the density functional theory B3LYP/6-31G* method. It has been found that partial energies ??E(H/Li) per single substitution of Li for H in the reactions C₆₀(OH)₂₀ + nLiAc ?? C₆₀(OH)_{20 ? n} (OLi) _n + nHAc and C₆₀(OH)₁₈ + nLiAc ?? C₆₀(OH)_{18 ? n} (OLi) _n + nHAc (Ac is acetate) and the energies averaged over the entire series of changes in n, as well as over its first and second halves, do not exceed a few kilocalories per mole. It has been predicted that at least one-half (or more than one-half) of OH groups can be replaced by OLi without noticeable changes in energy; however, with a further increase in n, substitutions become endothermic and require ever-increasing energy inputs. In the completely hydroxylated closo-dodecaborane dianion with an icosahedral [B₁₂] cage and more polar B-O-H bonds, analogous H/Li substitutions are slightly exothermic so that the reaction can proceed somewhat smoother and further (toward larger n values) than in fullerenols, other conditions being the same. In the inorganic clusters Si₂₀O₃₀(OH)₂₀ and Ti₂₀O₃₀(OH)₂₀ with the [Si₂₀] and [Ti₂₀] cages, respectively, and with even more polar Si-O-H and Ti-O-H moieties, the substitutions are even more exothermic (their partial energies ??E(H/Li) increase to 4?C6 kcal/mol). For sodium and potassium analogues, the qualitative pattern persists, but H/Na and H/K substitutions are somewhat less exothermic than the H/Li substitutions. The results are compared to the data of previous calculations of stepwise H/Li and H/Na substitutions in the reactions C60(OH)24 + nLAc ?? C60(OH)_{24 ? n} (OL) _n + nHAc (L = Li, Na).     

Investigation of hydrogen and methane adsorption/separation on silicon nanotubes: a hierarchical multiscale method from quantum mechanics to molecular simulation

A combination of ab initio quantum mechanical (QM) calculations and canonical Monte Carlo (CMC) simulations are employed to investigate possible usage of single-walled silicon nanotubes (SWSiNTs) as a novel media for hydrogen and methane adsorption as well as their separation from each other. By fitting the force field, a Morse potential model is selected as an efficient potential to describe the binding energies between both hydrogen-SiNTs and methane-SiNTs obtained from ab initio calculations. Then CMC simulations are performed to evaluate the adsorption and separation behaviors of H₂ and CH₄ on the three different sizes of SiNTs including (5, 5), (7, 7), and (9, 9) SiNTs at ambient temperatures and pressures from 1 up to 10 MPa. As a comparison, the adsorption and separation of H₂ and CH₄ on the (8, 8) CNTs which are isodiameter with (5, 5) SiNTs are also simulated. Results are indicative of remarkable enhancement of H₂ and CH₄ adsorption capacity on the SiNTs compared to the CNTs, which arise from stronger van der Waals (VDW) attractions. In the case of methane adsorption on SiNTs, the stored volumetric energy exceeds the goal of the US Freedom CAR Partnership by 2010, which can not be achieved by methane compression at such low pressures. Moreover, simulation results indicate that SiNTs preferentially adsorb methane relative to hydrogen in their equimolar mixture, which results in efficient separation of these gases from each other at 293 K.     

Synthesis of end-cap precursor molecules for (6, 6) armchair and (9, 0) zig-zag single-walled carbon nanotubes

We report the synthesis and purification of a C₆₀H₃₀ precursor molecule for the end-cap of a (6, 6) armchair and of a C₅₄H₂₄ precursor molecule for a (9, 0) zig-zag type single-walled nanotube. An approach to controlled growth of single-walled carbon nanotubes is suggested.     

Hydration of the ions Al3+ and Cu2+. An ab initio SCF study

Ab initio calculations are reported for the systems Al(H₂O)_n³⁺ and Cu(H₂O)_n²⁺ with n up to 7. The calculated binding energies increase monotically up to n = 6, with equal binding energies for n = 6 and 7 for the Al³⁺ cation. An estimate of the enthalpy of hydration of Al³⁺ is given, based on model calculations with one or two water molecules from the second solvation shell. An SN1 (dissociative) mechanism for the exchange of the water molecules from the first hydration shell of Al³⁺ appears energetically favorable if the leaving molecule remains in the second hydration shell.     

Molecular ionization-dissociation of fluoranthene at 266 nm: Energetic and dissociative pathways

Experimental studies have been carried out for nanosecond 266-nm laser-induced photoionization and dissociation of fluoranthene, C₁₆H₁₀ with pulse energies from 0.5 to 20 mJ using a time of flight mass spectrometer. The fragmentation patterns have been characterized and discussed with respect to the number of absorbed photons. They fall into three regimes. The first regime involves low energy processes, where the molecular parent ion promptly dissociates, resulting in the formation of Cm⁺Hn(m=11−15) by a process where up to two photons are absorbed. The second regime involves intermediate energy, where dissociative processes are activated by up to three-photon absorption and produce a second group of daughter ions: C10⁺Hn, C₉⁺Hn, and C₈⁺Hn. Finally, there is a third dissociative process, characterized by the absorption of up to four photons, producing C₇⁺Hn, C₆⁺Hn, C₅⁺Hn, C₄⁺Hn, and C₃⁺Hn. Most of the detected ions are of the form Cm⁺Hn with m < n. Total deprotonation has also been observed. The mechanism proposed involves the dissociation of the parent ion, which then dissociates by different competitive channels. Helium, neon and argon were used as carrier gases (CG). A detailed discussion is presented regarding the use of He as the CG. The laser pulse intensity allows the absorption of up to nine photons, observed through the formation of multiply charged ions of some of the CG atoms.     

Modification of the electronic properties of zigzag (n = 5–10) and armchair (n = 3, 5) carbon nanotubes by K atom adsorption

The adsorption of the potassium atom onto the surface of (n,0) zigzag nanotube (n = 5–10) and (n,n) armchair nanotubes (n = 3, 5) has been studied by density functional theory. The local density approximation calculation of adsorption energy (E _ads) emphasized on the dependency of E _ads to the diameter and chirality of the nanotube. E _ads decreases when the diameter increases. So the (5,0)-K system has the highest adsorption energy among all structures. Furthermore, a significant change was observed in the electronic properties of potassium-adsorbed single-walled carbon nanotube (SWCNT) and the metallic behavior of the nanotube improved. Therefore, our results showed that such modified SWCNTs can be applied in nanodevices such as transistors.     

Novel silicon nanorings: Persilacyclacenes at DFT

Structures, energies, and aromatic characters are compared and contrasted for a series of [n]persilacyclacenes with n = 6–12: Si₂₄H₁₂, Si₂₈H₁₄, Si₃₂H₁₆, Si₃₆H₁₈, Si₄₀H₂₀, Si₄₄H₂₂, and Si₄₈H₂₄, respectively, at B3LYP levels (n, number of fused benzenoid rings). These are a brand of silicon nanorings that bear a resemblance to the shortest zig‐zag silicon nanotubes (SiNTs), henceforth referred to as SiNRs. The NBO results show nearly sp²‐hybridization for virtually all Si atoms of our SiNRs. This is in contrast to most reports where sp³‐hybridization is proposed for typical SiNTs. Comparison between the optimized SiNRs and their corresponding planar (polyacenic) forms shows longer bond lengths for the former, due to their curvatures. Except for sterically hindered Si₂₄H₁₂ (n = 6), all even SiNRs (n = 8, 10, and 12), are more aromatic than the odd ones (n = 7, 9, and 11). Such a higher aromaticity is witnessed inside, outside, and on the surface of the scrutinized SiNRs. Also, except for Si₄₄H₂₂ (n = 11), the energy gaps (ΔE_HOMO?LUMO) for the odd set of SiNRs, as well as the even set, appear inversely proportional to their corresponding diameters, per se. Except for the sterically hindered SiNRs with n = 6 or 7, all the even SiNRs enjoy a higher stability (aromaticity) and conductivity for showing lower ΔE_HOMO?LUMO than the odd ones. Evidently, the ideal diameter for persilacyclacenes (SiNRs) studied is from 0.92 to 1.42 nm, corresponding to n = 8–12, respectively. Higher than 1.42 nm causes structural disorders while lower than 0.92 brings about bond localization due to the high‐steric effects. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Molecular and dissociative adsorption of H2O on zeolite Zn/ZSM-5 studied by diffuse-reflectance IR spectroscopy and quantum chemical calculations

Interest in water adsorption on cation-substituted zeolites is due to the possibility of the M ⁿ⁺ (H₂O) + [Si-O-Al]^?1 → MOH^{(n ? 1)+} + Si-O(H)-Al (M = metal, n = 1–3) reaction taking place. As a result of this reaction, the cation-substituted zeolite can exhibit Brønsted acid activity. The molecular adsorption of water on Zn/ZSM-5 zeolite at room temperature and the subsequent heterolytic dissociation of adsorbed water under heating have been investigated by diffuse-reflectance IR spectroscopy. For theoretical simulation of these processes, three different fragments of the ZSM-5 lattice corresponding to possible variants of the structure of the ionic site of the substituting cation have been examined. Calculations on the molecular and dissociative adsorption of water molecules on substituting Zn²⁺ cations have been performed by the DFT method. Two pathways of the dissociation of adsorbed H₂O molecules-endothermic and exothermic ones-have been discovered, and it has been demonstrated that the spatial separation of two lattice Al ions at the Zn²⁺ cation site significantly affects the adsorption energy.     

Time-dependent close coupling for vibrational excitation in three dimensions: Application to H+ - H2

Impact parameter calculations for the non-reactive H⁺ + H₂ (n_i = 0) → H⁺ + H₂ (n_f) collision are reported for energies 10 eV ? E_cm ? 200 eV describing the rotational motion of the molecule in the sudden limit. The time-dependent Schrödinger equation for the vibrational motion has been solved by close coupling techniques expanding the vibrational wavefunction into both harmonic and numerically exact H₂ bound states. The convergence in vibrational basis sets, where up to six vibrational levels are considered, becomes worse with decreasing energy and increasing inelasticity. Furthermore, the harmonic wavefunctions are not suitable over a large range of energies to calculate proper cross sections. The various integral and differential cross sections have been compared with the classical results of Giese and Gentry.     

Dissociative Photoionization of m-Xylene

The photoionization and dissociative photoionization of m-xylene (C₈H₁₀) were researched by using synchrotron radiation vacuum ultraviolet (SR-VUV) and supersonic expanding molecular beam reflectron time-of-flight mass spectrometer (RFTOF-MS) system. The photoionization efficiency spectra (PIEs) of parent ion C₈H₁₀⁺ and main fragment ions C₈H₉⁺ and C₇H₇⁺ were observed, and the ionization energy (IE) of m-xylene and appearance energies (AEs) of main fragment ions C₈H₉⁺ and C₇H₇⁺ were determined to be 8.60 ± 0.03 eV, 11.76 ± 0.04 eV and 11.85 ± 0.05 eV, respectively. Structures of reactant, transition states (TSs), intermediates (INTs), and products involved in two dominant dissociation channels were optimized at the B3LYP/6-311++G(d,p) level, and the relative energies were calculated at the G3 level. Based on the results, two major dissociative photoionization channels, C₇H₇⁺+CH₃ and C₈H₉⁺+H were calculated at the B3LYP/6-311++G(d,p) level. On the basis of theoretical and experimental results, the dissociative photoionization mechanisms of m-xylene were proposed. The C–H or C–C bond dissociation and hydrogen migration are the main processes in the dissociation channels of m-xylene cation.

     

Investigation of the defect manganese silicide MnnSi2n−m

Low-temperature magnetization studies upon melt-grown single crystals of the defect manganese silicide Mn_nSi_2n?m have shown this material to contain small quantities of plate-like MnSi precipitates. Metallographic and electron microprobe analyses have confirmed this result. The strongly magnetic MnSi precipitates dominate the diamagnetic Mn_nSi_2n?m matrix, and are responsible for the magnetic behavior reported in the literature. MnSi is metallic, and the plate-like metallic precipitates degrade the thermoelectric efficiency of the degenerate semiconductor Mn_nSi_2n?m.     

Evolution of (GaAl) n Clusters and Chemisorptions of H2 on (GaAl) n Clusters

The growth behavior of (GaAl) _n (n = 1–12) and the chemisorptions of hydrogen on the ground state geometries have been studied with the three-parameter hybrid generalized gradient approximation due to Becke-Lee–Yang–Parr (B3LYP). The dissociation energy, the second-order energy differences, and the HOMO–LUMO gaps indicate that the magic numbers of the calculated (GaAl) _n clusters are n = 4 and 6. To my knowledge, this is the first time that a systematic study of chemisorptions of hydrogen on gallium aluminum clusters. The onefold top site of aluminum atom is identified to be the most favorable chemisorptions site for one hydrogen chemisorptions on most (GaAl) _n clusters. In general, dissociative chemisorptions of a hydrogen molecule on a top site of aluminum atom is found common for all sizes clusters considered here except for (GaAl) _n (n = 1–3) clusters. The stability of the (GaAl) _n H _m complexes shows that both large second-order difference and large fragmentation energies for (GaAl)₁₀H₂ and (GaAl)₁₁H₂ make these species behaving like magic clusters.     

Carbon nanotubes, new material for purification of water-ethanol mixtures from isomers of propanol

The possibility of purification of water-ethanol mixtures from the unwanted admixtures of higher alcohol, in particular, of the isomeric propanols, with the help of carbon nanomaterial is discussed. Results of quantum-chemical calculations of n-propanol and isopropanol on the surface of the single-walled carbon nanotube of the “armchair” type are presented. Investigations are carried out within the frame of model of molecular cluster using semi-empirical quantum chemical MNDO method. The possibility of adsorption of molecules of propanol isomers on the outer surface of nanotubes of small diameter as well the absence of adsorption of ethanol molecules on them is shown. It indicates the possibility of selective sorption with carbon nanotubes. Main geometric, electronic, and energy characteristics of obtained adsorption complexes are evaluated. A conclusion is made on the possibility to use carbon nanotubes for superfine purification of water-ethanol mixtures from the unwanted admixtures of n- and isopropanol.     

Striking Size and Doping Effects of Ti−Si−O Clusters on Methane Conversion Reactions

The size and doping effects in methane activation by Ti−Si−O clusters have been explored by using a combination of gas-phase experiments and quantum chemical calculations. All [Ti_mSi_nO_2(m+n)]^.+ (m+n=2, 3, 8, 10, 12, 14) clusters can extract a hydrogen from methane. The associated energies and structures have been revealed in detail. Moreover, the doping and size effects have been discussed involving generalized Kohn-Sham energy decomposition analysis, natural population analysis, Wiberg bond indexes (WBI), molecular polarity index (MPI) and ionization potential (IP). It suggested that Ti−Si−O clusters with a low Ti : Si ratio is beneficial to adsorbing methane and inclination to the hydrogen atom transfer (HAT) process, while the clusters with a high Ti : Si ratio favors the generation of a terminal oxygen radical and results in high reactivity and turnover frequency. On the other hand, a cluster size of m+n=12 is recommended considering both the ionization potential and the turnover frequency of the reaction. Hopefully, these finding will be instructive for the design of high-performance Ti−Si−O catalyst toward methane conversion.     

Adsorption properties of N<Subscript>2</Subscript>O on (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes: a density functional study

Abstract  

The behavior of N₂O adsorbed on the external surface of H-capped (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes was studied by using density functional calculations. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 suite of programs. We present the nature of the N₂O interaction in selected sites of the nanotubes. Binding energies corresponding to adsorption of the N₂O are calculated to be in the range 4–21 kJ mol⁻¹. More efficient binding energies cannot be achieved by increasing the nanotube diameter. We also provide the effects of N₂O adsorption on the electronic properties of the nanotubes.     

Thermochemistry of Redistribution of Poly[oxymulti(dimethylsilylenes)], —[(Me2Si)mO]n—, to Polysiloxanes and Polysilanes. Theoretical Study

Poly[oxymulti(dimethylsilylenes)], —[(Me₂Si)_mO]_n—, are thermodynamically unstable and undergo exothermic base‐catalyzed bond redistribution producing polydimethylsiloxanes and polydimethylsilanes. The enthalpy and free energy of redistribution of model hydrogen‐substituted polyoxydisilylenes, —[(H₂Si)₂O]_n— were calculated by ab‐initio methods (DFT and CBS‐4). Thermochemistry of polyoxydisilylene disproportionation was compared with analogous hypothetical reaction of poly(ethylene oxide). The enthalpies of reactions were calculated to be ca. –10 kcal/SiSiO and –6 kcal/CCO, respectively. Calculations show that the thermodynamic stability of polysiloxanes and polyacetals, respectively, due to the n_O → σ*_XO hyperconjugation, where X = Si, C, is the main driving force for these reactions. The difference in reactivity between polyoxymultisilylenes and polyethers has a kinetic origin and may be explained by the difference in activation energies associated with heterolytic cleavage of the X—X and X—O bonds.     

A theoretical study on the protonation of cycloalkanes CnH2n (n = 3 to 6)

Ab initio self-consistent-field molecular orbital calculations have been carried out for the C_nH_2n (n = 3 to 6) cycloalkanes and various conformers of their protonated forms. The calculated protonation energies for the sequence of conformers of the protonated forms follow the experimentally observed trend. Correlations between optimum C? C? C bond angles at the protonation site and the calculated protonation energies have been observed, and these correlations may be of some use in estimating protonation energy-bond angle relations in other (strained) cyclic compounds when the central carbon atom of a C? C? C moiety is protonated.