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Showing papers in "Journal of the American Chemical Society in 1997"


Journal ArticleDOI
TL;DR: The synthesis of epitaxially grown, wurtzite CdSe/CdS core/shell nanocrystals is reported in this paper, where shells of up to three monolayers in thickness were grown on cores ranging in diameter from 23 to 39.
Abstract: The synthesis of epitaxially grown, wurtzite CdSe/CdS core/shell nanocrystals is reported Shells of up to three monolayers in thickness were grown on cores ranging in diameter from 23 to 39 A Shell growth was controllable to within a tenth of a monolayer and was consistently accompanied by a red shift of the absorption spectrum, an increase of the room temperature photoluminescence quantum yield (up to at least 50%), and an increase in the photostability Shell growth was shown to be uniform and epitaxial by the use of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and optical spectroscopy The experimental results indicate that in the excited state the hole is confined to the core and the electron is delocalized throughout the entire structure The photostability can be explained by the confinement of the hole, while the delocalization of the electron results in a degree of electronic accessibility that makes these nanocrystals

2,584 citations


Journal ArticleDOI
TL;DR: It is found that hybridization of surface-bound HS-ssDNA is dependent on surface coverage, and nonspecifically adsorbed DNA is largely removed from the surface.
Abstract: We have characterized thiol-derivatized, single-stranded DNA (5‘-HS-(CH2)6-CAC GAC GTT GTA AAA CGA CGG CCA G-3‘, abbreviated HS-ssDNA) attached to gold via a sulfur−gold linkage using X-ray photoelectron spectroscopy (XPS), ellipsometry, and 32P-radiolabeling experiments. We found that hybridization of surface-bound HS-ssDNA is dependent on surface coverage. The buffer concentration of the HS-ssDNA solution was found to have a profound effect on surface coverage, with adsorption greatly reduced at low salt concentration. More precise control over surface coverage was achieved by creating mixed monolayers of the thiol-derivatized probe and a spacer thiol, mercaptohexanol (MCH), by way of a two-step method, where first the gold substrate is exposed to a micromolar solution of HS-ssDNA, followed by exposure to a millimolar solution of MCH. A primary advantage of using this two-step process to form HS-ssDNA/MCH mixed monolayers is that nonspecifically adsorbed DNA is largely removed from the surface. Thus, th...

1,459 citations



Journal ArticleDOI
TL;DR: In this article, the 1s → 3d pre-edge features of high-spin ferrous and ferric model complexes in octahedral, tetrahedral, and square pyramidal environments were investigated and the allowable many-electron excited states were determined using ligand field theory.
Abstract: X-ray absorption Fe−K edge data on ferrous and ferric model complexes have been studied to establish a detailed understanding of the 1s → 3d pre-edge feature and its sensitivity to the electronic structure of the iron site. The energy position and splitting, and intensity distribution, of the pre-edge feature were found to vary systematically with spin state, oxidation state, geometry, and bridging ligation (for binuclear complexes). A methodology for interpreting the energy splitting and intensity distribution of the 1s → 3d pre-edge features was developed for high-spin ferrous and ferric complexes in octahedral, tetrahedral, and square pyramidal environments and low-spin ferrous and ferric complexes in octahedral environments. In each case, the allowable many-electron excited states were determined using ligand field theory. The energies of the excited states were calculated and compared to the energy splitting in the 1s → 3d pre-edge features. The relative intensities of electric quadrupole transitions...

1,181 citations


Journal ArticleDOI
TL;DR: The design, synthesis, and in vitro evaluation of the novel carbocycles as transition-state-based inhibitors of influenza neuraminidase (NA) are described and the presence of a large hydrophobic pocket in the region corresponding to the glycerol subsite of sialic acid is revealed.
Abstract: The design, synthesis, and in vitro evaluation of the novel carbocycles as transition-state-based inhibitors of influenza neuraminidase (NA) are described. The double bond position in the carbocyclic analogues plays an important role in NA inhibition as demonstrated by the antiviral activity of 8 (IC50 = 6.3 μM) vs 9 (IC50 > 200 μM). Structure−activity studies of a series of carbocyclic analogues 6a−i identified the 3-pentyloxy moiety as an apparent optimal group at the C3 position with an IC50 value of 1 nM for NA inhibition. The X-ray crystallographic structure of 6h bound to NA revealed the presence of a large hydrophobic pocket in the region corresponding to the glycerol subsite of sialic acid. The high antiviral potency observed for 6h appears to be attributed to a highly favorable hydrophobic interaction in this pocket. The practical synthesis of 6 starting from (−)-quinic acid is also described.

1,032 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used cyclic voltammetry, X-ray photoelectron spectroscopy, polarization modulation IR reflection absorption spectrograms, Auger spectrographic analysis, and Rutherford backscattering spectrographs to estimate the surface coverage of carbon-epoxy composites.
Abstract: Electrochemical reduction of a wide variety of aromatic diazonium salts on carbon electrodes (glassy carbon, highly oriented pyrolytic graphite) leads to the covalent attachment of the corresponding aromatic radicals The films thus deposited on glassy carbon surfaces require mechanical abrasion to be removed Cyclic voltammetry, X-ray photoelectron spectroscopy, polarization modulation IR reflection absorption spectroscopy, Auger spectroscopy, and Rutherford backscattering spectroscopy allow the characterization of the overlayer and an estimate of the surface coverage The latter can be controlled through diazonium concentration and electrolysis duration The mechanism of derivatization is discussed on the basis of the kinetic data obtained from cyclic voltammetry and preparative electrolysis This versatile method of surface modification may find applications in the field of carbon−epoxy composites as attested by the successful binding of grafted p-aminophenyl groups with epichlorhydrin

996 citations




Journal ArticleDOI
TL;DR: The homogeneous atom transfer radical polymerization (ATRP) of styrene using solubilizing 4,4'dialkyl substituted 2,2'bipyridines yielded well-defined polymers with Mw/Mn ≤ 1.10 as mentioned in this paper.
Abstract: The homogeneous atom transfer radical polymerization (ATRP) of styrene using solubilizing 4,4‘-dialkyl substituted 2,2‘-bipyridines yielded well-defined polymers with Mw/Mn ≤ 1.10. The polymerizations exhibited an increase in molecular weight in direct proportion to the ratio of the monomer consumed to the initial initiator concentration and also exhibited internal first-order kinetics with respect to monomer concentration. The optimum ratio of ligand-to-copper(I) halide for these polymerizations was found to be 2:1, which tentatively indicates that the coordination sphere of the active copper(I) center contains two bipyridine ligands. The exclusive role for this copper(I) complex in ATRP is atom transfer, since at typical concentrations that occur for these polymerizations (≈10-7−10-8 M), polymeric radicals were found not to react with the copper(I) center in any manner that enhanced or detracted from the observed control. ATRP also exhibited first-order kinetics with respect to both initiator and copper...

852 citations


Journal ArticleDOI
TL;DR: In this paper, the magnetic coupling constants (2J) of hydroxo- and alkoxo-bridged copper binuclear compounds have been evaluated to determine the accuracy of different density functional methods and to study the magnetic behavior of these compounds.
Abstract: Magnetic coupling constants (2J) of hydroxo- and alkoxo-bridged copper binuclear compounds have been evaluated to determine the accuracy of different density functional methods and to study the magnetic behavior of these compounds. Comparison between the calculated and experimental coupling constants for the complete structures of five compounds shows that the most successful computational strategy is the combination of the B3LYP method with the broken-symmetry approach. Calculations for model compounds of both families yield reasonable approximations to the values of magnetic coupling constants calculated for the full molecular structures. Our calculations show a correlation between the magnetic coupling constant and the Cu−O−Cu bridging angle and with the out-of-plane displacement of the hydroxo or alkoxo groups, in agreement with the experimental data. The counterions of the hydroxo-bridged complexes, when hydrogen bonded to the bridging hydroxo group, determine the extent of the out-of-plane displacem...

793 citations


Journal ArticleDOI
TL;DR: In this article, the P2W15Nb3O629 polyoxoanion and Bu4N+- stabilized Ir∼190-450 (hereafter, Ir(0)∼300) nanoclusters are presented.
Abstract: Following an overview of the primitive state of mechanistic studies of the formation of nanoclusters, with a focus on LaMer's classic work on the formation of sulfur sols, kinetic and mechanistic studies of the formation of our recently reported novel P2W15Nb3O629- polyoxoanion- and Bu4N+- stabilized Ir∼190-450 (hereafter, Ir(0)∼300) nanoclusters are presented. The work reported consists of the full experimental and other details of the following eight major components: (i) development of an indirectbut easy, continuous, highly quantitative and thus powerfulmethod to monitor the formation of the Ir(0) nanoclusters via their catalytic hydrogenation activity and through the concept of pseudoelementary reaction steps; (ii) application of the appropriate kinetic equations for nucleation and autocatalysis, and then demonstration that these equations fit the observed, sigmoidal-shaped kinetic curves quantitatively with resultant rate constants k1 and k2; (iii) confirmation by a more direct, GLC method that the...

Journal ArticleDOI
TL;DR: In this paper, triethylamine is added to an ethanol solution containing 1,3,5-benzenetricarboxylic acid (H3BTC) and zinc(II) nitrate hexahydrate.
Abstract: Diffusion of triethylamine into an ethanol solution containing 1,3,5-benzenetricarboxylic acid (H3BTC) and zinc(II) nitrate hexahydrate yields crystalline Zn2(BTC)(NO3)·(H2O)(C2H5OH)5, which was formulated by elemental microanalysis, solid-state NMR, and single-crystal X-ray diffraction [cubic, P213, a = 14.728(2) A, V = 3195(2) A3, Z = 4]. This compound possesses a 3-D structure with nearly 44% of the framework represented by an extended channel system having a 14 A cross-section, where highly mobile ethanol and water guest molecules reside. The multidentate functionality of BTC imparts rigidity to the structure, consequently allowing the guests to be removed or exchanged without destruction of the porous framework. X-ray powder diffraction, solid-state NMR (13C CP MAS and static), gas chromatography, and thermogravimetry analyses reveal that this material is highly selective to alcohols because of the coordination environment adopted by Zn(II) within its framework. Other molecular inclusions such as tet...

Journal ArticleDOI
TL;DR: Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing met athesis of the acyclic diene diethyl diallylmalonate as discussed by the authors.
Abstract: Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined: X = I < Br < Cl and PR3 = PPh3 ≪ PiPr2Ph < PCy2Ph < PiPr3 < PCy3. Additional studies were conducted with the catalyst (PCy3)2Cl2RuCH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways: the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the addition of CuCl to the reaction.



Journal ArticleDOI
TL;DR: Improved spectra are obtained relative to data recorded with other purging schemes and structural information obtained for a complex of the amino-terminal arginine rich domain of the N protein from bacteriophage λ and boxB RNA using the new methodology is presented.
Abstract: New pulse schemes for recording intermolecular NOEs in a molecular complex consisting of 15N,13C labeled and unlabeled components are presented. The pulse sequences select for magnetization transferred from protons on the unlabeled component to proximal protons of the labeled molecule. Filtering (suppression of signal from 13C labeled molecules) is accomplished using adiabatic 13C inversion pulses which are swept at a rate which is tuned according to the one-bond 1H−13C scalar coupling vs carbon chemical shift profile of the labeled molecule in the complex. Significantly improved spectra are obtained relative to data recorded with other purging schemes. Improvements are demonstrated in experiments where intermolecular NOEs between labeled RNA-unlabeled peptide and labeled protein−unlabeled peptide are recorded. A discussion of structural information obtained for a complex of the amino-terminal arginine rich domain of the N protein from bacteriophage λ and boxB RNA using the new methodology is presented.


Journal ArticleDOI
TL;DR: In this article, the synthesis and metal ion responsive properties of two 2,2‘-bipyridyl-phenylene-vinylene-based polymers are reported.
Abstract: The synthesis and metal ion responsive properties of two 2,2‘-bipyridyl-phenylene-vinylene-based polymers is reported. These polymers are designed to be partially conjugated in their metal-free state and fully conjugated when exposed to metal ions so that the ion-induced conjugation enhancement can be transduced into a measurable signal. It is found that these polymers exhibit highly ionochromic effects with a wide variety of transition and main group metal ions excluding metal ions of the alkali and alkaline earth groups. For instance, both absorption and fluorescence emission bands of the polymers upon exposure to metal ions can be red-shifted up to 120 nm, depending on the metal ions present and the polymers used.


Journal ArticleDOI
TL;DR: In this article, the assembling process of individual dye−polyion layers of Congo Red (CR)−poly(diallyldimethylammonium chloride) (PDDA) was investigated by using a quartz crystal microbalance (QCM).
Abstract: Electrostatic alternate adsorption was successfully employed for low-molecular-weight dyes, leading to a large variety of dye−polyion layer-by-layer assemblies. The assembling process of individual dye−polyion layers of Congo Red (CR)−poly(diallyldimethylammonium chloride) (PDDA) was investigated by using a quartz crystal microbalance (QCM). The in-situ QCM measurement revealed that the dye adsorption occurred at a rate similar to that of conventional polyion adsorption. Successful assembly of the CR layer was confirmed also by increasing intensities in the UV adsorption. The molecular aggregation within the dye layer was inferred from the observed absorption shift in the case of CR−PDDA. Apparently, stable film formation is promoted by dye aggregation in the adsorbed layer, although the extent of film growth was independent of conceivable aggregation in the solution. Successive frequency decreases due to film growth were observed for other representative dyes under optimized conditions. Comparison of the...

Journal ArticleDOI
TL;DR: In this paper, a catalytic asymmetric epoxidation method for olefins using potassium peroxomonosulfate (Oxone, Dupont) as oxidant and a fructose-derived ketone (1) as catalyst is described.
Abstract: This article describes a highly effective catalytic asymmetric epoxidation method for olefins using potassium peroxomonosulfate (Oxone, Dupont) as oxidant and a fructose-derived ketone (1) as catalyst. High enantioselectivies have been obtained for trans-disubstituted and trisubstituted olefins which can bear functional groups such as tributylsilyl ether, acetal, chloride, and ester. The enantiomeric excesses for cis-olefins and terminal olefins are not high yet. The current epoxidation shows that the catalyst efficiency is enhanced dramatically upon raising the pH. Mechanistic studies show that the epoxidation mainly proceeds via a spiro transition state, which provides a model for predicting the stereochemical outcome of the reaction. The planar transition state is likely to be the main competing pathway. The extent of the involvement of the planar mode is subject to the steric effect of the alkyl groups on the olefins.



Journal ArticleDOI
TL;DR: In this paper, a reversible charge transfer complex (CTC) with poly(3-alkylthiophenes) was shown to be a fluorescence quencher of mobile polaronic excitons, which explains why oxygen quenches luminescence with much greater efficiency than predicted by a collisional quenching model.
Abstract: Molecular oxygen forms a reversible charge transfer complex (CTC) with poly(3-alkylthiophenes). The complex is weakly bound but possesses a distinct absorption band in the visible region. The electronic properties of the neutral polymer are modulated by the CTC. The extent of modulation of these properties is proportional to oxygen pressure and is fully reversible. Analysis of poly(3-hexylthiophene) field effect transistors under increasing pressures of oxygen shows that the carrier concentration increases, conductivity increases, and the charge carrier mobility is lowered by the formation of the charge transfer complex. The CTC manifests itself as a fluorescence quencher of mobile polaronic excitons, and it explains why oxygen quenches luminescence with much greater efficiency than predicted by a collisional quenching model. Implications of the charge transfer complex on the photochemistry of poly(3-alkylthiophenes) and photosensitization of singlet oxygen are discussed.

Journal ArticleDOI
TL;DR: The use of pseudoephedrine as a chiral auxiliary for asymmetric synthesis is described in full in this article, where both enantiomers of pseudo-ephedrin are inexpensive commodity chemicals and can be N-acylated in high yields to form tertiary amides.
Abstract: The use of pseudoephedrine as a practical chiral auxiliary for asymmetric synthesis is described in full. Both enantiomers of pseudoephedrine are inexpensive commodity chemicals and can be N-acylated in high yields to form tertiary amides. In the presence of lithium chloride, the enolates of the corresponding pseudoephedrine amides undergo highly diastereoselective alkylations with a wide range of alkyl halides to afford α-substituted products in high yields. These products can then be transformed in a single operation into highly enantiomerically enriched carboxylic acids, alcohols, aldehydes, and ketones.

Journal ArticleDOI
TL;DR: The self-assembly of the tris-bipyridine ligands BI and BII with iron(II) salts yields polynuclear complexes displaying structures of cyclic double-helix type, termed circular helicates [n]cH (of order n).
Abstract: The self-assembly of the tris-bipyridine ligands BI and BII with iron(II) salts yields polynuclear complexes displaying structures of cyclic double-helix type, termed circular helicates [n]cH (of order n). With BI in which the bipyridine units in the ligand are connected by ethylene bridges, penta- or hexanuclear architectures [5]cH and [6]cH are obtained, depending on the anion present during the self-assembly process. The elongated tris-bipyridine ligand BII with oxypropylene bridges forms a tetranuclear circular helicate [4]cH, whose structure has been confirmed by crystal structure determination. The possible oligomeric combinations of tris-bipy ligands and iron(II) ions may be considered to constitute the potential members of a virtual combinatorial library, generated via dynamic combinatorial chemistry, from which a specific real constituent of the virtual set of circular helicates is expressed in given conditions.


Journal ArticleDOI
TL;DR: In this article, the rational design, synthesis via a new convergent approach, and the characterization of four generations of monodendrons based on the AB3 building block methyl 3,4,5-trishydroxybenzoate are described.
Abstract: The rational design, the synthesis via a new convergent approach, and the characterization of four generations of monodendrons based on the AB3 building block methyl 3,4,5-trishydroxybenzoate are described. The first generation monodendrons are crystalline. The second generation methyl 3,4,5-tris-3‘,4‘,5‘-[tris(n-dodecyloxy)benzyloxy]benzoate (12Gn-AG-CH3, n = 2), the corresponding benzyl alcohol (12G2-AG-CH2OH) and benzoic acid (12G2-AG), and all higher generation monodendrons with n = 3 and 4 exhibit in addition to the crystal phase an isomorphous cubic liquid-crystalline (LC) phase of Pm3n space group. The cubic unit cell parameters of 12G2-AG, 12G3-AG, and 12G4-AG are 68.3, 79.2, and 84.0 A, respectively. Computed electron density profiles demonstrated that in the cubic phase 12, 6, and 2 monodendrons of generation 2, 3 and 4 are self-assembled in supramolecular dendrimers resembling spherical micelles when they are centered at the corners and the body-center and respectively highly rounded tetrahedr...

Journal ArticleDOI
TL;DR: In this paper, a pseudorotaxane formed in solution by self-assembly of a wire-type electron acceptor and a macrocyclic electron donor (2,3-dinaphtho-30crown-10) can be unthreaded and rethreaded by chemical inputs.
Abstract: A pseudorotaxane formed in solution by self-assembly of a wire-type electron acceptor (2,7-dibenzyldiazapyrenium dication) and a macrocyclic electron donor (2,3-dinaphtho-30-crown-10) can be unthreaded and rethreaded by chemical inputs. Unthreading can be obtained by addition of stoichiometric amounts of acids or amines. After the unthreading process caused by addition of acid, rethreading can be obtained by addition of amine, and vice versa. The threading/unthreading processes are accompanied (and therefore can be monitored) by strong changes in the fluorescence properties of the system. The input (chemical)/output (fluorescence) characteristics of this molecular-level system correspond to those of an XOR logic gate.