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At least 433 records · Page 24Linked to original sources
PubMed2005

Dehydrogenation of ethylbenzene with nitrous oxide in the presence of mesoporous silica materials modified with transition metal oxides.

The novel mesoporous templated silicas (MCM-48, SBA-15, MCF, and MSU) were used as supports for transition metal (Cu, Cr, or Fe) oxides. The catalysts were synthesized using the incipient wetness impregnation, and characterized by low-temperature N2 sorption, DRIFT, photoacoustic IR spectroscopy, UV-vis diffuse reflectance spectroscopy, and temperature-programmed desorption of ammonia. It was shown that the preparation method used results in different distributions and dimensions of the transition metal oxide clusters on the inert support surface. The prepared catalysts were tested in the reaction of oxidative dehydrogenation of ethylbenzene in the presence of nitrous oxide. The iron-containing catalysts showed the highest catalytic activity. The presence of isolated Fe3+ was found to be the most important factor influencing the ethylbenzene conversion. The undesirable effect of the increase in selectivity toward CO2 was observed for the samples with the highest concentrations of acidic surface sites.

Acids↗
PubMed2005

Development of the ReaxFF reactive force field for describing transition metal catalyzed reactions, with application to the initial stages of the catalytic formation of carbon nanotubes.

With the aim of developing a computationally inexpensive method for modeling the high-temperature reaction dynamics of transition metal catalyzed reactions we have developed a ReaxFF reactive force field in which the parameters are fitted to a substantial quantum mechanics (QM) training set, containing full reaction pathways for relevant reactions. In this paper we apply this approach to reactions involving carbon materials plus Co, Ni, and Cu atoms. We find that ReaxFF reproduces the QM reaction data with good accuracy while also reproducing the binding characteristics of Co, Ni, and Cu atoms to hydrocarbon fragments. To demonstrate the applicability of ReaxFF we performed high-temperature (1500 K) molecular dynamics simulations on a nonbranched all-carbon feedstock in the presence and absence of Co, Ni, and Cu atoms. We find that the presence of Co and Ni leads to substantial amounts of branched carbon atoms, leading eventually to the formation of carbon-nanotube-like species. In contrast, we find that under the same simulation conditions Cu leads to very little branching and leads to products with no nanotube character. In the absence of metals no branching is observed at all. These results suggest that Ni and Co catalyze the production of nanotube-like species whereas Cu does not. This is in excellent agreement with experimental observations, demonstrating that ReaxFF can provide a useful and computational tractable tool for studying the dynamics of transition metal catalytic chemistry.

Catalysis↗
PubMed2005

Surface and subsurface hydrogen: adsorption properties on transition metals and near-surface alloys.

Periodic, self-consistent DFT-GGA calculations are used to study the thermochemical properties of both surface and subsurface atomic hydrogen on a variety of pure metals and near-surface alloys (NSAs). For surface hydrogen on pure metals, calculated site preferences, adsorption geometries, vibrational frequencies, and binding energies are reported and are found to be in good agreement with available experimental data. On NSAs, defined as alloys wherein a solute is present near the surface of a host metal in a composition different from the bulk composition, surface hydrogen generally binds more weakly than it binds to the pure-metal components composing the alloys. Some of the NSAs even possess the unusual property of binding hydrogen as weakly as the noble metals while, at the same time, dissociating H(2) much more easily. On both NSAs and pure metals, formation of surface hydrogen is generally exothermic with respect to H(2)(g). In contrast, formation of subsurface hydrogen is typically endothermic with respect to gas-phase H(2) (the only exception to this general statement is found for pure Pd). As with surface H, subsurface H typically binds more weakly to NSAs than to the corresponding pure-metal components of the alloys. The diffusion barrier for hydrogen from surface to subsurface sites, however, is usually lower on NSAs compared to the pure-metal components, suggesting that population of subsurface sites may occur more rapidly on NSAs.

Adsorption↗
PubMed2005

Perturbation of the PET process in fluorophore-spacer-receptor systems through structural modification: transition metal induced fluorescence enhancement and selectivity.

Several fluorescent signaling systems are built in the format fluorophore-spacer-receptor with ethylenediamine or N,N-dimethylethylenediamine as the receptor, anthracene as the fluorophore, and a methylene group as the spacer. The receptors are derivatized with different electron-withdrawing groups such as 4-nitrobenzene, 4-nitro-2-pyridine, and 2,4-dinitrobenzene, to perturb the photoinduced intramolecular electron transfer (PET) process from the nitrogen lone-pair to the fluorophore. The photophysical properties of these supramolecular systems and their fluorescence responses toward a number of quenching transition metal ions are reported. It is shown that the PET is highly efficient in the absence of a metal ion. With a metal ion input, the fluorescence can be recovered to a different extent depending on the nature of the metal and on the overall architecture of the system as well. Despite the possibility of strong interaction between the fluorophore and the metal ion, significant fluorescence enhancement is observed with quenching of paramagnetic transition metal ions. The complex stability data show that the stability constants for the metal ions showing fluorescence enhancement are of the order of 10(4) M(-1). This study shows that structurally simple fluorescent signaling systems for quenching transition metal ions can be built by maximizing the PET. It is also shown here that simple structural modification can make these systems highly specific for particular transition metal ions for potential applications in several contemporary areas of research.

Algorithms↗
PubMed2005

In situ one-pot synthesis of 1-dimensional transition metal oxide nanocrystals.

One-dimensional colloidal metal oxide nanocrystals are of great importance in materials chemistry, but reports on these materials are rare due to lack of well-defined synthetic protocols. In this paper, we present a general and highly effective one-pot synthetic protocol to produce 1-dimensional nanostructures of transition metal oxide (e.g., W(18)O(49), TiO(2), Mn(3)O(4), and V(2)O(5)) through thermally induced crystal growth processes from a mixture of metal chloride and surfactants.

Crystallization↗
PubMed2005

On the nature of metal-carbon bonding: AIM and ELF analyses of MCH(n) (n = 1-3) compounds containing early transition metals.

Ab initio and DFT calculations have been performed on a series of organometallic compounds, according to the formula MCH(n), where M = K, Ca, Sc, Ti, V, Cr, or Mn and n = 1-3. Various theoretical methods are compared, the B3LYP level yielding the same agreement with the experimental geometries available as the correlated MP2 and CISD methods, with the 6-311++G(3df,2p) basis set for C and H and Wachter's (15s11p6d3f1g)/[10s7p4d3f1g] basis set for transition metals. The main geometric and electronic features of the molecules studied are described, analyzing the M-C bonding characteristics in terms of the atoms in molecules theory (AIM) and the electron localization function (ELF). Although multiple bonding is expected from the Lewis bonding scheme, the results indicate an almost pure ionic bond for all of the systems studied. The net charge transfer from the metal to the carbon atom ranges from 0.5 to 1 e(-), and the electronic structure of the CH(n)(-) moiety is unaltered after the interaction with the metal cation, showing little or no effect on the shape of the electron pairing. The bond paths corresponding to a possible alpha-agostic bond for these systems are not present.

Algorithms↗
PubMed2006

Theory and application of dissociative electron capture in molecular identification.

The coupling of an electron monochromator (EM) to a mass spectrometer (MS) has created a new analytical technique, EM-MS, for the investigation of electrophilic compounds. This method provides a powerful tool for molecular identification of compounds contained in complex matrices, such as environmental samples. In particular, EM-MS has been applied to the detection of nitrated aromatic compounds, many of which are potent mutagens and/or carcinogens and are considered environmental hazards. EM-MS expands the application and selectivity of traditional MS through the inclusion of a new dimension in the space of molecular characteristics-the electron resonance energy spectrum. EM-MS also enhances detection sensitivity as well because the entire electron flux of the proper energy can be delivered into the negative ion resonance that is analytically most useful to solving the problem at hand. However, before this tool can realize its full potential, it will be necessary to create a library of resonance energy scans from standards of the molecules for which EM-MS offers a practical means of detection. Unfortunately, the number of such standards is very large and not all of the compounds are commercially available, making this library difficult to construct. Here, an approach supplementing direct measurement with chemical inference and quantum scattering theory is presented to demonstrate the feasibility of directly calculating resonance energy spectra. This approach makes use of the symmetry of the transition-matrix element of the captured electron to discriminate between the spectra of isomers. As a way of validating this approach, the resonance values for 25 nitrated aromatic compounds were measured along with their relative abundance. Subsequently, the spectra for the isomers of nitrotoluene were shown to be consistent with the symmetry-based model. The initial success of this treatment suggests that it might be possible to predict negative ion resonances and thus create a library of EM-MS standards.

Journal Article↗
PubMed2006

Carbon disulfide reactions with atomic transition-metal and main-group cations: gas-phase room-temperature kinetics and periodicities in reactivity.

The reactions of 46 atomic-metal cations with CS2 have been investigated at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and products were measured for the reactions of fourth-period atomic ions from K+ to Se+, of fifth-period atomic ions from Rb+ to Te+ (excluding Tc+), and of sixth-period atomic ions from Cs+ to Bi+. Primary reaction channels were observed leading to S-atom transfer, CS2 addition and, with Hg+, electron transfer. S-atom transfer appears to be thermodynamically controlled and occurs exclusively, and with unit efficiency, in the reactions with most early transition-metal cations (Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+) and with several main-group cations (As+, Sb+) and less efficiently with Se+, Re+ and Os+. Other ions, including most late transition and main-group metal cations, react with CS2 with measurable rates mostly through CS2 addition or not at all (K+, Rb+, Cs+). Traces of excited states (< 10%) were seen from an inspection of the observed product ions to be involved in the reactions with Mo+, Te+, Ba+ and Au+ and possibly Pt+ and Ir+. The primary products YS+, ZrS+, NbS+, HfS+, TaS+, WS+, ReS+ and OsS+ react further by S-atom transfer to form MS2(+), and TaS2(+) reacts further to form TaS3(+). CS2 addition occurs with the cations MCS2(+), MS+, MS2(+), CS2(+), and TaS3(+) to form M+(CS2)(n) (n < or = 4), MS+(CS2)(n) (n < or = 4), MS2(+)(CS2)(n) (n < or = 3), (CS2)2(+) and TaS3(+)(CS2). Up to four CS2 molecules add sequentially to bare metal cations and monosulfide cations, and three to disulfide cations. Equilibrium constant measurements are reported that provide some insight into the standard free energy change for CS2 ligation. Periodic variations in deltaG degrees are as expected from the variation in electrostatic attraction, which follows the trend in atomic-ion size and the trend in repulsion between the orbitals of the atomic cations and the occupied orbitals of CS2.

Carbon Disulfide↗
PubMed2006

Reactions of group 3 transition metal atoms with CS2 and OCS: Matrix isolation infrared spectra and density-functional calculations of SMCS, SM-(eta2-CS), SMCO, and SM-(eta2-CO) in solid argon.

Laser-ablated scandium, yttrium, and lanthanum atoms were reacted with CS2 and OCS molecules in an argon matrix. Products of the type SMCX and S-M(eta2-CX) (X = S or O) were formed on sample deposition. Photolysis favored the S-M(eta(2)-CX) complex, while annealing increased the more stable SMCX isomer. Product absorptions are identified by density-functional frequency calculations and isotopic substitutions. This work reports the first vibrational spectroscopic characterization of Sc, Y, and La reaction products with CS2 and OCS and the subsequent interconversion between SMCX and S-M(eta2-CX) structural isomers.

Argon↗
PubMed2006

Photoelectron spectroscopy of free multiply charged Keggin anions alpha-[PM12O40]3- (M = Mo, W) in the gas phase.

Two polyoxometalate Keggin-type anions, alpha-PM12O40(3-) (M = Mo, W), were transferred to the gas phase by electrospray; their electronic structure and stability were probed by photoelectron spectroscopy. These triply charged anions were found to be highly stable in the gas phase with large adiabatic electron detachment energies of 1.7 and 2.1 eV for M = Mo and W, respectively. The magnitude of the repulsive Coulomb barrier was measured as approximately 3.4 eV for both anions, providing an experimental estimate for the intramolecular Coulomb repulsion present in these highly charged anions. Density functional theory calculations were carried out and compared with the experimental data, providing insight into the electronic structure and valence molecular orbitals of the two Keggin anions. The calculations indicated that the highest occupied molecular orbital and other frontier orbitals for PM12O40(3-) are localized primarily on the mu2-oxo bridging ligands of the polyoxometalate framework, consistent with the reactivity on the mu2-oxo sites observed in solution. It was shown that the HOMO of PW12O40(3-) is stabilized relative to that of PMo12O40(3-) by approximately 0.35 eV. The experimental adiabatic electron detachment energies of PM12O40(3-) (i.e., the electron affinities of PM12O40(2-)) are combined with recent calculations on the proton affinity of PM12O40(3-) to yield O-H bond dissociation energies in PM12O39(OH)2- as approximately 5.1 eV.

Gases↗
PubMed2006

Structural and electronic characterization of the complexes obtained by the interaction between bare and hydrated first-row transition-metal ions (Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+)) and glycine.

The complexes formed by the simplest amino acid, glycine, with different bare and hydrated metal ions (Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+)) were studied in the gas phase and in solvent in order to give better insight into the field of the metal ion-biological ligand interactions. The effects of the size and charge of each cation on the organization of the surrounding water molecules were analyzed. Results in the gas phase showed that the zwitterion of glycine is the form present in the most stable complexes of all ions and that it usually gives rise to an eta(2)O,O coordination type. After the addition of solvation sphere, a resulting octahedral arrangement was found around Ni(2+), Co(2+), and Fe(2+), ions in their high-spin states, whereas the bipyramidal-trigonal (Mn(2+) and Zn(2+)) or square-pyramidal (Cu(2+)) geometries were observed for the other metal species, according to glycine behaves as bi- or monodentate ligand. Despite the fact that the zwitterionic structure is in the ground conformation in solution, its complexes in water are less stable than those obtained from the canonical form. Binding energy values decrease in the order Cu(2+) > Ni(2+) > Zn(2+) approximately Co(2+) > Fe(2+) > Mn(2+) and Cu(2+) > Ni(2+) > Mn(2+) approximately Zn(2+) > Fe(2+) > Co(2+) for M(2+)-Gly and Gly-M(2+) (H(2)O)(n) complexes, respectively. The nature of the metal ion-ligand bonds was examined by using natural bond order and charge decomposition analyses.

Cations, Divalent↗
PubMed2004

X-ray nanoscale profiling of layer-by-layer assembled metal/organophosphonate films.

The nanoscale structure of multilayer metal/phosphonate thin films prepared via a layer-by-layer assembly process was studied using specular X-ray reflectivity (XRR), X-ray fluorescence (XRF), and long-period X-ray standing wave (XSW) analysis. After the SiO(2) X-ray mirror surfaces were functionalized with a monolayer film terminated with phosphonate groups, the organic multilayer films were assembled by alternating immersions in (a) aqueous solutions containing Zr(4+), Hf(4+), or Y(3+) cations and then (b) organic solvent solutions of PO(3)-R-PO(3), where R was a porphyrin or porphyrin-square spacer molecule. The different heavy metal cations provided X-ray fluorescence marker layers at different heights within the different multilayer assemblies. The XSW measurements used a 22 nm period Si/Mo multilayer mirror. The long-period XSW generated by the zeroth-order (total external reflection) through fourth-order Bragg diffraction conditions made it possible to examine the Fourier transforms of the fluorescent atom distributions over a much larger q(z)() range in reciprocal space than previously achieved.

Membranes, Artificial↗
PubMed2002

pi-Ligands for generating transition metal-peptide complexes: coordination of amino acid derivatives to tungsten utilizing alkyne ligands.

[structure: see text] Amino acid derivatives bearing an alkyne (AA-CCH) at either the N- or C-terminus readily react with W(CO)3(S2CNMe2)2 to replace the carbon monoxides and form the novel bis-alkyne complexes W(AA-CCH)2(S2CNMe2)2; the solution behavior of these complexes shows that only the alkyne, and not the other functional groups on the amino acid, bonds to the tungsten.

Alkynes↗
PubMed2004

Allenes and transition metals: a diverging approach to heterocycles.

[reaction: see text] An alkynyl allene has been converted to heterocycles possessing an alpha-alkylidene cyclopentenone, a 4-alkylidene cyclopentenone, or a cross-conjugated triene. Thus, a common intermediate has been converted to three structurally unique compounds by changing only the reaction conditions and, therefore, controlling various reaction pathways.

Alkadienes↗
PubMed2005

Group 4 metals in polyketide synthesis: a convergent strategy for the synthesis of polypropionate-derived (E,E)-trisubstituted 1,3-dienes.

[reaction: see text] A convergent Group 4 metal-mediated coupling process is described for the synthesis of polypropionate-derived (E,E)-1,3-dienes. Both the stereochemistry of the internal alkyne and the presence/absence of a tethered alkoxide on this pi-component were found to play critical roles in dictating the regiochemical course of these reactions.

Alkenes↗
PubMed2002

In vitro inhibition of the enzymatic activity of tumor suppressor FHIT gene product by carcinogenic transition metals.

FHIT (Fragile Histidine Triad) is a human tumor suppressor gene. The Fhit protein is believed to inhibit tumor growth by inducing apoptosis through interaction with diadenosine triphosphate (Ap(3)A). The latter is first sequestered and eventually hydrolyzed by Fhit to ADP and AMP. Thus, the balance between the cellular Ap(3)A level and Fhit enzymatic activity may affect cell death or survival. Increasing the Ap(3)A level, e.g., by inhibition of the enzyme, should prevent apoptosis and thus sustain tumorigenesis. To test if certain carcinogenic transition metals could inhibit the enzymatic activity of Fhit, purified human Fhit protein [30 nM in 1.25 mM poly(vinylpyrrolidone)], expressed in and isolated from E. coli, was incubated at pH 6.8 (50 mM HEPES buffer in 150 mM NaCl) with 120 microM Ap(3)A in the presence of 5 mM Mg(II) (activating cation) and 0-100 microM Ni(II), Cu(II), Zn(II), Cd(II), Co(II), Cr(III), As(III), or As(V). The reaction mixtures were analyzed by HPLC. The results revealed a strong inhibitory potential of Cu(II) [0.4], followed by Ni(II) [3.5] >or= Zn(II) [7.0] >> Cr(III) [73] > Cd(II) [98] >> Co(II) [432] [the numbers in brackets are IC(50) values, microM]. As(III) and As(V) had no effect. As revealed by spectrophotometry, mass spectrometry, and gel electrophoresis, the exceptionally strong inhibition by Cu(II) was associated with Fhit dimerization through formation of a disulfide bond. The other metals and also H(2)O(2) and NO did not cause the dimerization. Thus, the effect of Cu(II) must be due to its reaction with Cys-39 bearing the only thiol group in Fhit monomer. Since Cys-39 is not readily accessible in the Fhit molecule, the reaction is most likely facilitated by conformational changes which follow the coordination of Cu(II) by the surface histidines 35, 94, and/or 96. The observed inhibition of Fhit may be mechanistically involved in metal-mediated toxicity and carcinogenesis.

Acid Anhydride Hydrolases↗
PubMed2005

Effects of naphthalene quinonoids on the induction of oxidative DNA damage and cytotoxicity in calf thymus DNA and in human cultured cells.

The purpose of this study was to examine the differences in the induction of DNA damage and cytotoxic effects by quinonoid derivatives of naphthalene in calf thymus DNA (ct-DNA) and in human T47D breast cancer cells. Results indicated that copper(II) and NADPH were essential for causing oxidant-mediated aldehydic DNA lesions (ADLs), including abasic sites and aldehydic base/sugar lesions, in ct-DNA exposed to 1,2-naphthalenediol (NCAT), 1,4-naphthalenediol (NHQ), 1,2-naphthoquinone (1,2-NQ), and 1,4-naphthoquinone (1,4-NQ). The ADLs induced by naphthalene quinonoids in ct-DNA decrease in the rank order NCAT congruent with 1,2-NQ > NHQ >> 1,4-NQ. Results from the analyses in cells indicated that after 1.5-5 h of exposure all naphthalene quinonoids induced a cytotoxic response in T47D cells at concentrations 10-100 microM or above, where NHQ and 1,4-NQ were approximately 5-10 times more efficient than NCAT and 1,2-NQ in the induction of cell death. In addition, NHQ, 1,2-NQ, and 1,4-NQ were not able to produce measurable levels of ADLs in cells at concentrations up to 1.25 mM, whereas NCAT (0.75-1.25 mM) induced a significant increase in the number of ADLs in T47D cells after 1.5 h of exposure when compared to control. The specific type of ADLs induced by NCAT is resistant to cellular excision repair pathway. Results from the measurements of reactive oxygen species (ROS) indicated that all naphthalene quinonoids induced increases in ROS formation in T47D cells. The induction of ROS formation in cells by naphthalene quinonoids decreases in the rank order 1,4-NQ congruent with 1,2-NQ > NHQ > NCAT. Overall, results from our investigation suggest that naphthalene quinonoids cause cell death at concentrations well below the concentrations at which they induce the formation of ADLs, perhaps by altering intracellular redox status.

Aldehydes↗