Chemistry Commons^™

A Carbocation Substituted Clay And Its Styrene Nanocomposite, Jinguo Zhang, Charles A. Wilkie

Chemistry Faculty Research and Publications

A substituted tropylium ion can be ion-exchanged onto montmorillonite to give a novel organically-modified clay. One can prepare a polystyrene nanocomposite of this clay by emulsion, but not bulk, polymerization. This is the first example of a clay that contains a carbocation and its use to prepare a polymer-clay nanocomposite. Both the clay and its nanocomposites exhibit outstanding thermal stability. Characterization by X-ray diffraction, transmission electron microscopy, cone calorimetry, thermogravimetric analysis and the evaluation of mechanical properties shows that a mixed intercalated-exfoliated nanocomposite is obtained.

Far-Infrared Spectroelectrochemistry: A Study Of Linear Molybdenum/Iron/Sulfur Clusters, Michael D. Ryan, Lin Li

Chemistry Faculty Research and Publications

The far-infrared spectroelectrochemistry of linear M/Fe/S (M=Mo, W) complexes was investigated in methylene chloride and dichloroethane. With CsI as spectral windows, bands above 200 cm⁻¹ can be observed in methylene chloride, except for a weak methylene chloride band at 450 cm⁻¹. Substitution of dichloroethane for methylene chloride, solvents of nearly identical electrochemical properties, allows one to observe solute bands in the 450-cm⁻¹ region. The far-infrared spectroelectrochemistry of [MoFe₂S₄Cl₄]²⁻ and its tungsten analogue was investigated. The disappearance of the oxidation bands and the appearance of bands due to the reduced …

Additional Xps Studies On The Degradation Of Poly(Methyl Methacryalte) And Polystyrene Nanocomposites, Jianxin Du, Jianqi Wang, Shengpei Su, Charles A. Wilkie

Chemistry Faculty Research and Publications

XPS studies have been undertaken on exfoliated nanocomposites of polystyrene and poly(methyl methacrylate). One can clearly see that carbon is lost and that oxygen, silicon and aluminum accumulate at the surface of the degrading polymer. The concentration of aluminum at the surface is very low at the beginning of the experiment but makes a large jump at the same temperature at which carbon is lost and oxygen begins to accumulate at the surface. It appears that the ratio of silicon to aluminum changes as the polymer is lost. A brief discussion is given to explain the origin of oxygen at …

Thermal Properties Of Thermoplastics, Charles A. Wilkie, Michael A. Mckinney

Chemistry Faculty Research and Publications

No abstract provided.

Polybutadiene Modified Clay And Its Polystyrene Nanocomposites, Shengpei Su, David D. Jiang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Vinylbenzyl chloride was allowed to react with a low-molecular-weight oligomer of butadiene in order to give a material that could then be used to alkylate an amine to form an ammonium salt. This ammonium salt was ion-exchanged onto a clay to give a new organically modified clay, which was used to prepare polystyrene-clay nanocomposites by solution and bulk polymerization and by melt blending. The nanocomposites were characterized by using X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry, and the evaluation of mechanical properties. These systems show good nanodispersion and improvement in thermal and mechanical properties. J. Vinyl Addit. Technol. …

Structural Effects Of Carbon Monoxide Coordination To Carbon Centers. Π And Σ Bindings In Aliphatic Acyl Versus Aromatic Aroyl Cations, M. G. Davlieva, Sergey V. Lindeman, Ivan S. Neretin, Jay K. Kochi

Chemistry Faculty Research and Publications

The binding of carbon monoxide to carbon centers has been examined with two series of aromatic and aliphatic oxocarbonium ions that are successfully isolated as crystalline and highly reactive (hygroscopic) aroylium and acylium salts with poorly coordinating counteranions. X-Ray crystallographic analyses at −150 °C afford precise structural parameters for the characteristic linear carbonyl bond (r_CO) and the bond to the carbon centers (r_Cα). The correlations of these structural parameters evaluated for alkyl (Me, Et and i-Pr) and aryl (p-Me, 2,4,6-trimethyl, p-MeO and p-fluorophenyl) oxocarbonium ions with the corresponding carbonyl …

Investigation Of Nanodispersion In Polystyrene-Montmorillonite Nanocomposites By Solid State Nmr, Serge Bourbigot, David L. Vanderhart, Jeffery W. Gilman Sr., Walid Awad, Rick D. Davis, Alexander B. Morgan, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites result from combinations of materials with vastly different properties in the nanometer scale. These materials exhibit many unique properties such as improved thermal stability, reduced flammability, and improved mechanical properties. Many of the properties associated with polymer–clay nanocomposites are a function of the extent of exfoliation of the individual clay sheets or the quality of the nanodispersion. This work demonstrates that solid-state NMR can be used to characterize, quantitatively, the nanodispersion of variously modified montmorillonite (MMT) clays in polystyrene (PS) matrices. The direct influence of the paramagnetic Fe3, embedded in the aluminosilicate layers of MMT, on polymer protons within …

The Role Of The Dielectric Barrier In Narrow Biological Channels: A Novel Composite Approach To Modeling Single-Channel Currents, Artem B. Mamonov, Rob D. Coalson, Abraham Nitzan, Maria G. Kurnikova

Chemistry Faculty Research and Publications

A composite continuum theory for calculating ion current through a protein channel of known structure is proposed, which incorporates information about the channel dynamics. The approach is utilized to predict current through the Gramicidin A ion channel, a narrow pore in which the applicability of conventional continuum theories is questionable. The proposed approach utilizes a modified version of Poisson-Nernst-Planck (PNP) theory, termed Potential-of-Mean-Force-Poisson-Nernst-Planck theory (PMFPNP), to compute ion currents. As in standard PNP, ion permeation is modeled as a continuum drift-diffusion process in a self-consistent electrostatic potential. In PMFPNP, however, information about the dynamic relaxation of the protein and the …

Synthesis Of Tin Oxide Nanocrystalline Phases Via Use Of Tin (Ii) Halide Precursors, Hongmei Deng, Frank K. Lamelas, Jeanne Hossenlopp

Chemistry Faculty Research and Publications

Nanocrystalline tin oxides are synthesized via precipitation from heated solutions as well as from a novel above-solution vapor deposition route that occurs at low temperatures and atmospheric pressure. Crystalline phases are characterized via powder X-ray diffraction. Samples precipitated from reactions of SnCl2 are found to exist primarily as mixtures of tetragonal SnO and tetragonal SnO2 or tetragonal SnO2 and tin(II) oxyhydroxide (Sn6O4- (OH)4), depending on reaction conditions. A mixed tin(II)/tin(IV) sample is shown to produce a rarely observed form of the intermediate oxide Sn3O4 upon annealing in air at 600 °C. SnBr2 exclusively forms tetragonal SnO2 via precipitation. Variation in …

Kinetic And Spectroscopic Characterization Of The H178a Methionyl Aminopeptidase From Escherichia Coli, Alicja J. Copik, Sabina I. Swierczek, W. Todd Lowther, Ventris M. D'Souza, Brian W. Matthews, Richard C. Holz

Chemistry Faculty Research and Publications

To gain insight into the role of the strictly conserved histidine residue, H178, in the reaction mechanism of the methionyl aminopeptidase from Escherichia coli (EcMetAP-I), the H178A mutant enzyme was prepared. Metal-reconstituted H178A binds only one equivalent of Co(II) or Fe(II) tightly with affinities that are identical to the WT enzyme based on kinetic and isothermal titration calorimetry (ITC) data. Electronic absorption spectra of Co(II)-loaded H178A EcMetAP-I indicate that the active site divalent metal ion is pentacoordinate, identical to the WT enzyme. These data indicate that the metal binding site has not been affected by altering H178. …

Regio- And Stereoselective Ruthenium Catalyzed Hydrovinylation Of 1,3-Dienes: Application To The Generation Of A 20s-Steroidal Sidechain, Zhengjie He, Chae S. Yi, William A. Donaldson

Chemistry Faculty Research and Publications

The addition of ethylene to 1,3-dienes and 1-vinylcycloalkenes, catalyzed by two ruthenium complexes, proceeds in a regioselective fashion to afford 3-methyl-1,4-dienes as products. For a steroidal-based 1-vinylcycloalkene, the addition is stereospecific, giving a product with a 20(S) configuration.

Co-Catalytic Metallopeptidases As Pharmaceutical Targets, Richard C. Holz, Krzysztof P. Bzymek, Sabina I. Swierczek

Chemistry Faculty Research and Publications

Understanding the reaction mechanism of co-catalytic metallopeptidases provides a starting point for the design and synthesis of new molecules that can be screened as potential pharmaceuticals. Many of the enzymes that contain co-catalytic metallo-active sites play important roles in cellular processes such as tissue repair, protein maturation, hormone level regulation, cell-cycle control and protein degradation. Therefore, these enzymes play central roles in several disease states including cancer, HIV, stroke, diabetes, bacterial infections, neurological processes, schizophrenia, seizure disorders, and amyotrophic lateral sclerosis. The mechanism of AAP, an aminopeptidase from Aeromonas proteolytica, is one of the best-characterized examples of a metallopeptidase …

Synthesis Of (+)-Decarestrictine L, Julie Lukesh, William A. Donaldson

Chemistry Faculty Research and Publications

A synthesis of (+)-decarestrictine L 1, a cholesterol biosynthesis inhibitory metabolite isolated from Penicillium simplicissimum, is described. Beginning from tri-O-acetyl- -glucal, alkylation with trimethylaluminum introduced the axial methyl group at C-2 in a stereoselective fashion. Chain extension at the C-6 carbon was accomplished by generation of the primary tosylate, followed by displacement with cyanide anion. The synthesis of (+)-1 was completed in 13 steps and 6.3% overall yield.

Nanocomposites Based On Poly (Ε-Caprolactone) (Pcl)/Clay Hybrid: Polystyrene, High Impact Polystyrene, Abs, Polypropylene And Polyethylene, Xiaoxia Zheng, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polystyrene, high impact polystyrene, acrylonitrile–butadiene–styrene terploymer, polypropylene and polyethylene have been prepared using an organically-modified clay that contains polycaprolactone—PCL-modified clay. Depending upon the mode of preparation of the PCL-modified clay, all three types of nanocomposites, immiscible, intercalated and exfoliated, may be produced. The materials have been characterized by X-ray diffraction, transmission electron microscopy, cone calorimetry, thermogravimetric analysis, and the evaluation of mechanical properties.

Molecular Recognition Of No/No+ Via Multicenter (Charge-Transfer) Binding To Bridged Diarene Donors. Effect Of Structure On The Optical Transitions And Complexation Thermodynamics, Sergiy V. Rosokha, Sergey V. Lindeman, Rajendra Rathore, Jay K. Kochi

Chemistry Faculty Research and Publications

Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular “Venus flytrap”. The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of …

An Xps Investigation Of Thermal Degradation And Charring On Poly(Vinyl Chloride)–Clay Nanocomposites, Jianxin Du, Dongyan Wang, Charles A. Wilkie, Jiangqi Wang

Chemistry Faculty Research and Publications

More information concerning the thermal degradation and charring of nanocomposites of poly(vinyl chloride), dioctyl phthalate and clay has been obtained by the use of X-ray photoelectron spectroscopy and the acquisition of the carbon (C1s), chlorine (Cl2p), and oxygen (O1s) spectra. In the cases of polystyrene–clay and poly(methyl methacrylate)–clay nanocomposites, it has been shown that the clay migrates to the surface as the temperature is raised and the polymer degrades, thereby confirming the barrier properties as a mechanism by which these materials function. For PVC–clay nanocomposites the surface at high temperatures is dominated by carbon, and not the oxygen of the …

A Stibonium-Modified Clay And Its Polystyrene Nanocomposite, Dongyan Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Triphenylhexadecylstibonium trifluoromethylsulfonate has been prepared and ion-exchanged with sodium montmorillonite to obtain a new organically-modified clay. The clay has higher thermal stability than an ammonium clay; only a portion of the alkyl chain is lost during degradation and all of the antimony is retained. This clay has been used to prepare a polystyrene nanocomposite in which the clay is not uniformly distributed throughout the polymer. Nonetheless the polymer does insert into the clay layers and the d-spacing of the clay expands from 2.0 to 3.0 nm. The enhanced thermal stability of this system may mean that it could be …

In Situ Reactive Blending To Prepare Polystyrene-Clay And Polypropylene-Clay Nanocomposites, Dongyan Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polystyrene and polypropylene with organically-modified clay may be prepared by melt blending in a Brabender mixer the clay and the polymer. The presence of maleic anhydride increases the likelihood of nanocomposite formation for polystyrene but is less important for polypropylene. The materials that result are immiscible materials, in that the clay is not uniformly distributed throughout the polymer matrix, but there is polymer inserted between the clay layers. The results from cone calorimetry suggest that nanocomposite formation has occurred, since there is a significant reduction in the peak heat release rate.

Synergy Between Conventional Phosphorus Fire Retardants And Organically-Modified Clays Can Lead To Fire Retardancy Of Styrenics, Grace Chigwada, Charles A. Wilkie

Chemistry Faculty Research and Publications

Polystyrene–clay nanocomposites combined with phosphorous-containing fire retardants have been prepared and used to explore the thermal stability and mechanical properties of the polymer formed. The amounts of fire retardants and clay used were varied to study the effect of each on thermal stability and mechanical properties of the polymer. The samples were prepared by bulk polymerization and analyzed by X-ray diffraction, thermogravimetric analysis, cone calorimetry, Fourier Transform infrared spectroscopy and the evaluation of mechanical properties. The thermal stability of the polymers is enhanced by the presence of the phosphorus-containing fire retardants.

Iron-Mediated Preparation Of Vinylcyclopropanes. Scope, Mechanism, And Applications, Young K. Yun, Kamil Godula, Yeyu Cao, William A. Donaldson

Chemistry Faculty Research and Publications

The addition of stabilized carbon nucleophiles to tricarbonyl(1-methoxycarbonylpentadienyl)iron(1+) cation (1a) proceeds via attack at C2 on the face of the ligand opposite the Fe(CO)₃ group to generate tricarbonyl(pentenediyl)iron complexes 2. Oxidation of complexes 2 affords vinylcyclopropanecarboxylates in good yield. In general, the relative stereochemistry about the cyclopropane ring reflects reductive elimination with retention of configuration. In cases where the C2 substituent is bulky (i.e., 2b) the major cyclopropane product 9b represents ring closure with inversion at C3. A mechanism involving π−σ−π rearrangement of the initially oxidized (pentenediyl)iron species is proposed to account for these results. …

Preparation And Flammability Properties Of Polyethylene-Clay Nanocomposites, Jinguo Zhang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Polyethylene (PE)–clay nanocomposites have been prepared using melt blending in a Brabrender mixer. X-ray diffraction and transmission electron microscopy were used to characterize the nano-structure of these composites while the thermal stability was evaluated from thermogravimetric analysis and the flammability parameters using cone calorimetry. It is found that the PE–clay nanocomposites have a mixed immiscible-intercalated structure and there is better intercalation when maleic anhydride is combined with the polymer and clay to be melt blended. The reduction in peak heat release rate is 30–40%.

Flame Retardancy Of Polystyrene Nanocomposites Based On An Oligomeric Organically-Modified Clay Containing Phosphate, Xiaoxia Zheng, Charles A. Wilkie

Chemistry Faculty Research and Publications

Novel modified clays, which may enable the formation of flame retarded polystyrene nanocomposites by melt or solution blending, have been prepared using an ammonium salt which contains an oligomeric material consisting of vinylbenzyl chloride, styrene and vinyl phosphate reacting with dimethylhexadecylamine. These nanocomposites have been characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry and the evaluation of mechanical properties. Melt blending is an effective, economical way to produce intercalated nanocomposites with greatly reduced peak heat release rate and a decreased total heat release; the polymer does not all undergo thermal degradation.

Exfoliated Poly(Methyl Methacrylate) And Polystyrene/ Nanocomposites Occur When The Clay Cation Contains A Vinyl Monomer, Shengpei Su, Charles A. Wilkie

Chemistry Faculty Research and Publications

The bulk polymerization of methyl methacrylate and styrene in the presence of an organically modified clay containing a vinyl group that can be involved in the polymerization produces exfoliated nanocomposites. These nanocomposites have been characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, mechanical properties, and cone calorimetry. The onset temperature of thermal degradation increases with the mechanical properties. The peak heat release rate is significantly reduced for nanocomposites containing 3 or 5% clay. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1124–1135, 2003

Kinetic And Structural Characterization Of Manganese(Ii)-Loaded Methionyl Aminopeptidases, Ventris M. D'Souza, Sabina I. Swierczek, Nathaniel J. Cosper, Lu Meng, Shane Ruebush, Alicja J. Copik, Robert A. Scott, Richard C. Holz

Chemistry Faculty Research and Publications

Manganese(II) activation of the methionyl aminopeptidases from Escherichia coli (EcMetAP-I) and the hyperthermophilic archaeon Pyrococcus furiosus (PfMetAP-II) was investigated. Maximum catalytic activity for both enzymes was obtained with 1 equiv of Mn(II), and the dissociation constants (K_d) for the first metal binding site were found to be 6 ± 0.5 and 1 ± 0.5 μM for EcMetAP-I and PfMetAP-II, respectively. These K_d values were verified by isothermal titration calorimetry (ITC) and found to be 3.0 ± 0.2 and 1.4 ± 0.2 μM for EcMetAP-I and PfMetAP-II, respectively. The …

Doming Modes And Dynamics Of Model Heme Compounds, Dennis D. Klug, Marek Z. Zgierski, John S. Tse, Zhenxian Liu, James R. Kincaid, Kazimierz Czarnecki, Russell J. Hemley

Chemistry Faculty Research and Publications

Synchrotron far-IR spectroscopy and density-functional calculations are used to characterize the low-frequency dynamics of model heme FeCO compounds. The “doming” vibrational mode in which the iron atom moves out of the porphyrin plane while the periphery of this ring moves in the opposite direction determines the reactivity of oxygen with this type of molecule in biological systems. Calculations of frequencies and absorption intensities and the measured pressure dependence of vibrational modes in the model compounds are used to identify the doming and related normal modes.

The Aminopeptidase From Aeromonas Proteolytica: Structure And Mechanism Of Co-Catalytic Metal Centers Involved In Peptide Hydrolysis, Richard C. Holz

Chemistry Faculty Research and Publications

Enzymes containing multi-metal active sites are central to numerous biological processes and, consequently, characterization of their structure and function is a problem of outstanding importance. One of the least-explored groups of enzymes is the hydrolases that contain dinuclear metal centers. These enzymes play key roles in carcinogenesis, tissue repair, and protein degradation processes. In addition, some of these enzymes can catalyze the hydrolysis of phosphorus(V) compounds found in nerve gases and agricultural neurotoxins. The determination of detailed reaction mechanisms for these enzymes is required for the design of highly potent, specific inhibitors that can function as potential pharmaceuticals. Hydrolytic enzymes …

Enantioselective Synthesis Of The C11–C17 Segment Of Soraphen A1Α Via Organoiron Methodology, Yeyu Cao, Ahmad F Eweas, William A. Donaldson

Chemistry Faculty Research and Publications

The C11–C17 segment of the antifungal agent soraphen A_1α was prepared from glyceraldehyde acetonide in nine steps. The C12 stereocenter is derived from glyceraldehyde, while the C17 stereocenter as introduced by 1,6-asymmetric control via the coordinated Fe(CO)₃.

The C11–C17 segment of the antifungal agent soraphen A1α, with required inverted stereochemistry at C17, was prepared. The C12 stereocenter is derived from glyceraldehyde, while the C17 stereocenter is introduced by 1,6-asymmetric induction via a coordinated Fe(CO)₃.

Crown Ether-Modified Clays And Their Polystyrene Nanocomposites, Hongyang Jao, Jin Zhu, Alexander B. Morgan, Charles A. Wilkie

Chemistry Faculty Research and Publications

Crown ether-modified clays were obtained by the combination of sodium and potassium clays with crown ethers and cryptands. Polystyrene nanocomposites were prepared by bulk polymerization in the presence of these clays. The structures of nanocomposites were characterized by X-ray diffraction and transmission electron microscopy. Their thermal stability and flame retardancy were measured by thermogravimetric analysis and cone calorimetry, respectively. Nanocomposites can be formed only from the potassium clays; apparently the sodium clays are not sufficiently organophilic to enable nanocomposite formation. The onset temperature of the degradation is higher for the nanocomposites compared to virgin polystyrene, and the peak heat release …

The 1.20 Å Resolution Crystal Structure Of The Aminopeptidase From Aeromonas Proteolytica Complexed With Tris: A Tale Of Buffer Inhibition, William Desmarais, David L. Bienvenue, Krzysztof P. Bzymek, Richard C. Holz, Gregory A. Petsko, Dagmar Ringe

Chemistry Faculty Research and Publications

The aminopeptidase from Aeromonas proteolytica (AAP) is a bridged bimetallic enzyme that removes the N-terminal amino acid from a peptide chain. To fully understand the metal roles in the reaction pathway of AAP we have solved the 1.20 Å resolution crystal structure of native AAP (PDB ID = 1LOK). The high-quality electron density maps showed a single Tris molecule chelated to the active site Zn²⁺, alternate side chain conformations for some side chains, a sodium ion that mediates a crystal contact, a surface thiocyanate ion, and several potential hydrogen atoms. In addition, the high precision of the atomic …

Synthetic Studies Directed Toward The Phorboxazoles: Preparation Of The C3–C15 Bisoxane Segment And Two Stereoisomers, Patrick Bernard Greer, William A. Donaldson

Chemistry Faculty Research and Publications

A synthetic approach to the C3–C15 segment of the cytotoxic marine metabolite phorboxazoles is described. This segment consists of a methylene linked bisoxane structure. The first pyran ring was constructed by a Lewis acid catalyzed diene–aldehyde cyclocondensation. The β-C-glucoside substitution pattern of this ring was established by a stereoselective allylation. Ozonolysis of vinyl group and enantioselective allylation of the racemic aldehyde generated two separable homoallylic alcohols (−)-22 and (+)-23. The Mosher's esters of each alcohol were determined to be >90% de. Reaction of (−)-22 with acryloyl chloride, followed by ring closing metathesis gave the …