C-BN single-walled nanotubes from hybrid connection of BN/C nanoribbons : prediction by ab initio density functional calculations

We demonstrated for the first time by ab initio density functional calculation and molecular dynamics simulation that C0.5(BN)0.5 armchair single-walled nanotubes (NT) are gapless semiconductors and can be spontaneously formed via the hybrid connection of graphene/BN Nanoribbons (GNR/BNNR) at room temperature. The direct synthesis of armchair C0.5(BN)0.5 via the hybrid connection of GNR/BNNR is predicted to be both thermodynamically and dynamically stable. Such novel armchair C0.5(BN)0.5 NTs possess enhanced conductance as that observed in GNRs. Additionally, the zigzag C0.5(BN)0.5 SWNTs are narrow band gap semiconductors, which may have potential application for light emission. In light of recent experimental progress and the enhanced degree of control in the synthesis of GNRs and BNNR, our results highlight an interesting avenue for synthesizing a novel specific type of C0.5(BN)0.5 nanotube (gapless or narrow direct gap semiconductor), with potentially important applications in BNC-based nanodevices.

Magnetism of C adatoms on BN nanostructures : implications for functional nanodevices

Spin-polarized density functional calculations reveal that magnetism can be induced by carbon adatoms on boron nitride nanotubes (BNNTs) and BN hexagonal sheets. As a result of the localization of impurity states, these hybrid sp-electron systems are spin-polarized, with a local magnetic moment of 2.0 μB per C adatom regardless of the tube diameter and the bonding between the C atom and the BNNTs/BN sheets. An analysis of orbital hybridization indicates that two valence electrons participate in the bonding and the remaining two electrons of the C adatom are confined at the adsorption site and contribute to the magnetism accordingly. The effective interaction distance between the C-induced magnetic moments is evaluated. In terms of the diffusion barrier and the adsorption energy of C adatoms on the BN nanotubes/ sheets, a fabrication method for BN-C-based functional nanodevices is proposed, and a series of virtual building blocks for functional devices are illustrated.

Reactions of Nb2 and Nb3 with CO, D2, N2, and O2 : reconciling experimental kinetics with density functional theory-calculated reaction profiles

 Calculated energy profiles for the reactions of neutral Nb₂ and Nb₃ metal clusters with CO, D₂, N₂, and O₂ are presented. In each reaction path, both a physisorption energy minimum, where the reactant remains intact, and a chemisorption energy minimum, where the reactant has dissociated, are calculated and linked by saddle points. We calculate branching ratios for the forward (dissociative) and reverse reactions which we compare with the experimental kinetic data. It is found that a combination of average thermal energies and barrier heights leads to wide variation in branching ratios which compares favourably to previously determined experimental reaction rates.

Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel

We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (MnγAl anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site.

Synthesis and structure of bis(para-methoxyphenyl)selenoxide and its monohydrate. theoretical considerations of the hydration of diorganoselenium oxides

The title compound was prepared by base hydrolysis of (p-MeOC₆H₄)₂SeCl₂ in water and isolated as the crystalline monohydrate, (p-MeOC₆H₄)₂SeO·H₂O, in which the water molecule is associated via hydrogen-bonding. Water-free (p-MeOC₆H₄)₂SeO was obtained crystalline after drying and recrystallisation from toluene. Both crystal phases were investigated by single crystal X-ray diffraction. Preliminary DFT calculations at the B3LYP/LANL2DZdp level of theory suggest that the hydrogen bonded complexes R₂SeO·H₂O (R = H, Me, Ph) are by 2.79, 3.36 and 11.10 kcal mol^-1 more stable than the corresponding elusive diorganoselenium dihydroxides R₂Se(OH)₂. The hydrogen bond energies of R₂SeO·H₂O (R = H, Me, Ph) are 5.98, 7.18 and 5.89 kcal mol^-1.

The reactivity of bis(para-methoxyphenyl)telluroxide towards triflic acid and diphenylphosphinic acid. Theoretical considerations of the protonation and hydration process of diorganotelluroxanes

The reaction of (p-MeOC₆H₄)₂TeO with two equivalents of HO₃SCF₃ and HO₂PPh₂ provided the tetraorganoditelluroxanes (F₃CSO₃)(p-MeOC₆H₄)₂TeOTe(p-MeOC₆H₄)₂(O₃SCF₃) (1) and (Ph₂PO₂)(p-MeOC₆H₄)₂TeOTe(p-MeOC₆H₄)₂(O₂PPh₂)·2 Ph₂PO₂H (2) in good yields. Compounds 1 and 2 were characterized by solution and solid-state ³¹P and ¹²⁵Te NMR spectroscopy, IR spectroscopy, electrospray mass spectrometry, conductivity measurements and single crystal X-ray diffraction. In solution, compound 1 undergoes an electrolytic dissociation and reversibly reacts with traces of water to give the mononuclear cation [(p-MeOC₆H₄)₂TeOH]⁺ and triflate anions. Theoretical aspects of the protonation and hydration of model telluroxanes R₂TeO (R = H, Me, Ph) were investigated by preliminary DFT calculations and compared to the corresponding selenoxanes R₂SeO. The tellurium dihydroxides R₂Te(OH)₂ seem to be more stable than the hydrogen-bonded complexes R₂TeO·H₂O.

Half metallicity in a zigzag double-walled nanotube nanodot : an ab initio prediction

Ab initio density functional calculations were performed to study finite-length zigzag (7, 0) @ (16, 0) double-walled carbon nanotubes (DWCNTs) with H-termination at the open ends. We find that such a DWCNT nanodot displays a very large magnetic moment at the zigzag edges and the ground state displays symmetric anti-ferromagnetic coupling. When an external electric field is applied along the direction of tube axis, a gap is opened for one spin channel, whereas another spin channel remains metallic, i.e. half metallicity occurs. Our results suggest an important new avenue for the development of CNT-based spintronic materials with enhanced properties. © 2008 Elsevier B.V. All rights reserved.

Reactions with oleum under harsh conditions : characterization of the unique [M(S2O7)3]2- ions (M=Si, Ge, Sn) in A2[M(S2O7)3] (A=NH4, Ag)

The reactions of group 14 tetrachlorides MCl4 (M=Si, Ge, Sn) with oleum (65 % SO3) at elevated temperatures lead to the unique complex ions [M(S2O7)3]2−, which show the central M atoms in coordination with three chelating S2O72− groups. The mean distances M[BOND]O within the anions increase from 175.6(2)–177.5(2) pm (M=Si) to 186.4(4)–187.7(4) pm (M=Ge) to 201.9(2)–203.5(2) pm (M=Sn). These distances are reproduced well by DFT calculations. The same calculations show an increasing positive charge for the central M atom in the row Si, Ge, Sn, which can be interpreted as the decreasing covalency of the M[BOND]O bonds. For the silicon compound (NH4)2[Si(S2O7)3], 29Si solid-state NMR measurements have been performed, with the results showing a signal at −215.5 ppm for (NH4)2[Si(S2O7)3], which is in very good agreement with theoretical estimations. In addition, the vibrational modes within the [MO6] skeleton have been monitored by Raman spectroscopy for selected examples, and are well reproduced by theory. The charge balance for the [M(S2O7)3]2− ions is achieved by monovalent A+ counter ions (A=NH4, Ag), which are implemented in the syntheses in the form of their sulfates. The sizes of the A+ ions, that is, their coordination requirements, cause the crystallographic differences in the crystal structures, although the complex [M(S2O7)3]2− ions remain essentially unaffected with the different A+ ions. Furthermore, the nature of the A+ ions influences the thermal behavior of the compounds, which has been monitored for selected examples by thermogravimetric differential thermal analysis (DTA/TG) and XRD measurements.

Modelling ion-pair geometries and dynamics in a 1-ethyl-1-methylpyrrolidinium-based ion-conductive crystal

Full conformational and energy explorations are conducted on an organic ionic plastic crystal, 1-ethyl-1-methylpyrrolidium tetrafluoroborate [C2 mpyr][BF4 ]. The onsets of various stages of dynamic behaviour, which appear to account for low-temperature solid-solid phase transitions, are investigated by using quantum-chemical simulations. It is suggested that pseudorotation of the pyrrolidine ring occurs in the first instance; the partial rotation of the entire cation subsequently occurs and may be accompanied by reorientation of the ethyl chain as the temperature increases further. A cation-anion configuration, whereby BF4 (-) interacts with the C2 mpy cation from the side of the ring, is the most likely structure in the low-temperature phase IV region. These interpretations are supported by (13) C nuclear magnetic resonance chemical-shift analysis.

Diagnosis of the redox levels of TCNQF4 compounds using vibrational spectroscopy

The vibrational spectroscopy of TCNQF4, TCNQF41- and TCNQF42- has been investigated by means of density functional theory. Band assignments in infrared and Raman spectra have been clarified and a series of diagnostics developed for redox level characterisation of TCNQF4 compounds. In the C£C stretching region (1460-1600 cm-1), TCNQF40 and TCNQF 41- show two bands, with the more energetic being at 1600 cm-1 in TCNQF40 and at approximately 1535 cm-1 in TCNQF41-; in TCNQF42- both modes absorb below 1500 cm-1, often merging to give a single band. In the C-F and endocyclic C-C stretching region (1290 and 1360 cm-1), TCNQF40 and TCNQF41- show strong bands, whereas TCNQF42- absorbs weakly or not at all. (Additional bands, e.g. from co-crystallised solvent molecules, may complicate this region.) In the nitrile stretching region (2000-2250 cm-1), modes are highly sensitive to nitrile coordination by metal cations. All three redox levels can produce bands above 2200 cm -1, however bands below 2150 cm-1 are usually due to TCNQF42-. This sensitivity to coordination is likely to affect the spectra of many organic molecular ions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.