Showing papers in "Journal of the American Chemical Society in 1999"

A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties

Abstract: BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or ...

Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks

Abstract: Novel organic−inorganic hybrid mesoporous materials have been synthesized, in which organic and inorganic oxide moieties are distributed homogeneously at the molecular level in the framework, forming a covalently bonded network. They are highly ordered at the mesoscale, with two- and three-dimensional hexagonal symmetries and well-defined external morphologies. Nitrogen adsorption measurements show a uniform pore-size distribution with pore diameters of 31 and 27 A, and high surface areas of 750 and 1170 m2/g. The synthetic procedure to polymerize the organosilane monomer containing two trialkoxysilyl groups in the presence of surfactant can be applied to the synthesis of a variety of highly ordered organic−inorganic hybrid mesoporous materials.

Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals

Abstract: To explore the relationship between local electromagnetic field enhancement and the large SERS (surface enhanced Raman scattering) enhancement that enables the observation of single molecule Raman spectra, we measure both resonant Rayleigh scattering spectra and rhodamine 6G Raman spectra from single Ag particles. Our apparatus combines the techniques of dark-field optical microscopy for resonant Rayleigh measurements, and grazing incidence Raman spectroscopy. The Rayleigh spectra show that the citrate-reduced Ag colloid is extremely heterogeneous. Only the larger particles, in part created by salt induced aggregation, show a large SERS effect. In agreement with the work of Nie and Emory, we find that a few nanocrystals show huge single molecule R6G SERS intensities. While all SERS active particles have some resonant Rayleigh scattering at the 514.5 nm laser wavelength, there is no correlation between the resonant Rayleigh spectra and the SERS intensity. We discuss a model in which huge SERS intensities r...

Development of a Universal Alkoxyamine for “Living” Free Radical Polymerizations

Abstract: Examination of novel alkoxyamines has demonstrated the pivotal role that the nitroxide plays in mediating the “living” or controlled polymerization of a wide range of vinyl monomers. Surveying a variety of different alkoxyamine structures led to α-hydrido derivatives based on a 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy, 1, skeleton which were able to control the polymerization of styrene, acrylate, acrylamide, and acrylonitrile based monomers. For each monomer set, the molecular weight could be controlled from 1000 to 200 000 amu with polydispersities typically 1.05−1.15. Block and random copolymers based on combinations of the above monomers could also be prepared with similar control. In comparison with 2,2,6,6-tetramethylpiperidinoxy (TEMPO), these new systems represent a dramatic increase in the range of monomers that can be polymerized under controlled conditions and overcome many of the limitations associated with nitroxide-mediated “living” free radical procedures. Monomer selection and functional...

Highly Active Palladium Catalysts for Suzuki Coupling Reactions

Abstract: Mixtures of palladium acetate and o-(di-tert-butylphosphino)biphenyl (4) catalyze the room-temperature Suzuki coupling of aryl bromides and aryl chlorides with 0.5−1.0 mol % Pd. Use of o-(dicyclohexylphosphino)biphenyl (2) allows Suzuki couplings to be carried out at low catalyst loadings (0.000001−0.02 mol % Pd). The process tolerates a broad range of functional groups and substrate combinations including the use of sterically hindered substrates. This is the most active catalyst system in terms of reaction temperature, turnover number, and steric tolerance which has been reported to date.

Iron and cobalt ethylene polymerization catalysts bearing 2,6-bis(imino)pyridyl ligands : synthesis, structures, and polymerization studies

Abstract: The synthesis, characterization, and ethylene polymerization behavior of a series of iron and cobalt halide complexes, LMXn (M = Fe, X = Cl, n = 2, 3, X = Br, n = 2; M = Co, X = Cl, n = 2), bearing chelating 2,6-bis(imino)pyridyl ligands L [L = 2,6-(ArNCR1)2C5H3N] is reported. X-ray diffraction studies show the geometry at the metal centers to be either distorted square pyramidal or distorted trigonal bipyramidal. Treatment of the complexes LMXn with methylaluminoxane (MAO) leads to highly active ethylene polymerization catalysts converting ethylene to highly linear polyethylene (PE). LFeX2 precatalysts with ketimine ligands (R1 = Me) are approximately an order of magnitude more active than precatalysts with aldimine ligands (R1 = H). Catalyst productivities in the range 3750−20600 g/mmol·h·bar are observed for Fe-based ketimine catalysts, while Co ketimine systems display activities of 450−1740 g/mmol·h·bar. Molecular weights (Mw) of the polymers produced are in the range 14000−611000. Changing reaction ...

Olefin Metathesis-Active Ruthenium Complexes Bearing a Nucleophilic Carbene Ligand

Abstract: The reaction of [Cp*RuCl]4 (1; Cp* = η5-C5Me5) with the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) affords a coordinatively unsaturated Cp*Ru(IMes)Cl (3) complex in 86% isolated yield. Solution calorimetric results in this system provide information concerning the electron donor properties of the carbene ligand, which are very similar to those of PCy3. Structural information from single-crystal X-ray studies for complex 3 allows the determination of steric parameters associated with this ligand. The thermochemical information is used to predict the magnitude of the enthalpic driving force behind substitution reactions involving RuCl2(C(H)Ph)(PCy3)2 (1) and the carbene ligand, IMes, affording the RuCl2(C(H)Ph)(PCy3)(IMes) (6) complex in high yield. A similar mixed carbene/phosphine ruthenium complex, RuCl2(C(H)Ph)(PPh3)(IMes), can also be isolated from RuCl2(C(H)Ph)(PPh3)2 and the IMes ligand. A single-crystal X-ray diffraction study has been performed on 6. The thermal stabilit...

A Recyclable Ru-Based Metathesis Catalyst

Abstract: A Ru carbene (8, Scheme 2) that contains an internal metal−oxygen chelate is an active metathesis catalyst and is readily obtained by the sequential treatment of Cl2Ru(PPh3)3 with (2-isopropoxyphenyl)diazomethane and PCy3 This Ru-carbene complex offers excellent stability to air and moisture and can be recycled in high yield by silica gel column chromatography The structures of this and related complexes have been unambiguously established by NMR and single-crystal X-ray diffraction studies

Fundamental Properties of the CH···O Interaction: Is It a True Hydrogen Bond?

Abstract: Ab initio calculations are used to analyze the CH···O interaction between FnH3-nCH as proton donor and H2O, CH3OH, and H2CO as acceptor. The interaction is quite weak with CH4 as donor but is enhanced by 1 kcal/mol with each F added to the donor. The CH···O interaction behaves very much like a conventional OH···O H-bond in most respects, including shifts in electron density that accompany the formation of the bond and the magnitudes of the various components of the interaction energy. The two sorts of H-bonds also gravitate toward a similar equilibrium geometry and are comparably sensitive to deformations from that structure. In a quantitative sense, while both CH···O and OH···O prefer a linear configuration, the former is somewhat more easily bent and is less sensitive to stretches from its equilibrium H-bond length. Whereas the OH bond has been shown to stretch and undergo a red shift in its vibrational frequency upon formation of a H-bond, the CH bond of the molecules studied here follows the opposite ...

From Condensed Lanthanide Coordination Solids to Microporous Frameworks Having Accessible Metal Sites

Abstract: The combination of terbium nitrate and 1,4-benzenedicarboxylic acid (H2BDC) in the presence of triethylamine yields the compound Tb2(BDC)3·(H2O)4, which has an extended nonporous structure constructed from copolymerized BDC and Tb(III) units. The multidentate functionality of BDC and the tendency of Tb to have a high coordination number has allowed water to act as a terminal ligand to Tb in the structure. Upon thermally liberating the water ligands, a microporous material, Tb2(BDC)3, is achieved, which has extended 1-D channels and the same framework structure as that of the as-synthesized solid as evidenced by XRPD. Water sorption isotherm data proves that Tb2(BDC)3 has permanent microporosity, and points to the presence of accessible metal sites within the pores, which also allows the sorption of ammonia to give Tb2(BDC)3·(NH3)4. Luminescence lifetime measurements confirm that resorbed water and sorbed ammonia are bound to Tb and that they give distinctly different decay constants.

Mesocellular siliceous foams with uniformly sized cells and windows

Abstract: Molecular sieves with uniform large pores are desirable for chemical reactions and for use in separations involving large molecules.1 Periodic cubic and hexagonal mesoporous silica phases with uniform large pores have been synthesized by using nonionic triblock and star diblock copolymers as templates.2 Control over the pore size is achieved by adjusting the hydrophobic volumes of the self-assembled aggregates.2,3 In this paper, we describe how adding a sufficiently large amount of an organic cosolvent induces a phase transformation from the highly ordered p6mm mesostructure of SBA-15-type mesoporous silicas to remarkable mesostructured cellular foams (mesocellular foams, MCFs) composed of uniformly sized, large spherical cells that are interconnected by uniform windows to create a continuous 3-D pore system. The interconnected nature of the large uniform pores makes these new mesostructured silicas promising candidates for supports for catalysts and in separations involving large molecules, and they may be of interest in low-dielectric applications. The MCFs have been synthesized in aqueous acid by using dilute Pluronic P123 solutions in the presence of 1,3,5-trimethylbenzene (TMB) as organic cosolvent.4 X-ray diffraction (XRD) experiments5 reveal well-resolved peaks at small angles, as shown in Figure 1 for a sample with a cell diameter of 33 nm. Careful analyses of the scattering data for MCFs show that the higher order peaks cannot be indexed to any plane or space group (e.g., p6mm) or to a lamellar diffraction pattern. In fact, after subtraction of the background,6 the X-ray data are in good agreement with simulated scattering7 due to monodisperse spheres (cells) of diameter D (see Table 1), while attempts to fit the X-ray data to

Computational Alanine Scanning To Probe Protein−Protein Interactions: A Novel Approach To Evaluate Binding Free Energies

Abstract: Noncovalent interactions are important in many physiological processes of complexation which involve all components of the living cells. Here we report an approach to computationally study the interaction free energies in protein−protein complexes which allows from a single simulation an estimate of the individual contribution of each residue to the binding. We developed this new techniquecomputational alanine scanningand applied it to study the interactions of the oncoprotein Mdm2 to the N-terminal stretch of tumor suppressor protein p53. Excellent agreement has been found between the calculated and experimental data. This residue mutation methodology could prove to be a useful general design tool for moleculesnucleotides, peptides, lipids, or any other organic compoundoptimized for interactions or stability, since one can qualitatively estimate the free energy consequences of many mutations from a single molecular dynamics trajectory.

A New Nonhydrolytic Single-Precursor Approach to Surfactant-Capped Nanocrystals of Transition Metal Oxides

Abstract: The advent of new methods to prepare semiconductor and metal nanocrystals, specifically the injection of molecular precursors into hot organic surfactants, has yielded markedly improved samples with good size control, narrow size distribution, and good crystallinity of individual and dispersable nanocrystals. It is of considerable interest to apply these methods to the synthesis of transition metal oxide nanoparticles, which typically are prepared by methods involving water as solvent or reactant. The authors report the first results of a new nonhydrolytic single-precursor approach to the synthesis of dispersable nanocrystals of transition metal oxides.

Programmable one-pot oligosaccharide synthesis

Abstract: The reactivity of a number of p-methylphenyl thioglycoside (STol) donors which are either fully protected or have one hydroxyl group exposed has been quantitatively determined by HPLC in conjunction with the development of a broadly applicable approach for a facile one-pot synthesis of oligosaccharides. The influence on reactivity of the structural effects of different monosaccharide cores and different protecting groups on each glycoside donor is characterized and quantified. In addition, a correlation between glycosyl donor reactivity and the chemical shift of the anomeric proton by 1 H NMR has been established. A database of thioglycosides as glycosyl donors has been created using this reactivity data. The utility is demonstrated by the easy and rapid one-pot assembly of various linear and branched oligosaccharide structures. In addition, a computer program as been described for use as a database search tool and guide for the selection of building blocks for the one-pot assembly of a desired oligosaccharide or a library of individual oligosaccharides.

What Do the Kohn−Sham Orbitals and Eigenvalues Mean?

Abstract: Kohn−Sham orbitals and eigenvalues are calculated with gradient-corrected functionals for a set of small molecules (H2O, N2, CrH66-, and PdCl42-), varying basis sets and functionals. The calculated Kohn−Sham (KS) orbital shapes, symmetries, and the order and absolute energy of the associated eigenvalues are investigated and compared with those of Hartree−Fock (HF) and one-electron extended Huckel (eH) calculations, as well as experimental ionization potentials. The shape and symmetry properties of the KS orbitals are very similar to those calculated by HF and eH methods. The energy order of the occupied orbitals is in most cases in agreement among the various methods. The order of empty orbitals of a minimal basis set is sometimes interchanged, within that group or with some orbitals resulting from a larger basis calculation. Overall the KS orbitals are a good basisas Baerends suggestedfor qualitative interpretation of molecular orbitals. For the Kohn−Sham eigenvalues we find an approximately linear depen...

Sol−Gel Synthesis of KVII[CrIII(CN)6]·2H2O: A Crystalline Molecule-Based Magnet with a Magnetic Ordering Temperature above 100 °C

Abstract: Recently, there has been much interest in the synthesis of molecule-based magnets, which are magnets whose solid-state structures consist of arrays of molecular units. 1,2 Such solids do not at present have real-world uses, in part because none of them possesses a key characteristic: for most commercial applications, a magnet must retain its magnetism well above room temperature. Of the four molecule-based magnets that remain magnetic at room temperature, one decomposes at 350 K, 3 and three demagnetize near 315 K. 4,5 All four of these solids are amorphous, and their solid state structures are unknown, although the latter three probably adopt a structure like that of the pigment Prussian blue. We now describe the use of sol -gel methods to synthesize several crystalline molecule-based magnets with the Prussian blue structure. The gelation process appears to be important to the growth of the crystalline phase. Most notable among the current results is the discovery of a molecule-based magnet, KV II[CrIII (CN)6]‚2H2O, with the unprecedented magnetic ordering temperature of 376 K (103°C). The four known molecule-based magnets that keep their magnetism at room temperature are listed below, along with the temperatures at which the magnetism is lost:

Opls all-atom model for amines : resolution of the amine hydration problem

Abstract: Classical force-field parameters have been developed for amines primarily by fitting to experimental data for pure liquids and to hydrogen-bond strengths from gas-phase ab initio calculations. The resultant parameters were used to calculate relative free energies of hydration for ammonia, methylamine, dimethylamine, and trimethylamine using free energy perturbation calculations in Monte Carlo simulations (MC/FEP). The results including the fact that the most favorable ΔGhyd occurs for methylamine are in excellent agreement with the experimental data, in contrast to numerous prior computational reports. The calculations reveal two opposing trends in water: increased contribution from hydrogen-bond acceptance and diminished contribution from hydrogen-bond donation with increasing methylation of the amines. The proper balance of hydrogen-bond strengths, which is achieved with the OPLS-AA force field, is essential for correct ordering of the free energies of hydration. MC simulations for the pure liquids of ...

Top down versus bottom up protein characterization by tandem high- resolution mass spectrometry

Abstract: Characterization of larger proteins by mass spectrometry (MS) is especially promising because the information complements that of classical techniques and can be obtained on as little as 10-17 mol of protein. Using MS to localize errors in the DNA-derived sequence or modifications (posttranslational, derivatized active sites, etc.) usually involves extensive proteolysis to yield peptides of <3 kDa, with separation and MS/MS to compare their sequences to those expected (the “bottom up” approach). In contrast, an alternative “top down” approach limits the dissociation (proteolysis or MS/MS) to yield larger products from which a small set of complementary peptides can be found whose masses sum to those of the molecule. Thus a disagreement with the predicted molecular mass can be localized to a fragment(s) without examining all others, with further dissociation of the fragments in the same way providing further localization. Using carbonic anhydrase (29 kDa) as an example, Fourier transform mass spectrometry ...

Nanocages Derived from Shell Cross-Linked Micelle Templates

Abstract: Over the past decade, there has been a surge of interest in nanomaterials, which include structures with at least one dimension under 100 nm. It has been found that the ability to tailor the composition, structure, properties, and function of organic materials with control on the nanometer scale is leading to the production of nanomaterials that exhibit interesting properties, and which may be applicable in broadly ranging technologies. Of particular recent interest are hollow-sphere structures, due to their potential for encapsulation of large quantities of guest molecules or largesized guests within the “empty” core domain. For example, the microencapsulation of biologically active components has been investigated for the development of artificial cells. 1

Electron Capture Dissociation of Gaseous Multiply-Charged Proteins Is Favored at Disulfide Bonds and Other Sites of High Hydrogen Atom Affinity

Abstract: Disulfide bonds in gaseous multiply-protonated proteins are preferentially cleaved in the mass spectrometer by low-energy electrons, in sharp contrast to excitation of the ions by photons or low-energy collisions. For S−S cyclized proteins, capture of one electron can break both an S−S bond and a backbone bond in the same ring, or even both disulfide bonds holding two peptide chains together (e.g., insulin), enhancing the sequence information obtainable by tandem mass spectrometry on proteins in trace amounts. Electron capture at uncharged S−S is unlikely; cleavage appears to be due to the high S−S affinity for H• atoms, consistent with a similar favorability found for tryptophan residues. RRKM calculations indicate that H• capture dissociation of backbone bonds in multiply-charged proteins represents nonergodic behavior, as proposed for the original direct mechanism of electron capture dissociation.

Preparation of Structurally Well-Defined Polymer−Nanoparticle Hybrids with Controlled/Living Radical Polymerizations

Abstract: Spherical nanoparticles can be assembled into periodic, closepacked layers using sedimentation or slow crystallization procedures, 4,5 and these arrays act as Bragg gratings and diffract light of specific wavelengths. 6 Such materials could be developed into diffractive optics and photonic band gap materials. Other nanoparticle assembly methods have been investigated, including the use of DNA conjugates, 7-9 assembly onto monolayer surfaces, 10,11

Kinetics of RNA Degradation by Specific Base Catalysis of Transesterification Involving the 2‘-Hydroxyl Group

Abstract: A detailed understanding of the susceptibility of RNA phosphodiesters to specific base-catalyzed cleavage is necessary to approximate the stability of RNA under various conditions. In addition, qua...

Novel Electron-Rich Bulky Phosphine Ligands Facilitate the Palladium-Catalyzed Preparation of Diaryl Ethers

Abstract: A general method for the palladium-catalyzed formation of diaryl ethers is described. Electron-rich, bulky aryldialkylphosphine ligands, in which the two alkyl groups are either tert-butyl or 1-adamantyl, are the key to the success of the transformation. A wide range of electron-deficient, electronically neutral and electron-rich aryl bromides, chlorides, and triflates can be combined with a variety of phenols with the use of sodium hydride or potassium phosphate as base in toluene at 100 °C. The bulky yet basic nature of the phosphine ligand is thought to be responsible for increasing the rate of reductive elimination of the diaryl ether from palladium.

Designed Two-Dimensional DNA Holliday Junction Arrays Visualized by Atomic Force Microscopy

Abstract: A two-dimensional DNA crystal has been designed and constructed from Holliday junction analogues that contain two helical domains twisted relative to each other. The Holliday junction is not an inherently rigid system, but it can be made less flexible if it is combined into a larger construct. We have fused four junctions into a rhombus-like molecule consisting of four six-turn helices, two on an upper layer and two on a lower layer; the branch points, which define vertices, are separated by four double helical turns each. Ligation of the rhombus-like motifs produces no cyclic species, when assayed by ligation-closure experiments. Self-assembly of the rhombuses in one dimension leads to a linear pattern. The rhombuses can be directed to self-assemble by hydrogen bonding into a two-dimensional periodic array, whose spacing is six turns in each direction. The expected spacing is seen when the array is observed by atomic force microscopy (AFM). Variation of the dimensions of the repeat unit from six turns × ...