Methanol adsorption on the beta-Ga2O3 surface with oxygen vacancies: theoretical and experimental approach

Methanol adsorption on beta-Ga2O3 surface has been studied by Fourier transform infrared spectroscopy (FTIR) and by means of density functional theory (DFT) cluster model calculations. Adsorption sites of tetrahedral and octahedral gallium ions with different numbers of oxygen vacancies have been compared. The electronic properties of the adsorbed molecules have been monitored by computing adsorption energies, optimized geometry parameters, overlap populations, atomic charges, and vibrational frequencies. The gallia-methanol interaction has different behaviors according to the local surface chemical composition. The calculations show that methanol can react in three different ways with the gallia surface giving rise to a nondissociative adsorption, a dissociative adsorption, and an oxidative decomposition. The surface without oxygen vacancies is very reactive and produces the methanol molecule decomposition. The molecule is nondissociatively adsorbed by means of a hydrogen bond between the alcoholic hydrogen atom and a surface oxygen atom and a bond between the alcoholic oxygen atom and a surface gallium atom. Two neighbor oxygen vacancies on tetrahedral gallium sites produce the dissociation of the methanol molecule and the formation of a bridge bond between two surface gallium atoms and the methoxy group.     

A microfluidic channel flow cell for electrochemical ESR

The design, fabrication, and characterization of microfluidic channel flow devices for in situ simultaneous hydrodynamic electrochemical ESR is reported. The microelectrochemical reactors consist of gold film electrodes situated within rectangular ducts of height 350 microm and widths in the range 500-2000 microm. The small dimensions of the channels result in minimal dielectric loss when centralized within a cylindrical TE011 resonant cavity, leading to a high level of sensitivity. This is demonstrated by using the one-electron oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in acetonitrile as a model system, wherein the ESR spectra obtained for the corresponding stable radical cation are of a high signal-to-noise ratio. Signal intensity is measured as a function of flow rate for this system, and the behavior is validated by means of 3-dimensional numerical modeling of the hydrodynamic flow profile.     

Ion induced lamellar-lamellar phase transition in charged surfactant systems

We propose a model for the liquid-liquid (L(alpha)-->L(alpha(') )) phase transition observed in osmotic pressure measurements of certain charged lamellae-forming amphiphiles. The model free energy combines mean-field electrostatic and phenomenological nonelectrostatic interactions, while the number of dissociated counterions is treated as a variable degree of freedom that is determined self-consistently. The model, therefore, joins two well-known theories: the Poisson-Boltzmann theory for ionic solutions between charged lamellae and the Langmuir-Frumkin-Davies adsorption isotherm modified to account for charged adsorbing species. Minimizing the appropriate free energy for each interlamellar spacing, we find the ionic density profiles and the resulting osmotic pressure. While in the simple Poisson-Boltzmann theory the osmotic pressure isotherms are always smooth, we observe a discontinuous liquid-liquid phase transition when the Poisson-Boltzmann theory is self-consistently augmented by the Langmuir-Frumkin-Davies adsorption. This phase transition depends on the area per amphiphilic head group, as well as on nonelectrostatic interactions of the counterions with the lamellae and interactions between counterion-bound and counterion-dissociated surfactants. Coupling the lateral phase transition in the bilayer plane with electrostatic interactions in the bulk, our results offer a qualitative explanation for the existence of the L(alpha)-->L(alpha(') ) phase transition of didodecyldimethylammonium bromide (DDABr), but the transition's apparent absence for the chloride and the iodide homologs. More quantitative comparisons with experiment require better understanding of the microscopic basis of the phenomenological model parameters.     

Neutral additives enhance the metal-chelate affinity adsorption of nucleic acids: role of water activity

Immobilized metal-chelate affinity chromatography has been widely used in the purification of proteins, and we have recently found that it can also be applied to purification of nucleic acids through interactions involving exposed bases, especially purines. Here we report that the inclusion of moderate quantities of neutral solutes in the buffer substantially enhances the binding affinity of nucleic acids for immobilized metal-chelate affinity adsorbents. Addition of 20% (v/v) of solutes such as ethanol, methanol, isopropanol, n-propanol, and dimethyl sulfoxide enhances the initial affinity of binding of total yeast RNA by 4.4-, 3.8-, 3.7-, 3.0-, and 2.8-fold, respectively for Cu(II)-iminodiacetic acid (IDA) agarose adsorbent, and the weaker adsorption by Cu(II)-nitrilotriacetic acid (NTA) agarose was even more strongly enhanced. The adsorption affinities of the smaller oligodeoxynucleotide molecules A20, G20, C20 and T20 also increase with the addition of ethanol, suggesting that the effect is not significantly mediated by conformational changes. Binding enhancement generally correlates with reduction of water activity by the various solutes, as predicted by several models of solution thermodynamics, consistent with an entropic contribution by displacement of waters from the metal-chelate. Interestingly, the enhancement was not seen with the proteins bovine serum albumin and lysozyme.     

Cube-type nitrido complexes containing titanium and zinc/copper

Several azaheterometallocubane complexes containing [MTi3N4] cores have been prepared by the reaction of [{Ti(eta5-C5Me5)(mu-NH)}3(mu3-N)] (1) with zinc(II) and copper(I) derivatives. The treatment of 1 with zinc dichloride in toluene at room temperature produces the adduct [Cl2Zn{(mu3-NH)3Ti3(eta5-C5Me5)3(mu3-N)}] (2). Attempts to crystallize 2 in dichloromethane gave yellow crystals of the ammonia adduct [(H3N)Cl2Zn{(mu3-NH)Ti3(eta5-C5Me5)3(mu-NH)2(mu3-N)}] (3). The analogous reaction of 1 with alkyl, (trimethylsilyl)cyclopentadienyl, or amido zinc complexes [ZnR2] leads to the cube-type derivatives [RZn{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (R = CH2SiMe3 (5), CH2Ph (6), Me (7), C5H4SiMe3 (8), N(SiMe3)2 (9)) via RH elimination. The amido complex 9 decomposes in the presence of ambient light to generate the alkyl derivative [{Me3Si(H)N(Me)2SiCH2}Zn{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (10). The chloride complex 2 reacts with lithium cyclopentadienyl or lithium indenyl reagents to give the cyclopentadienyl or indenyl zinc derivatives [RZn{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (R = C5H5 (11), C9H7 (12)). Treatment of 1 with copper(I) halides in toluene at room temperature leads to the adducts [XCu{(mu3-NH)3Ti3(eta5-C5Me5)3(mu3-N)}] (X = Cl (13), I (14)). Complex 13 reacts with lithium bis(trimethylsilyl)amido in toluene to give the precipitation of [{Cu(mu4-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}2] (15). Complex 15 is prepared in a higher yield through the reaction of 1 with [{CuN(SiMe3)2}4] in toluene at 150 degrees C. The addition of triphenylphosphane to 15 in toluene produces the single-cube compound [(Ph3P)Cu{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (16). The X-ray crystal structures of 3, 8, 9, and 15 have been determined.     

Network Design Via Iterative Rounding Of Setpair Relaxations

A typical problem in network design is to find a minimum-cost sub-network H of a given network G such that H satisfies some prespecified connectivity requirements. Our focus is on approximation algorithms for designing networks that satisfy vertex connectivity requirements. Our main tool is a linear programming relaxation of the following setpair formulation due to Frank and Jordan: a setpair consists of two subsets of vertices (of the given network G); each setpair has an integer requirement, and the goal is to find a minimum-cost subset of the edges of G sucht hat each setpair is covered by at least as many edges as its requirement. We introduce the notion of skew bisupermodular functions and use it to prove that the basic solutions of the linear program are characterized by “non-crossing families” of setpairs. This allows us to apply Jain’s iterative rounding method to find approximately optimal integer solutions. We give two applications. (1) In the k-vertex connectivity problem we are given a (directed or undirected) graph G=(V,E) with non-negative edge costs, and the task is to find a minimum-cost spanning subgraph H such that H is k-vertex connected. Let n=|V|, and let ε<1 be a positive number such that k≤(1−ε)n. We give an -approximation algorithm for both problems (directed or undirected), improving on the previous best approximation guarantees for k in the range . (2)We give a 2-approximation algorithm for the element connectivity problem, matching the previous best approximation guarantee due to Fleischer, Jain and Williamson. * Supported in part by NSERC researchgran t OGP0138432. † Supported in part by NSF Career Award CCR-9875024.     

The design and synthesis of inhibitors of pantothenate synthetase

Pantothenate synthetase catalyses the ATP-dependent condensation of D-pantoate and beta-alanine to form pantothenate. Ten analogues of the reaction intermediate pantoyl adenylate, in which the phosphodiester is replaced by either an ester or sulfamoyl group, were designed as potential inhibitors of the enzyme. The esters were all modest competitive inhibitors, the sulfamoyls were more potent, consistent with their closer structural similarity to the pantoyl adenylate intermediate.     

Synthesis and structures of metallocene complexes of iron(II) and ruthenium(II) derived from 9,9′-spirobifluorene

Reaction between 9,9′-spirobifluorene and [CpM]⁺ (where M = Fe and Ru) equivalents gives the complexes [CpRu(η⁶-SBF)][PF₆] (1), [(CpRu)₂(η⁶,η⁶-SBF)][PF₆]₂ (2) and [(CpFe)₂(η⁶,η⁶-SBF)][PF₆]₂ (3), respectively. Single crystal X-ray structures of 1 and 3 show that the metal atoms exhibit distorted η⁶-coordination to SBF phenyl moieties primarily as a consequence of steric interactions between Cp and SBF. The structure of 3 contains each of the possible C₂ enantiomers whereas NMR spectroscopy shows signals consistent with a 1:1 mixture of C₂ and C₁ stereoisomers for both 2 and 3. In conjunction with electrochemical data the observations are consistent with SBF acting as a molecule containing two independent biphenyl moieties.     

Annular tautomerism: experimental observations and quantum mechanics calculations

The use of MP2 level quantum mechanical (QM) calculations on isolated heteroaromatic ring systems for the prediction of the tautomeric propensities of whole molecules in a crystalline environment was examined. A Polarisable Continuum Model was used in the calculations to account for environment effects on the tautomeric relative stabilities. The calculated relative energies of tautomers were compared to relative abundances within the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB). The work was focussed on 84 annular tautomeric forms of 34 common ring systems. Good agreement was found between the calculations and the experimental data even if the quantity of these data was limited in many cases. The QM results were compared to those produced by much faster semiempirical calculations. In a search for other sources of the useful experimental data, the relative numbers of known compounds in which prototropic positions were often substituted by heavy atoms were also analysed. A scheme which groups all annular tautomeric transformations into 10 classes was developed. The scheme was designed to encompass a comprehensive set of known and theoretically possible tautomeric ring systems generated as part of a previous study. General trends across analogous ring systems were detected as a result. The calculations and statistics collected on crystallographic data as well as the general trends observed should be useful for the better modelling of annular tautomerism in the applications such as computer-aided drug design, small molecule crystal structure prediction, the naming of compounds and the interpretation of protein—small molecule crystal structures.