Poster Abstracts - 2009
A series of unique oxidation catalysts have been synthesized, and cryptographically characterized. The crystal structures, along with a discussion of their catalytic activity, are presented.
Moqing Hu, Worcester Polytechnic Institute
Porous solids that derive their porosity from rigid frameworks of molecules are of interest because these materials exhibit large pore volumes, permanent porosity, high thermal stability and feature open channels with tunable dimensions and topology. Such framework solids show great promise as host materials for application in the separation, sensing, and storage of guest molecules. Current approaches to construct porous solids have relied largely on a limited number of cubic architectures to generate open frameworks. Consequently, there is a growing need to explore alternative framework architectures and to characterize their structures, stabilities, and host-guest behavior. Toward this end, we are developing a new class of organic building blocks in an effort to prepare framework solids with diamond and other non-cubic architectures. 4-(Imidazolyl)benzoic acids represent a promising new class of ligands that has yet to be investigated in this regard. Herein, we report our initial efforts to synthesize derivatives of 4-(imidazolyl)benzoic acids, prepare crystalline framework solids via hydrothermal methods, and characterize their structures and porous properties.
Pranoti Navare, Worcester Polytechnic Institute
Resolution of pharmaceuticals via crystallization on chemically modified surfaces
Pranoti S. Navare, John C. MacDonald - Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, MA 01609.
Chirality is important in development of pharmaceuticals because more than half of drugs marketed worldwide are chiral. Enantiomers of chiral drugs often differ considerably in their pharmacological and toxicological effects. The FDA requires that enantiomers be separated when racemic mixtures are produced during synthesis. Crystallization has been used to separate enantiomers in select cases. For example, enantiomers in racemic mixtures crystallize from solution either separately by forming equal amounts of enantiomeric crystals referred to as conglomerates. Unfortunately, crystallization energetically favors formation of racemic crystals (90-95%) over conglomerates (5-10%) because molecules generally pack more efficiently in racemic crystals. We aim to develop surface-based methods that promote formation of conglomerates as a means to separate enantiomers. Toward this goal, we are investigating crystallization of chiral drugs on surfaces functionalized with achiral and chiral molecules that exhibit varying hydrophobicity/hydrophilicity. These surfaces act as templates that bias homochiral molecular aggregation at the surface, thus inducing nucleation of conglomerates over racemic crystals. We also are investigating whether chiral surfaces promote chiral discrimination where one enantiomer crystallizes preferentially, leading to enantiomeric excess. Compounds being investigated include 3-phenyllactic acid and N-acetylleucine. Initial results of experiments with 3-phenyllactic acid are presented. Important findings include oriented growth of crystals indicating that surfaces might template growth via hydrogen bonding. We demonstrate that groups exposed at the surface and solvent significantly affect formation of conglomerates versus racemic crystals.
Jeffrey Wistrom, Tufts University
Dipicolyl-based nickel complexes as urease models: steric effects on complexation, crystallographic confirmation of carbon dioxide fixtion, and urea- and acetamide-assisted solvolysis.
Jeffrey Wikstrom, Alexander Filatov, and Elena Rybak-Akimova
The 2Ni-2OH core of the active site of the urease enzyme has long been an attractive target for synthetic biomimetic coordination complexes, but functional models have been rare. Utilizing a sterically hindered tert-butyl derivative of 2,2'-dipicolylamine, we have successfully synthesized, and crystallographically characterized, a novel 2Ni-2OH dimeric complex. Each nickel sits in a pentacoordinate environment, leaving one coordination site available for substrate binding. Crystallographic studies of less sterically hindered ligands (benzyl and isopropyl derivatives) demonstrate the steric effect on coordination geometry. The 2Ni-2OH complex reacts readily with atmospheric carbon dioxide to form a carbonate-bridged dimer of dimers with a novel nickel-carbonate coordination mode, and in nitrile solvents (benzonitrile, acetonitrile) undergoes solvolysis with dissolved acetamide, forming N'-benzimidoyl-benzamidine or N-(1-Iminoethyl)acetamidine, respectively.
Shawn Burdette, University of Connecticut
The novel catechol-BODIPY dyad, 8-(3,4-dihydroxyphenyl)-2,6-diethoxycarbonyl-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (FerriBRIGHT) was rationally designed with the aid of computational methods. FerriBRIGHT could be prepared by standard one-pot synthesis of BODIPY fluorophores from 3,4-dibenzyloxybenzaldehyde (1) and 3,5-dimethyl-4-ethoxycarbonylpyrrole (3); however, isolating the dipyrrin intermediate 8-(3,4-dibenzyloxyphenyl)-2,6-diethoxycarbonyl-1,3,5,7-tetramethyl-4,4-diaza-s-indacene (7) prior to reaction with excess BF3·OEt2 led to marked improvements in the isolated overall yield of the desired compound. In addition to these improvements in fluorophore synthesis, microwave assisted palladium catalyzed hydrogenolysis of benzyl ethers was used to reduce reaction times and catalyst loading in preparation of the desired compound.
When FerriBRIGHT is exposed to excess FeCl3, CuCl2, [Co(NH3)5Cl]Cl2, 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) or ceric ammonium nitrate (CAN) in methanol, a significant enhancement of fluorescence was observed. FerriBRIGHT-Q, the product resulting from the oxidation of the pendant catechol to the corresponding quinone, was found to be the emissive species. FerriBRIGHT-Q was synthesized independently, isolated and fully characterized to allow for direct comparison with the spectroscopic data acquired in solution. Biologically relevant reactive oxygen species (ROS), such as H2O2, ·OH, 1O2, O2·-, and bleach (NaOCl) failed to cause any changes in the emission intensity of FerriBRIGHT. In accordance with the quantum mechanical calculations, the quantum yield of fluorescence for FerriBRIGHT (QYfl ~ 0) and FerriBRIGHT-Q (QYfl = 0.026, lex/lem = 490/510 nm) suggests that photoinduced electron transfer (PeT) between the catechol and the BODIPY dye is attenuated upon oxidation, which results in fluorescence enhancement. Binding studies of FerriBRIGHT with Ga(NO3)3, a redox inactive analog of Fe(III), provided conditional binding constant logB12' = 13.3 for a [Ga(FerriBRIGHT)2]1- complex. A 2.8 fold enhancement of fluorescence intensity upon addition of Ga(III) to FerriBRIGHT suggests the possibility for metal ion sensing with this new class of compounds.
Chandrima Chatterjee, Yale University
Investigating Hexafluoroacetylacetone Structure by X-ray Diffraction
Chandrima Chatterjee, Christopher D. Incarvito, Lori A. Burns, and Patrick H. Vaccaro
Department of Chemistry, Yale University, New Haven, CT 06520-8107, USA
The cis-enol forms of β-diketones are stabilized by strong intramolecular hydrogen bonds that mediate attendant proton-transfer events. Theoretical  and experimental  investigations on acetylacetone (AA), one of the simplest members of this class, have demonstrated that the proton-transfer process is governed by a double-minimum potential well, where the minima correspond to two equivalent asymmetric Cs structures separated by a barrier of finite height. Since the intrinsic strength of such bonding motifs can be influenced by chemical substitution, it is expected that electron-withdrawing fluoromethyl groups in hexafluoroacetylacetone (HFAA) should weaken the hydrogen bond relative to that of AA. Despite numerous efforts to unravel the ground-state structure of HFAA, its detailed geometry is yet to be ascertained. While most theoretical endeavors point to the Cs structure as the global minimum configuration, gas-phase electron diffraction studies [3,4] have suggested a symmetric (C2v) enol tautomer. Motivated by such contradictory conclusions, the ground-state manifold of HFAA has been re-examined using low-temperature single-crystal X-ray diffraction techniques. Hexafluoroacetylacetone has a normal melting point of 177K and, therefore, exists as a liquid at room temperature. After introducing the liquid in a 0.1mm diameter glass capillary, it was mounted vertically on a Rigaku R-AXIS SPIDER diffractometer and cooled to 93K by a cryogenic stream of nitrogen vapor. Once a polycrystalline mass was formed, the single crystal was grown in situ by the zone-melting method , with a heated filament producing a molten zone that was slowly translated along the length of the capillary. The HFAA crystal structure emerging from collected diffraction data clearly favors an asymmetric H-bond. In addition, the wider separation of 2.683 Å between the donor and acceptor oxygen atoms implies a weaker hydrogen bond compared to AA. Our X-ray analys is has been corroborated by high-level quantum chemical calculations, a detailed discussion of which will be presented in this paper.
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Yaron Segal, Yale University
Structure and Composition of the Crystalline Oxide on Silicon Interface
Yaron Segal, Fred J Walker, James W Reiner, Alexie M Kolpak, Sohrab Ismail-Beigi, Charles H Ahn
Applied Physics and Center for Research on Interface Structure and Phenomena, Yale University, New Haven, Connecticut
Zhan Zhang Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois
The successful integration of crystalline ferroelectric and multiferroic oxides with covalent semiconductors requires atomic level control over the structure of the interface structure between the two materials. Silicon and Barium Oxide are representatives of the two classes of materials that form this problem. BaO was grown epitaxially on Si using a sub-monolayer strontium interfacial passivation layer. We present a structure determination of this interface using X-Ray synchrotron radiation and first principles calculations, supported by RHEED and TEM data. We observe a unique phenomenon in which the Si surface's 2X1 symmetry continues into the BaO film, creating a non-cubic 2X1 phase of BaO near the interface. The structural effect of different interface stoichiometries was examined using DFT calculations. Comparison with the X-Ray diffraction data demonstrates the interface is O-rich. The electronic structure of the interfacial layer is examined. This structure affects the mobility and interface state spectrum of the Si channel in this structure.
Qian Sun, Yale University
X-Ray Diffraction of III-Nitride Wide Bandgap Semiconductors
Qian Sun, Benjamin Leung, and Jung Han
Department of Electrical Engineering, Yale University, New Haven, CT 06511
Group III-nitride semiconductors, AlN, GaN, InN, and their alloys, have energy band gaps spanning from deep UV to near IR spectra range. III-nitride thin films are widely used to fabricate UV, blue, and green light-emitting diodes (LEDs) and lasers, as well as high power, high frequency, and high temperature transistors. Due to the lack of GaN bulk substrates, however, GaN heteroepitaxy on foreign substrates (normally with a large lattice mismatch) generates a high density of structural defects, and the film is often under a large strain. The defects and residual strain greatly affect the material property and the device performance. X-ray diffraction (XRD), as a rapid and non-destructive technique, is commonly utilized to characterize the material quality and device structures, giving a quick yet crucial feedback to crystal growers for the further improvement. In this study, we first show how to determine the thickness and lattice parameters of AlGaN/GaN heterostructure and InGaN/GaN multiple quantum wells by triple-axis XRD, together with the strain status and the alloy composition by reciprocal space mapping (RSM). Then we present a study of the effect of growth conditions on the microstructure of N-polar c-plane (000-1) GaN thin films as compared with Ga-polar (0001) GaN, including the mosaic tilt and twist (accommodated by screw and edge type dislocations, respectively), by measuring x-ray rocking curves (XRCs) on a series of planes (10-1-n) with various angles from the surface. Also, we carried out a detailed XRD analysis on nonpolar a-/m-plane GaN and semipolar (11-22) GaN, which are the emerging areas of the most interest in III-nitride community. A special emphasis is placed on obta.ining a complete picture of their mosaic microstructure often with a strong anisotropy by implementing XRC measurements on both on-axis and off-axis planes at various azimuths. In addition, the high density of basal plane stacking faults which are the maj or challenge in nonpolar and semipolar GaN research, is evaluated by a modified Williamson-Hall analysis.