Hyperbranched polyesters: synthesis, characterization, and molecular simulations

New types of hyperbranched polyesters were synthesized by the reaction of 2,2-bis(hydroxymethyl) propionic acid as an AB2-type monomer with pentaerythritol, trimethylol propane, or glycerol as the core moiety. The obtained globular networks were characterized by NMR and MALDI-TOF spectroscopic techniques. Molecular weights determined by MALDI-TOF were confirmed by gel permeation chromatography. Fourier transform infrared (FTIR) spectroscopy was used for the quantitative evaluation of hydrogen bonding as well as to study the structure-property relationship. To investigate the changes and types of intermolecular H-bonding interactions in hyperbranched polyesters with a variation in molecular structure, the deconvolution of FTIR spectra was carried out using Origin 6.0 software through the Gaussian curve-fitting method. Molecular simulations were performed through molecular mechanics and molecular dynamics (MD) calculations using the DISCOVER module. Cohesive energy density, solubility parameters, and surface properties of the hyperbranched polyesters were calculated. Further, vibrational analysis was computed using MD simulations for all the hyperbranched polyesters developed in this work.     

Formation of 8-nitroguanine and 8-oxoguanine due to reactions of peroxynitrite with guanine

Reactions of peroxynitrite with guanine were investigated using density functional theory (B3LYP) employing 6-31G** and AUG-cc-pVDZ basis sets. Single point energy calculations were performed at the MP2/AUG-cc-pVDZ level. Genuineness of the calculated transition states (TS) was tested by visually examining the vibrational modes corresponding to the imaginary vibrational frequencies and applying the criterion that the TS properly connected the reactant and product complexes (PC). Genuineness of all the calculated TS was further ensured by intrinsic reaction coordinate (IRC) calculations. Effects of aqueous media were investigated by solvating all the species involved in the reactions using the polarizable continuum model (PCM). The calculations reveal that the most stable nitro-product complex involving the anion of 8-nitroguanine and a water molecule i.e. 8NO(2)G(-) + H(2)O can be formed according to one reaction mechanism while there are two possible reaction mechanisms for the formation of the oxo-product complex involving 8-oxoguanine and anion of the NO(2) group i.e. 8OG + NO(2)(-). The calculated relative stabilities of the PC, barrier energies of the reactions and the corresponding enthalpy changes suggest that formation of the complex 8OG + NO(2)(-) would be somewhat preferred over that of the complex 8NO(2)G(-) + H(2)O. The possible biological implications of this result are discussed.     

Closo-alanes (Al4H4, AlnHn+2, 4 <or= n <or= 8): a new chapter in aluminum hydride chemistry

Anion photoelectron spectroscopy and density functional theory were employed to study aluminum hydride clusters, AlnHm- (4 相似文献   

Structure of SiAu16: can a silicon atom be stabilized in a gold cage?

Nanostructures of Au and Si as well as Au-Si hybrid structures are topics of great current interest from both scientific and technological points of view. Recent discovery of Au clusters having fullerene-like geometries and the possibility of endohedral complexes with Si atoms inside the Au cage opens new possibilities for designing Au-Si nanostructures. Using ab initio simulated annealing method we have examined the stability of Si-Au16 endohedral complex. Contrary to what we believed, we find that the endohedral configuration is metastable and the structure where Si atom binds to the exterior surface of the Au16 cage is the lowest energy structure. The bonding of Si to Au cluster mimics its behavior of that in bulk and liquid phase of Au. In addition, doping of Si in high concentration would cause fracture and embrittlement in gold nanostructures just as it does in the bulk phase. Covalent bonding between Au-Au and Au-Si is found to be a dominant feature in the stability of the Au-Si nanostructures. Our study provides insight that may be useful in fabricating hybrid Au-Si nanostructures for applications microelectronics, catalysis, biomedicine, and jewelry industry.     

Synthesis of flower-like gold nanoparticles and their electrocatalytic activity towards the oxidation of methanol and the reduction of oxygen

This article describes the synthesis of branched flower-like gold (Au) nanocrystals and their electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Gold nanoflowers (GNFs) were obtained by a one-pot synthesis using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) as a reducing/stabilizing agent. The GNFs have been characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and electrochemical measurements. The UV-visible spectra show two bands corresponding to the transverse and longitudinal surface plasmon (SP) absorption at 532 and 720 nm, respectively, for the colloidal GNFs. The GNFs were self-assembled on a sol-gel-derived silicate network, which was preassembled on a polycrystalline Au electrode and used for electrocatalytic applications. The GNFs retain their morphology on the silicate network; the UV-visible diffuse reflectance spectra (DRS) of GNFs on the silicate network show longitudinal and transverse bands as in the case of colloidal GNFs. The GNFs show excellent electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Oxidation of methanol in alkaline solution was observed at approximately 0.245 V, which is much less positive than that on an unmodified polycrystalline gold electrode. Reduction of oxygen to H2O2 and the further reduction of H2O2 to water in neutral pH were observed at less negative potentials on the GNFs electrode. The electrocatalytic activity of GNFs is significantly higher than that of the spherically shaped citrate-stabilized Au nanoparticles (SGNs).     

Investigation of hot electrons and hot phonons generated within an AlN/GaN high electron mobility transistor

We review our recent results obtained on an AlN/GaN-based high-electron-mobility transistor. The temperature of the electrons drifting under a relatively-high electric field is significantly higher than the lattice temperature (i.e., the hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. We have also measured the decay time constant for LO phonons to be about 4.2 ps. An electric field present in a GaN/AlN heterostructure can bring both the first-order and second-order Raman scattering processes into strong resonances. The resonant Stokes and anti-Stokes Raman scattering results in the increase and decrease of non-equilibrium longitudinal-optical phonon temperatures, respectively. Moreover, the phonon temperature measured from the Raman scattering is increased with an applied electric field at a much higher rate than the lattice temperature due to the presence of field-induced non-equilibrium longitudinal-optical phonons.     

The Reissner-Mindlin plate is the Γ-limit of Cosserat elasticity

The linear Reissner-Mindlin membrane-bending plate model is the rigorous Γ-limit for zero thickness of a linear isotropic Cosserat bulk model with symmetric curvature. For this result we use the natural nonlinear scaling for the displacements u and the linear scaling for the infinitesimal microrotations Ā ∈ so(3) affecting the in-plane drill rotations. No boundary conditions on the microrotations are prescribed. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chemically controlled self-assembly of protein nanorings

The exploitation of biological macromolecules, such as nucleic acids, for the fabrication of advanced materials is a promising area of research. Although a greater variety of structural and functional uses can be envisioned for protein-based materials, systematic approaches for their construction have yet to emerge. Consistent with theoretical models of polymer macrocyclization, we have demonstrated that, in the presence of dimeric methotrexate (bisMTX), wild-type Escherichia coli dihydrofolate reductase (DHFR) molecules tethered together by a flexible peptide linker (ecDHFR(2)) are capable of spontaneously forming highly stable cyclic structures with diameters ranging from 8 to 20 nm. The nanoring size is dependent on the length and composition of the peptide linker, on the affinity and conformational state of the dimerizer, and on induced protein-protein interactions. Delineation of these and other rules for the control of protein oligomer assembly by chemical induction provides an avenue to the future design of protein-based materials and nanostructures.     

Multiferroic materials based on organic transition-metal molecular nanowires

We report on the density functional theory aided design of a variety of organic ferroelectric and multiferroic materials by functionalizing crystallized transition-metal molecular sandwich nanowires with chemical groups such as -F, -Cl, -CN, -NO(2), ═O, and -OH. Such functionalized polar wires exhibit molecular reorientation in response to an electric field. Ferroelectric polarizations as large as 23.0 μC/cm(2) are predicted in crystals based on fully hydroxylized sandwich nanowires. Furthermore, we find that organic nanowires formed by sandwiching transition-metal atoms in croconic and rhodizonic acids, dihydroxybenzoquinone, dichloro-dihydroxy-p-benzoquinone, or benzene decorated by -COOH groups exhibit ordered magnetic moments, leading to a multiferroic organometallic crystal. When crystallized through hydrogen bonds, the microscopic molecular reorientation translates into a switchable polarization through proton transfer. A giant interface magnetoelectric response that is orders of magnitude greater than previously reported for conventional oxide heterostructure interfaces is predicted.