The influence of nanotubes with various chirality on solvent structure

The structural and dynamic properties of solutions of electrolytes close to single-walled carbon nanotubes was studied by the method of molecular dynamics. The influence of nanotube chirality on the structure of solutions was analyzed.     

The influence of substrate structure on the kinetics of the hydrolysis of starch by glucoamylase

In this research the influence of substrate structure on the kinetics of the enzymatic hydrolysis of starch by glucoamylase was evaluated. For this purpose, two substrates of different form and molecular weight were used. In one case, the kinetics of the hydrolysis corresponds to a typical Michaelis-Menten behavior; in the other, a decrease of the hydrolysis rate occurred once a determined substrate concentration was surpassed. The structural differences between the starches, which caused important differences on the rheological properties of their solutions, justify the observed differences in their behavior. Branching of the substrate exerts two opposite effects on the hydrolysis rate because it allows the increase of the number of available points for the enzymatic attack, although the branching increases the steric hindrances and, consequently, the mass transfer resistances. The balance between these two effects is clearly dependent on the substrate concentration.     

Solvation bifunctional catalysis of the hydrolysis of sulfonyl chlorides by hydration complexes of 2-propanol: influence of the substrate structure

The temperature dependence of rate constants for the pseudo-first-order hydrolysis of 2-methylbenzenesulfonyl chloride, 1,5-naphthalenedisulfonyl chloride, and 4-acetamidobenzenesulfonyl chloride in water-2-propanol mixtures is studied in the i-PrOH mole fraction range x₂ = 0–0.10. The concentration dependences of the enthalpy of activation and the entropy contribution ( = 100(–TS)/(G, %) for the hydrolyses of all sulfonyl chlorides are nonmonotonic, depending on the sulfonyl chloride structure and the degree of complementarity of the hydration complexes to the solvent structure. The sulfonyl chlorides are hydrolyzed via two pathways: one of them involves a water dimer as a bifunctional catalyst along with a water molecule as a nucleophile, and the other involves an alcohol hydrate.Translated from Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 35–43.Original Russian Text Copyright © 2005 by Ivanov, Mikhailov, Gnedin, Lebedukho, Korolev.     

Silicon carbide nanotubes and nanotubular fibers: Synthesis, stability, structure, and classification

Published data on silicon carbide nanotubes (SiC-NT) are analyzed. According to theoretical calculations, single-layer SiC-NTs do not dissociate, but they have not yet been detected experimentally. According to the experimental data, metastable SiC-NTs with walls consisting of several layers and nanotube fibers were produced. The optimized structure of single-layer SiC-NTs was calculated by the RHF/6-31G quantum-chemical method. The possibility of obtaining SiC-NTs by gas-phase chemical deposition from methyltrichlorosilane in the temperature range of 800–1000 °C was investigated. Nanofibers and polygrained SiC nanotubes were obtained, but ordinary layer SiC nanotubes were not detected. To remove the inconsistencies it was first proposed to classify the nanotubes according to the structure of their walls, separating all the SiC-NTs into three types: 1) ordinary layer nanotubes with rolled layers, similar to carbon nanotubes; 2) polynanocrystalline nanotubular fibers or nanotubes with walls consisting of linked differently oriented nanograins; 3) monocrystalline synthetic nanotubes with ideal crystalline walls. It was concluded that the ordinary SiC-NTs of the first type are unstable with the exception of one-or two-layer nanotubes; stable SiC-NTs of the first type and SiC-NTs of the third type have not yet been discovered; only nanotubular fibers of the second type were obtained experimentally. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 3–13, January–February, 2006.     

Synthesis of InP nanotubes

Indium phosphide (InP) nanotubes have been synthesized via the vapor-liquid-solid (VLS) growth mechanism. The nanotubes are crystalline and have the (bulk) zinc blende structure and therefore represent a new class of tube materials. The tubes show photoluminescence, which is considerably blue-shifted with respect to bulk emission, indicating that the optical properties are not dominated by defect states. They are formed at higher temperatures than those at which nanowires are fabricated. A simple model for the formation of the nanotubes is presented. The wall thickness can be controlled by the synthesis temperature and is in the range of 2-20 nm.     

Synthesis of carbon nanotubes on water-soluble supports

Gas-phase pyrolytic synthesis of carbon to give multilayer carbon nanotubes on Ni catalysts supported by NaCl and K₂SO₄ mineral water-soluble substrates was studied. The morphology of the nanotubes synthesized was examined.     

Synthesis of alumina nanotubes using carbon nanotubes as templates

Alumina nanotubes have been fabricated using carbon nanotubes (CNTs) as templates at 1473 K. The Al₂O₃ nanotubes are polycrystals. They are less than 100 nm in outer diameter and tens of nanometer in inner diameter, which is close to the outer diameters of the templates. Under certain conditions, AlN and Al₂O₃ nanowires can also be fabricated in this reaction system. Discussions on the growth mechanisms of these nanotubes and nanowires are presented.     

Effect of the temperature on the composition and ratio of hydrolysis products of 1,2-dichlorotetramethyldisilane

The hydrolysis of 1,2-dichlorotetramethyldisilane was studied at different temperatures. At reduced temperatures, the hydrolysis gave permethylcyclo(oxadisilanes) [(Me₂Si)₂O]_n (n = 2 and 3) and α,ω-dihydroxypermethyloligo(oxadisilanes) HO[(SiMe₂)₂O]_mH (m = 1–5). The formation of the latter was proved by the GC-MS method. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 722–724, April, 2006.     

Electronic structure of GaN nanotubes

Nanotube properties are strongly dependent on their structures. In this study, gallium nitride nanotubes (GaNNTs) are analyzed in armchair and zigzag conformations. The wurtzite GaN (0001) surface is used to model the nanotubes. Geometry optimization is performed at the PM7 semiempirical level, and subsequent single-point energy calculations are carried out via Hartree–Fock and B3LYP methods, using the 6-311G basis set. Semiempirical and ab initio methods are used to obtain strain energy, charge distribution, dipole moment, |HOMO-LUMO| gap energy, density of states and orbital contribution. The gap energy of the armchair structure is 3.82 eV, whereas that of the zigzag structure is 3.92 eV, in agreement with experimental data.     

Vanadium-oxide nanotubes: Synthesis and template-related electrochemical properties

The electrochemistry of mixed-valent-containing vanadium-oxide nanotubes (VO_x-NTs) is first reported. Using dodecylamine and hexadecylamine as templates, two kinds of VO_x-NTs were synthesized and characterized. Both VO_x-NTs contain V₂O₃, VO₂ and V₂O₅ species. Dodecylamine-templated VO_x-NTs (C₁₂-VO_x-NTs) are much more stable than hexadecylamine-templated VO_x-NTs (C₁₆-VO_x-NTs). C₁₂-VO_x-NTs demonstrate stable redox behavior at ∼−0.2 V, which is arising from the electrochemical transfer between V(III) and V(IV), whereas C₁₆-VO_x-NTs exhibit unstable and weak redox peaks. In both cases, the reduction reaction of V(V)–V(IV) is never observed. The oxidation peak for V(IV) appeared only after a relatively long time immersion, suggesting that some small molecules such as water intercalate into the layered nanostructures before the electrochemical reaction. The difference may be an origin of the different template-induced the electroactivity of vanadium-oxide species in the nanotube-microenvironment.     

Ethylenediamine-anchored gold nanoparticles on multi-walled carbon nanotubes: Synthesis and characterization

Binding of gold nanoparticles (Au-NP) at amine-functionalised multi-walled carbon nanotubes (MWNTs) is proposed. The MWNTs are functionalised with acylchloride groups, which further react with ethylenediamine to form amine-functionalised MWCNTs. These amines are able to bind preformed colloidal Au-NPs. The Au/MWNT composite material facilitates electron-transfer reactions with free-diffusing redox compounds.     

荧光素二苯甲酰酯的晶体结构与水解性质的研究

以荧光素和苯甲酰氯为原料制备了荧光素二苯甲酰酯, 并用X射线单晶法测定了晶体和分子结构。晶体空间群为P21/C, a=0.8459(2), b=1.8221(5),c=1.7491(8)nm; β=103.79(3)°; V=2.618nm3, Z=4。酯水解和酯酶水解实验表明其水解速度明显低于荧光素的二醋酸酯、二丙酸酯、二丁酸酯和二已酸酯。根据酯链上苯环的空间位阻和共轭效应, 以及质谱分析结构对这一特性作了解释。     

Polyesters of phosphoric acid: Synthesis and kinetics of hydrolysis

Three major methods have been elaborated in our laboratory for preparation of polymers with poly(alkylene phosphates) backbones: ring-opening polymerization, poly-condensation and transesterification, and polyaddition. The second method is based on the reaction of the commercially available compounds, namely dialkyl (or diaryl)-H-phosphonates and glycols. Reaction of the aliphatic H-phosphonates with aliphatic glycols is a reversible process, whereas polycondensation of diphenyl H-phosphonates with aliphatic and cycloaliphatic diols is practically irreversible. This latter method has recently been developed and is described in this paper. Poly H-phosphonates with M̄_n up to 40–10³ were prepared. Polymers are easily oxidized and quantitatively converted into the relatively stable poly(alkylene phosphates). Some physical properties of these polymers and kinetics of their hydrolysis is discussed.     

Synthesis, photochemistry, and photophysics of butadiene derivatives: influence of the methyl group on the molecular structure and photoinduced behavior

Novel butadiene derivatives display diverse photochemistry and photophysics. Excitation of 2-methyl-1-(o-vinylphenyl)-4-phenylbutadiene leads to the dihydronaphthalene derivative, whereas photolysis of the corresponding model o-methyl analogue results in the formation of the naphthalene-like derivative, deviating from the nonmethylated analogue of the prior starting compound and producing benzobi- and -tricyclic compounds. The effect of the methyl substituents is even more dramatic in the case of the dibutadienes. The parent unsubstituted compound undergoes photoinduced intramolecular cycloaddition giving benzobicyclo[3.2.1]octadiene, whereas the photochemical reaction of the corresponding dimethylated derivative shows only geometrical isomerization due to the steric effect of the substituents. Methyl groups on the butadiene backbones reduce the extent of conjugation, causing a blue-shift of the characteristic absorption band. The fluorescence efficiency is dramatically decreased, as a consequence of nonplanarity and reduced rigidity of the molecules due to the crowding by the methyl and phenyl groups together. Four molecules of very similar structures show dramatically different photoinduced behavior, revealing how changes of the nature and position of the substituents are valuable in understanding the photophysics and photochemistry of these types of compounds.     

Synthesis and the crystal structure of [Hg6As4](InCl6)Cl: influence of covalent and electrostatic factors on the supramolecular structure

A new inorganic supramolecular complex [Hg₆As₄](InCl₆)Cl was synthesized. The complex crystallizes in the cubic space group Pa3 with the unit cell parameter a = 12.109(1) and Z = 4. The crystal structure of the complex is based on the ³[Hg₆As₄]⁴⁺ three-dimensional cationic host framework. The guest InCl₆ ^3– and Cl^– anions are located in the cavities of the framework. The host—guest interactions were examined by quantum-chemical calculations of the equilibrium geometry of the InCl₆ ^3– anion and analysis of the periodic zero-potential surface of the supramolecular complex. It was demonstrated that the electrostatic factors dominate over the covalent factors in the organization of the supramolecular structure of [Hg₆As₄](InCl₆)Cl.     

The influence of carbon nanotubes on the relative stability of diborane molecular forms

Relative stability of three molecular forms of diborane in the model single-walled carbon nanotubes has been studied by means of the PBE/3ζ hybrid DFT method. The influence of the force field of the nanotube results in the decrease of the В–В bond length, decrease of the energy difference between the orthogonal and the planar forms, and appearance of high negative charge at the В₂Н₄ molecule. The strengthening of these effects is not related to the length and geometry [(n,n) or (n,0)] of the nanotube, but is determined only by the nanotube diameter.     

Nitrogen-doped carbon nanotubes: growth, mechanism and structure

Nitrogen-doped bamboo-structured carbon nanotubes have been successfully grown using a series of cobalt/molybdenum catalysts. The morphology and structure of the nanotubes were analysed by transmission electron microscopy and Raman spectroscopy. The level of nitrogen doping, as determined by X-ray photoelectron spectroscopy, was found to range between 0.5 to 2.5 at.%. The growth of bamboo-structured nanotubes in the presence of nitrogen, in preference to single-walled and multi-walled nanotubes, was due to the greater binding energy of nitrogen for cobalt in the catalyst compared to the binding strength of carbon to cobalt, as determined by density functional theory.