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A successful attempt in the functionalization and solubilization of boron nitride nanotubes is reported, and a functionalization mechanism based on interactions of amino functional groups with nanotube surface borons is proposed.
The complexes [Pd(eta2-dmfu)(P-N)] [P-N = 2-(PPh2)C6H4-1-CH=NR, R = C(6)H(4)OMe-4; CHMe2; C6H3Me2-2,6; C6H3(CHMe2)-2,6] react with an excess of BrC6H4R1-4 (R1= CF3; Me) yielding the oxidative addition products [PdBr(C6H4R1-4)(P-N)] at different rates depending on R [C6H4OMe-4 > C6H3(CHMe2)-2,6 > CHMe2 approximately C6H3Me2-2,6] and R1 (CF3>> Me). In the presence of K2CO3 and activated olefins (ol = dmfu, fn), the latter compounds react with an excess of 4-R2C6H4B(OH)2 (R2= H, Me, OMe, Cl) to give [Pd(eta2-ol)(P-N)] and the corresponding biaryl through transmetallation and fast reductive elimination. The transmetallation proceeds via a palladium(II) intermediate with an O-bonded boron anion, the formation of which is markedly retarded by increasing the bulkiness of R. The intermediate was isolated for R = CHMe2, R1 = CF3 and R2= H. The boron anion is formulated as a diphenylborinate anion associated with phenylboronic acid and/or as a phenylboronate anion associated with diphenylborinic acid. In general, the oxidative addition proceeds at a lower rate than transmetallation and represents the rate-determining-step in the coupling reaction of aryl bromides with arylboronic acids catalyzed by [Pd(eta2-dmfu)(P-N)].
Two small homogeneous markers for electron spectroscopic imaging (ESI) containing eight dodecaborane cages linked to a poly-alpha, epsilon-L-lysine dendrimer were synthesized; one of these was made water soluble by the attachment of a polyether. The markers were coupled to the sulfhydryl group of (monovalent) antibody fragments (Fab') by a homobifunctional cross-linker. While the coupling ratios of the poorly water-soluble compound did not exceed 20%, the polyether-containing variant reacted quantitatively. Its suitability for immunolabelling was tested in a study of the mechanism of the transcellular transport of an administered heterologous protein (bovine serum albumin, BSA) through ileal enterocytes of newborn piglets by endocytotic vesicles in comparison to conventional immunogold reagents. The post-embedding technique was employed. The boronated Fab' gave rise to considerably higher tagging frequencies than seen with immunogold, as could be expected from its form- and size-related physical advantages and the dense packing of BSA in the vesicles. The new probe, carrying the antigen-combining cleft at one end and the boron clusters at the opposite end of the oval-shaped conjugate, add to the potential of ESI-based immunocytochemistry.
The chemical adsorption of H atoms on an (8,0) zigzag boron nitride nanotube is studied using the density functional theory with the supercell method. One to four H atoms per 32 B and 32 N are considered. The results show that H atoms prefer to adsorb on the top sites of adjacent B and N atoms to form an armchair chain along the tube axis. An even-odd oscillation behavior of the adsorption energy of H atoms on the tube is found, and the average adsorption energy of even H atoms is obviously bigger than that of odd H atoms. The results can be understood with the frontier orbital theory. Based on this adsorption behavior, several high-symmetric structures of H adsorbed boron nitride nanotubes with 50% and 100% coverages are studied. The pairs of lines' pattern with 50% coverage has the biggest average adsorption energy per H(2) among the chosen configurations, corresponding to approximately 4 wt % hydrogen storage.
Collision and adsorption of hydrogen with high incident kinetic energies on a single-walled boron nitride (BN) nanotube have been investigated. Molecular-dynamics (MD) simulations indicate that at incident energies below 14 eV hydrogen bounces off the BN nanotube wall. On the other hand, at incident energies between 14 and 22 eV each hydrogen molecule is dissociated at the exterior wall to form two hydrogen atoms, but only one of them goes through the wall. However, at the incident energies between 23 and 26 eV all of the hydrogen atoms dissociated at the exterior wall are found to be capable of going inside the nanotube and then to recombine to form hydrogen molecules inside the nanotube. Consequently, it is determined that hydrogen should have the incident energy >22 eV to go inside the nanotube. On the other hand, we find that the collisions using the incident energies >26 eV could result in damaging the nanotube structures. In addition our MD simulations find that hydrogen atoms dissociated at the wall cannot bind to either boron or nitrogen atoms in the interior wall of the nanotube.
In this work, we have calculated boron-, aluminum-, titanium-, and nickel-doped La13 clusters by DMOL method based on the density-functional theory. Two doping modes are employed: surface and center doping. The boron, aluminum, and nickel atoms prefer to occupy the surface sites while the titanium atom prefers to occupy the center site. The doped La13 clusters with these four kinds of atoms have lower binding energy than pure La13 clusters. The icosahedral isomers are of lower binding energy than cubotahedral and decahedral isomers for La12B(-1), La12Al(-1), and La12Ni, while doping makes the cubotahedral La12Ti stable with a binding energy a little lower than icosahedral La12Ti. There are electronic shell effects in icosahedral La12B(-1) and La12Al(-1). The icosahedral La12B(-1) is promising to be formed during the doped process experimentally. Furthermore, we have also discussed the distorted structures of center doping by bond lengths, density of states, and charge transfers.
Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure ( approximately 10 GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity (TRIP) was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure (by a factor of 3-4) while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.
A set of three 14mer oligodeoxyribonucleotides of sequence d(5'-CTATGGCCTCAG*CT-3'/3'-GATACCGGAGTCGA-5') containing G* variants either as 2'-deoxyguanosine phosphate (unmodified), N7-cyanoborane 2'-deoxyguanosine phosphate (base-modified) or 2'-deoxyguanosine boranophosphate (backbone-modified) were synthesized by template-directed primer extension. Both the N7-cyanoborane 2'-deoxyguanosine triphosphate and 2'-deoxyguanosine alpha-boranotriphosphate nucleotides are good substrates for Sequenase. We infer that a single Sp boranophosphate linkage (which has a stereochemistry equivalent to the corresponding Rp thiophosphate analog) is formed in the backbone-modified 14mer. Thermally induced helix-coil transitions were monitored for the hybridized duplexes using UV and circular dichroism (CD) spectroscopy. The CD spectra of the two types of boron-modified hybrids closely resemble the unmodified parent duplex, forming B-type helices in 150 mM NaCl, 1 mM EDTA, 10 mM phosphate, pH 7.4, buffer. UV melting results indicate that both hybrids have stabilities comparable with the parent duplex as measured by Tm or delta G degree 25. These studies indicate that singly modified base- or backbone-boronated DNA are good analogs of normal DNA.
We report the preparation method of and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature T(c)(10B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in T(c) between Mg11B2 and Mg10B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.
The equation of state of boron has been measured up to 100 GPa by single-crystal x-ray diffraction with helium as the pressure transmitting medium. Rhombohedral beta-boron is the stable structure up to 100 GPa under hydrostatic conditions. Nonhydrostatic stress stabilizes a different rhombohedral structure. At about 100 GPa a pressure-induced amorphization is observed. The amorphous phase can be quenched to ambient pressure. An explanation is proposed based on the different stability under pressure between intraicosahedra and intericosahedra bonds.
Using first-principles calculations, we develop a theory for ultrahigh impurity doping in semiconductors. Our study of B in Si explains why boron solubility in epitaxial growth could exceed the solid solubility to reach the kinetic solubility, and, with adequate surface passivation, to reach even higher values. We further show that the partial ionization at high B concentration, C(B), observed by experiment is predominantly an electron chemical potential effect, not a boron clustering effect. Our calculated hole concentration over a wide C(B) range is in reasonable agreement with experiments.
Structural competition in boron group elements has been studied by means of ab initio calculations. For boron we predict a structural change alpha-B-->alpha-Ga accompanied by a nonmetal-metal transition at a pressure of about 74 GPa. For Al and Ga we find an icosahedron based elemental modification (alpha-B) 0.22 and 0.05 eV/atom, respectively, higher in energy than the corresponding metallic ground state structures. In particular, the low energy difference for Ga raises expectations into the experimental feasibility of new modifications for these elements, especially in nanosized systems.
We determine the fraction f of B atoms belonging to boroxol rings in vitreous boron oxide through a first-principles analysis. After generating a model structure of vitreous B2O3 by first-principles molecular dynamics, we address a large set of properties, including the neutron structure factor, the neutron density of vibrational states, the infrared spectra, the Raman spectra, and the 11B NMR spectra, and find overall good agreement with corresponding experimental data. From the analysis of Raman and 11B NMR spectra, we yield consistently for both probes a fraction f of approximately 0.75. This result indicates that the structure of vitreous boron oxide is largely dominated by boroxol rings.
Gibbs free-energy calculations based on density functional theory have been used to determine the possible source of failure of boron carbide just above the Hugoniot elastic limit (HEL). A range of B4C polytypes is found to be stable at room pressure. The energetic barrier for shock amorphization of boron carbide is by far the lowest for the B12(CCC) polytype, requiring only 6 GPa approximately = P(HEL) for collapse under hydrostatic conditions. The results clearly demonstrate that the collapse of the B12(CCC) phase leads to segregation of B12 and amorphous carbon in the form of 2-3 nm bands along the (113) lattice direction, in excellent agreement with recent transmission electron microscopy results.
AN INVESTIGATION WAS UNDERTAKEN TO DETERMINE WHETHER ANY OF THE FOLLOWING FUNGI HAD A REQUIREMENT FOR BORON (B): Saccharomyces cerevisiae, Aspergillus niger, Neurospora crassa, and Penicillium chrysogenum. Boron was unessential, and hence a study was made of the concentrations of B that reduced the growth of S. cerevisiae and P. chrysogenum and the mode of action of the B toxicity. Fifty and 4000 mg B/liter, respectively, significantly (5% level) reduced the growth of the latter 2 species.In both, glycolysis appeared to be inhibited by toxic levels of B, since the cells accumulated fructose-1,6-diP and ADP, but were low in glyceraldehyde-3-P and ATP. With S. cerevisiae growing on glucose, 150 mg B/liter significantly reduced CO(2) evolution. When glyceraldehyde was substituted for glucose, CO(2) evolution and O(2) consumption were unaffected by this level of B.Aldolase was suspected of being inhibited by high B, and this was confirmed using a crude aldolase extract from S. cerevisiae and purified rabbit muscle aldolase. The inhibition of aldolase by B was uncompetitive.With aldolase activity being reduced by toxic levels of B, the fungi were apparently unable to utilize carbohydrates at a rate sufficient to maintain the metabolic processes involved in growth and reproduction.
Low concentrations of boron were found to affect the bioelectric field potentials of hypocotyls excised from 4-day-old mung bean (Phaseolus aureus cv. Oklahoma 612) seedlings grown in darkness and irradiated with red light. The significance of this effect is discussed in relation to the possible role of boron in some membrane function.
A pronounced interaction between calcium, magnesium, and boron was found in growth studies with Pinus radiata cell cultures. Quantitative isoactivity data for the interaction was analyzed in terms of selected simple and plausible theoretical models. The data was found to be consistent with a model in which a critical acceptor molecule is activated only by binding both Ca and B at separate sites; Mg competitively displaces Ca to inactivate the acceptor. It was found that B is, surprisingly, not bound strongly (K(diss) = 450 +/- 80 micromolar) and that the affinity for Ca is two orders of magnitude stronger than for Mg. Therefore only a small proportion of the acceptor will be boronated under natural conditions. Moderate levels of mannitol were found to aggravate B deficiency due to its effective removal by direct chemical complexation. At higher concentrations of mannitol (or other sugars), where osmotic contribution is significant, little B was needed to overcome growth inhibition-a result consistent with B having a primary role in cell wall biosynthesis.
A new nanotube structural form is reported that resembles a double helix in multiwall boron nitride nanotubes (MW-BNNT) grown by a carbon-free chemical-vapor-deposition process as documented by evidence obtained by transmission electron diffraction and microscopy. The double-helix structure is found in MW-BNNTs exhibiting the same chirality in its different walls. The MW-BNNTs deviate from the structure of ideal nested coaxial cylindrical tubes. Most significantly, bright- and dark-field electron imaging reveals regular zigzag dark and bright spots on the side walls of the nanotubes. The repeating distance between the bright, or dark, spots is related to the chiral angle of the nanotube. Electron diffraction patterns recorded from individual nanotubes show additional diffraction spots belonging to the 201 zone axes, which are not allowed in a perfectly cylindrical nanotube. These additional diffraction spots become asymmetrical as smaller sections of the nanotube are probed. A series of diffraction patterns recorded along the tube axis showed that the imperfections giving rise to these spots move in a regular fashion around the circumference of the tube. It is shown that all experimental evidence supports the structure model of two helices; one is polygonal in cross section and highly crystalline and the other is circular and less ordered. It is further suggested that the double-helix structure is a result of stronger wall-wall interactions associated with the ionic bonding in boron nitride.