Structural Preferences of Single‐Walled Silica Nanostructures: Nanospheres and Chemically Stable Nanotubes

Structural preferences of single‐walled and coordinatively saturated spherical and tubular nanostructures of silica have been determined by ab initio calculations. Two families of spherical (SiO₂)_n clusters derived from Platonic solids and Archimedean polyhedra are depicted, with n ranging from 4–120. The analogue of a truncated icosidodecahedron, I_h‐symmetric Si₁₂₀O₂₄₀, is favored in energy, closely followed by the I_h‐symmetric Si₆₀O₁₂₀‐truncated icosahedron. The silica nanotubes derived from spherical clusters are capped by Si₂O₂ rings, whereas the tubular section consists of single oxygen bridges. Periodic studies performed with open‐ended silica nanotubes and the α‐quartz polymorph of silica, along with a comparisons to fullerenes and carbon nanotubes, suggest that tubes with diameters of approximately 1 nm should be chemically stable.     

How Good is Fluorine as a Hydrogen‐Bond Acceptor in Fluorinated Single‐Walled Carbon Nanotubes?

Ab initio (RI-MP2/TZVPP) computations were employed to investigate the interaction between hydrogen-bond donors H2O and CH3OH and covalently bound fluorine in organofluorine compounds. While the CFHO interaction energy is around 3 kcal mol(-1) for unstrained systems, the linear correlation between pyramidalization angle at the carbon atom and the interaction energy suggests that increased binding can be obtained in strained systems. This is confirmed for the dihydrodifluoropyrene-methanol pair, but a large portion of the binding energy is due to the interaction of the pi system with the oxygen atom. Density functional periodic boundary condition computations (PBC-PBE/6-31G*) of the structures of (5,5) and (10,10) armchair (C2F)n fluorinated SWNTs (F-SWNTs) indicate that the pyramidalization at the fluorine-binding carbon atoms are too similar to that of CH3F to enhance the hydrogen-bond acceptor properties of fluorine significantly. The solubility of F-SWNTs in alcohols therefore could be due to a combination of hydrogen bonds and van der Waals interactions with the pi systems.     

Defective α‐Fe2O3(0001): An ab Initio Study

By using density functional theory calculations at the PBE+U level, we investigated the properties of hematite (0001) surfaces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (${\mu _{\rm{O}} }$ ), the vacancy formation and adsorption energies were determined as a function of ${\mu _{\rm{O}} }$ . Under oxygen‐rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen‐poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all defects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geometry of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies.     

Redox Switches for Single‐Molecule Magnet Activity: An Ab Initio Insight

A dinuclear Co^II complex ( 1 ) featuring unprecedented anodic and cathodic switches for single‐molecule magnet (SMM) activity has been recently investigated (J. Am. Chem. Soc. 2013 , 135, 14670). The presence of sandwiched radicals in different oxidation states of this compound mediates magnetic coupling between the high‐spin (S=3/2) cobalt ions, which gives rise to SMM activity in both the oxidized ([ 1 (OEt₂)]⁺) and reduced ([ 1 ]^?) states. This feature represents the first example of a SMM exhibiting fully reversible, dual ON/OFF switchability. Here we apply ab initio and broken‐symmetry DFT calculations to elucidate the mechanisms responsible for magnetic properties and magnetization blocking in these compounds. It is found that due to the strong delocalization of the magnetic molecular orbital, there is a strong antiferromagnetic interaction between the radical and cobalt ions. The lack of high axiality of the cobalt centres explains why these compounds possess slow relaxation of magnetization only in an applied dc magnetic field.     

Simultaneous Aromatic–Beryllium Bonds and Aromatic–Anion Interactions: Naphthalene and Pyrene as Models of Fullerenes,Carbon Single‐Walled Nanotubes,and Graphene

The possibility of forming stable BeR₂:ArH:Y^? (R=H, F, Cl; ArH=naphthalene, pyrene; Y=Cl, Br) ternary complexes in which the beryllium compounds and anions are located on the opposite sides of an extended aromatic system is explored by means of MP2/aug‐cc‐pVDZ ab initio calculations. Comparison of the electron‐density distribution of these ternary complexes with the corresponding BeR₂:ArH and ArH:Y^? binary complexes reveals the existence of significant cooperativity between the two noncovalent interactions in the triads. The energetic effects of this cooperativity are quantified by evaluation of the three‐body interaction energy Δ³E in the framework of the many‐body interaction‐energy (MBIE) approach. Although an essential component of the interaction energies is electrostatic and is well reflected in the changes in the molecular electrostatic potential of the aromatic system on complexation, strong polarization effects, in particular for the BeR₂:ArH interactions, also play a significant role. The charge transfers associated with these polarization effects are responsible for significant distortion of both the BeR₂ and the aromatic moieties. The former are systematically bent in all the complexes, and the latter are curved to a degree that depends on the nature of the R substituents of the BeR₂ subunit.     

Experimental and Computational Investigations of the Properties of Fluorinated Single‐Walled Carbon Nanotubes

Fluorination of single‐walled carbon nanotubes by reaction with elemental fluorine at elevated temperatures provides fluorinated single‐walled carbon nanotubes (F‐SWNT), which have the highest degree of functionalization (up to F/C=1/2) of any derivatized carbon‐nanotube material reported to date. Also, F‐SWNTs have received more scrutiny than any other functionalized carbon nanotubes. This Minireview covers experimental and computational investigations of F‐SWNTs with a focus on the nature and the strength of the C–F linkage.     

Ground and Electronically Excited Singlet‐State Structures of 5‐Fluoroindole Deduced from Rotationally Resolved Electronic Spectroscopy and ab Initio Theory

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S₁) of 5‐fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration‐less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L_b.     

Covalent Attachment of Anderson‐Type Polyoxometalates to Single‐Walled Carbon Nanotubes Gives Enhanced Performance Electrodes for Lithium Ion Batteries

Single‐walled carbon nanotubes (SWNTs) covalently functionalized with redox‐active organo‐modified polyoxometalate (POM) clusters have been synthesized and employed as electrode materials in lithium ion batteries. The Anderson cluster [MnMo₆O₂₄]^9? is functionalized with Tris (NH₂C(CH₂OH)₃) moieties, giving the new organic–inorganic hybrid [N(nC₄H₉)₄]₃[MnMo₆O₁₈{(OCH₂)₃CNH₂}₂]. The compound is then covalently attached to carboxylic acid‐functionalized SWNTs by amide bond formation and the stability of this nanocomposite is confirmed by various spectroscopic methods. Electrochemical analyses show that the nanocomposite displays improved performance as an anode material in lithium ion batteries compared with the individual components, that is, SWNTs and/or Anderson clusters. High discharge capacities of up to 932 mAh g^?1 at a current density of 0.5 mA cm^?2 can be observed, together with high long‐term cycling stability and decreased electrochemical impedance. Chemisorption of the POM cluster on the SWNTs is shown to give better electrode performance than the purely physisorbed analogues.     

Electronic and magnetic structures and conductivity of strontium ferrite: An ab initio LSDA + U approach

Using the first-principle nonempirical linear muffin-tin orbital method in the tight-binding approximation (TB-LMTO) to the LSDA + U approximation, the electronic and magnetic structures and defect formation in strontium ferrite Sr₃Fe₂O₆ are studied. It is found that Sr₃Fe₂O₆ is a G type antiferromagnetic with the semiconductor electronic structure. The calculated band gap of 1.82 eV agrees well with experimental value (～2 eV). The ferrite spectrum corresponds to that of a semiconductor with a band gap of charge transfer. Iron ions in Sr₃Fe₂O₆ are in a high-spin state and have configuration t _2g ^↑3 e _g ^↑2 e _g ^↓1 . The calculated local magnetic moment on the iron ions is 3.9 μ_B. The presence of iron ions with a magnetic moment approaching 4 μ_B in Sr₃Fe₂O₆ is explained by strong hybridization of 3d orbitals of iron and 2p orbitals of oxygen. The high-spin state of iron ions is described by d ⁵ + d ⁶ L states with predominant contribution d ⁶L, where L is a hole on oxygen. Based on ab initio LSDA + U calculations, various types and configurations of defects in the oxygen sublattice (oxygen vacancies, anti-Frenkel defects) are studied and a model for ionic transport in Sr₃Fe₂O₆ is proposed.     

Homocatenation of Aluminum: Alkane‐like Structures of Li2Al2H6 and Li3Al3H8

A new class of aluminum homocatenated compounds (Li_nAl_nH_2n+2) is proposed based on quantum chemical calculations. In these compounds, Al abstracts an electron from Li, becoming valence isoelectronic with C, Si, and Ge, thus mimicking respective structural features of Group 14 hydrides. Using the Coalescence Kick search program coupled with density functional theory calculations, we investigated the potential energy surfaces of Li₂Al₂H₆ and Li₃Al₃H₆. Then single‐point‐energy coupled‐cluster calculations were performed for the lowest energy structures found. Indeed, the global minima established for Li₂Al₂H₆ and Li₃Al₃H₆ contain the Al₂H₆^2? and Al₃H₆^3? kernels, which are isostructural with ethane (C₂H₆), disilane (Si₂H₆), digermane (Ge₂H₆) and propane (C₃H₈), trisilane (Si₃H₈), trigermane (Ge₃H₈) molecules, respectively. Structural, energetic, and electronic characteristics of the Li₂Al₂H₆ and Li₃Al₃H₈ compounds are presented and the viability of their synthesis is discussed.     

Electron-spin magnetic moments of the 2Σ+ ions Li2+, Li2−, and Be2+: An ab initio ROHF study

For the ²Σ⁺ ground states of the ions Li₂⁺, Li₂⁻, and Be₂⁺, the dependence of the magnetic moment (parametrized by g-shifts) on the bond length R was studied at the ROHF level. The Δ g-values were calculated via a perturbative approach (complete to second order in Breit-Pauli interactions) using quadruple-zeta AO basis sets augmented by semidiffuse and polarization functions. All Δ g-values in these systems are negative. The parallel component Δ g_∥ generally changes little with R, remaining close to the g-shift of the corresponding ²S atomic dissociation product. For Li₂⁺ and Be₂⁺, the perpendicular component Δ g _⟂ is more sensitive to geometry than is Δ g_∥, mainly because of the second-order magnetic coupling with excited ²Π states. For Li₂⁻, Δ g _⟂ and Δ g_∥ are similar due to the large size of the 2σ_u, SOMO, resulting in g-values close to that of a free electron. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 511–521, 1997     

Elementary Photoelectronic Processes at a Porphyrin Dye/Single‐Walled TiO2 Nanotube Hetero‐interface in Dye‐Sensitized Solar Cells: A First‐Principles Study

A unique one‐dimensional (1D) sandwich single‐walled TiO₂ nanotube (STNT) is proposed as a photoanode nanomaterial with perfect morphology and large specific surface area. We have thoroughly examined the elementary photoelectronic processes occurring at the porphyrin dye/STNT hetero‐interface in dye‐sensitized solar cells (DSSCs) by theoretical simulation. It is desirable to investigate the interfacial photoelectronic processes to elucidate the electron transfer and transport mechanism in 1D STNT‐based DSSCs. We have found that the photoexcitation and interfacial charge separation mechanism can be described as follows. A ground‐state electron of the dye molecule (localized around the electron donor) is first promoted to the excited state (distributed electron donor), and then undergoes ultrafast injection into the conduction band of the STNT, leaving a hole around the oxidized dye. Significantly, the injected electron in the conduction band is transported along the STNT by means of Ti 3d orbitals, offering a unidirectional electron pathway toward the electrode for massive collection without the observation of trap states. Our study not only provides theoretical guidelines for the modification of TiO₂ nanotubes as a photoanode material, but also opens a new perspective for the development of a novel class of TiO₂ nanotubes with high power‐generation efficiency.     

Single‐Walled TiO2 Nanotubes: Enhanced Carrier‐Transport Properties by TiCl4 Treatment

In the present work we report significant enhancement of the photoelectrochemical properties of self‐ organized TiO₂ nanotubes by a combined “de‐coring” of classic nanotubes followed by an appropiate TiCl4 treatment. We show that, except for the expected particle decoration, a key effect of the TiCl4 treatment is that the electron transport characteristics in TiO₂ nanotubes can be drastically improved, for example, we observe an enhancement of up to 70 % in electron‐transport times.     

All‐Nitrogen Analogue of Ozone: Li3N3 Species

A theoretical study of ozone isoelectronic Li₃N₃ species has been performed. Ab initio electronic structure calculations prove the viability of the ozone‐like Li₃N₃ molecule that might become synthesized. The predicted Li₃N₃ species with a novel N₃^3? molecular motif possess structural and chemical bonding features similar to that of O₃ molecules and can thus be considered as an “all‐nitrogen ozone”.     

Computational Study of the Stability of Nanotube Fragments

Extremely short (<1 nm) fragments of zig‐zag carbon nanotubes are studied with ab‐initio techniques to determine their geometric and electronic structure as well as their magnetic susceptibility. It is found that for lengths of a few carbon–carbon bonds, each fragment can be viewed as composed of crowns, that is, zig‐zag rings of carbon atoms along the circumference of the tube. In this case, two kinds of electronic structures are found, depending on whether the number of carbon atoms in each crown is even or odd. Systems comprising three or more crowns either have a high spin ground state or involve a charge transfer across the length of the fragment. Conjugation changes qualitatively when the length of the fragment approaches and surpasses its girth. Indications regarding the predicted chemical stability and electronic response are provided and interpreted in terms of current densities induced within each crown by a magnetic field along the tube axis.