Reactivity of TCNE or TCNQ derivatives of quinonoid zwitterions: platinum-induced HCN elimination vs oxidative-addition

The reaction of the functional, zwitterionic quinonoid molecule (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,1,2,2-tetracyanoethyl)cyclohexa-1,4-dien-1-olate, [C(6)H-2-{C(CN)(2)C(CN)(2)H}]-4,6-(···NH n-Bu)(2)-1,3(···O)(2) (2), which has been previously prepared by regioselective insertion of TCNE into the C-H bond adjacent to the C···O bonds of the zwitterionic benzoquinone monoimine (6E)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate, C(6)H(2)-4,6-(···NHn-Bu)(2)-1,3-(···O)(2) (1), with 2 equiv of [Pt(C(2)H(4))(PPh(3))(2)], afforded the Pt(0) complex [Pt(PPh(3))(2)(4)] (6) (4 = 2-HCN; (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,2,2-tricyanoethenyl)cyclohexa-1,4-dien-1-olate), in which a tricyanoethenyl moiety is π-bonded to the metal. A metal-induced HCN elimination reaction has thus taken place. The same complex was obtained directly by the reaction of 1 equiv of the Pt(0) complex [Pt(C(2)H(4))(PPh(3))(2)] with the olefinic ligand [C(6)H-2-{C(CN)═C(CN)(2)}]-4,6-(···NHn-Bu)(2)-1,3-(···O)(2)) (4), previously obtained by the reaction of 2 with NEt(3) in THF. A similar reactivity pattern was observed between 2 and 2 equiv of the Pd(0) precursor [Pd(dba)(2)] in the presence of dppe, which led to [Pd(dppe)(4)] (7), which was also directly obtained from 4 and 1 equiv [Pd(dba)(2)]/dppe. In contrast to the behavior of the TCNE derivative 2, the reaction of the TCNQ derivative (6E)-4-(butylamino)-6-(butyliminio)-2-(dicyano(4-(dicyanomethyl)phenyl)methyl)-3-oxocyclohexa-1,4-dien-1-olate, [C(6)H-2-{C(CN)(2)p-C(6)H(4)C(CN)(2)H}]-4,6-(···NHn-Bu)(2)-1,3-(···O)(2)) (3), with 2 equiv of [Pt(C(2)H(4))(PPh(3))(2)] led to formal oxidative-addition of the C-H bond of the C(CN)(2)H moiety to give the Pt(II) hydride complex trans-[PtH(PPh(3))(2){N═C═C(CN)p-C(6)H(4)C(CN)(2)-2-[C(6)H-4,6-(···NHn-Bu)(2)-1,3-(···O)(2))}] (8). The molecular structures of 3, 4, 6·0.5(H(2)O), and 8·3(CH(2)Cl(2)) have been determined by single-crystal X-ray diffraction.     

Capillary-pressure driven adhesion of rigid-planar surfaces

A thin film of viscous-Newtonian fluid sandwiched between parallel-plane walls, is examined both experimentally and theoretically for gap spacings initially much smaller than the capillary length to determine the conditions for adhesion. The problem is parameterized by the variables F, which is the ratio of an external load generated by one of the parallel-plane surfaces, to the product of surface tension and a characteristic length scale, and the static contact angle α. An analytical solution for the change in gap height as a function of elapsed time is derived in the limit of small Reynolds and zero capillary numbers. The load is suspended for a long but finite elapsed time as the gap spacing approaches a critical value, and for gap spacing values less than the critical one the load is suspended indefinitely. Experiments are performed with typical elapsed times of O(100-1000s) using fluids with viscosity, O(1000 cSt), but different surface tension and contact angle for gap spacings O(10-100μm) with loads of either 2.7N or 4.9N. There is good agreement between the theory and experiments.     

Topochemical transketalization reaction driven by hydrogen bonding

An unusual thermal isomerization of 1,2;3,4-di-O-isopropylidene-myo-inositol to 1,2;5,6-di-O-isopropylidene-myo-inositol was observed in the solid state. A detailed study revealed that this ketal migration is a topochemically driven one. X-ray structure revealed that strong intermolecular hydrogen bonding in the crystal preorganizes the hydroxyl and ketal carbon in a transition-state-like arrangement favorable for the reaction. The 5-OH of each molecule faces the trans ketal carbon of a neighboring molecule at a close distance and an angle of approach near to linearity, making a trigonal-bipyramidal-like arrangement in the crystal, which could facilitate the reaction. This is the first report of a topochemical transketalization reaction.     

Halogen bonding driven self-assembly of fluorocarbons and hydrocarbons

Halogen bonding, the interaction between halogen atoms and lone pair possessing atoms, overcomes the low affinity existing between perfluorocarbon and hydrocarbon derivatives. This interaction effectively drives the self-assembly of materials wherein perfluorocarbon and hydrocarbon layers alternate thanks to a remarkable module segregation. The approach has been exploited in supramolecular polymer coating.     

EWOD driven cleaning of bioparticles on hydrophobic and superhydrophobic surfaces

Environmental air monitoring is of great interest due to the large number of people concerned and exposed to different possible risks. From the most common particles in our environment (e.g. by-products of combustion or pollens) to more specific and dangerous agents (e.g. pathogenic micro-organisms), there are a large range of particles that need to be controlled. In this article we propose an original study on the collection of electrostatically deposited particles using electrowetting droplet displacement. A variety of particles were studied, from synthetic particles (e.g. Polystyrene Latex (PSL) microsphere) to different classes of biological particle (proteins, bacterial spores and a viral simulant). Furthermore, we have compared ElectroWetting-On-Dielectric (EWOD) collecting efficiency using either a hydrophobic or a superhydrophobic counter electrode. We observe different cleaning efficiencies, depending on the hydrophobicity of the substrate (varying from 45% to 99%). Superhydrophobic surfaces show the best cleaning efficiency with water droplets for all investigated particles (MS2 bacteriophage, BG (Bacillus atrophaeus) spores, OA (ovalbumin) proteins, and PSL).     

Structure and bonding of ethoxy species adsorbed on transition metal surfaces

The interaction of the ethoxy radical with Cu(111), Ag(111), Pd(111) and Au(111) has been studied using a periodic density functional approach. The most stable adsorption site is the fcc with adsorption energies in the 1.1–2.2 eV interval. All analyses consistently indicate that ethoxy becomes negatively charged, that the presence of ethoxy slightly perturbs the electronic structure of the metallic surface, that the interaction is essentially of electrostatic character and not directional predicting a rather mobile species. The calculated adsorption energies are found to correlate almost linearly with the total net charge on the ethoxy moiety thus confirming that the electrostatic interactions dominate the bonding between this organic species and the underlying metallic surfaces.     

On obtaining localized bonding schemes from delocalized molecular-orbital wave functions

A straightforward procedure is proposed for expanding a molecular orbital determinantal wave function into a set of determinantal wave functions composed of atomic orbitals localized at the atoms of a molecule. By employing this method, atomic orbital determinants and their weights can be derived for a molecule from the computed molecular-orbital wave function. The procedure permits the interpretation of a molecular orbital determinantal wave function in terms of bonding schemes related to the classic resonance structures used by organic chemists. By using the unrestricted molecular orbital determinant, bonding schemes and their weights are obtained for butadiene, the butadiene radical cation and the acrylonitrile radical anion. Their dominant bonding schemes are in accord with the relevant resonance structures for these molecules. For the butadiene radical cation and the acrylonitrile anion they are shown to be compatible with the accepted mechanisms of the electrochemical coupling reactions of butadiene and acrylonitrile. Received: 7 August 1996 / Accepted: 18 March 1997     

Surface bonding on silicon surfaces as probed by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been used to obtain the Raman signal of surface species on silicon single crystal surfaces without the necessity for surface enhancement by addition of Ag nanoparticles. By illuminating the hydrogen terminated silicon surface covered with a droplet of 4-vinylpyridine with UV light, a 4-ethylpyridine modified silicon surface can be easily obtained. By bringing a scanning tunneling microscope (STM) Au tip with a nanoscale tip apex to a distance of ca. 1 nm from the m...     

Multi-wall carbon nanotubes bonding on silica-hydride surfaces for open-tubular capillary electrochromatography

Prepared multi-wall carbon nanotube (MWNT) materials, including untreated MWNT, HNO₃-treated MWNT and HNO₃-HCl-treated MWNT were covalently attached onto a silica-hydride-modified capillary by hydrosilation, using the abundant double bonds between the pentagon carbons in the MWNT structure. These MWNT-incorporated capillaries were characterized by SEM, ATR-IR and electroosmotic flow (EOF) measurements in phosphate buffers with a pH range of 3.7–9.3 and in the mixtures of acetonitrile modifier. The untreated capillary was assumed to carry some carboxylate groups formed on the non-acid-treated MWNTs, as it had higher EOF values than the hydride capillary. As the MWNTs were treated with HNO₃ and HCl solutions, the capillaries had increasingly higher EOF values. To examine the existence of an electrochromatography mechanism in the modified capillaries, a mixture of nucleosides and thymine was probed to check the velocity factor and retention factor. In addition to the π–π interaction between the probe solutes and the MWNT immobilized stationary phases; a reversed-phase mechanism could contribute to the chromatographic retention. For acidic tetracyclines, increasing the loadability of MWNTs resulted in a high retention factor and improved the separation resolution.     

Non-specific adhesion on biomaterial surfaces driven by small amounts of protein adsorption

This work explores how long-range non-specific interactions, resulting from small amounts of adsorbed fibrinogen, potentially influence bioadhesion. Such non-specific interactions between protein adsorbed on a biomaterial and approaching cells or bacteria may complement or even dominate ligand–receptor mating. This work considers situations where the biomaterial surface and the approaching model cells (micron-scale silica particles) exhibit strong electrostatic repulsion, as may be the case in diagnostics and lab-on-chip applications. We report that adsorbed fibrinogen levels near 0.5 mg/m² produce non-specific fouling. For underlying surfaces that are less fundamentally repulsive, smaller amounts of adsorbed fibrinogen would have a similar effect. Additionally, it was observed that particle adhesion engages sharply and only above a threshold loading of fibrinogen on the collector. Also, in the range of ionic strength, I, below about 0.05 M, increases in I reduce the fibrinogen needed for microparticle capture, due to screening of electrostatic repulsions. Surprisingly, however, ionic strengths of 0.15 M reduce fibrinogen adsorption altogether. This observation opposes expectations based on DLVO arguments, pointing to localized electrostatic attractions and hydration effects to drive silica–fibrinogen adhesion. These behaviors are benchmarked against microparticle binding on silica surfaces carrying small amounts of a polycation, to provide insight into the role of electrostatics in fibrinogen-driven non-specific adhesion.     

Metric engineering of perfluorocarbon-hydrocarbon layered solids driven by the halogen bonding

The halogen bonding driven self-assembly of 1,4-dicyanobutane and 1,6-dicyanohexane with a range of perfluorinated telechelic diiodoalkanes yields layered 1D, infinite chain co-crystals. The structure and metrics of these co-crystals are remarkably predictable.     

Dipole moment and conformational properties of poly(methacrylic acid) in solvents with hydrogen bonding

Dielectric polarization of solutions of un-ionized linear poly(methacrylic acid) in polar associated liquids is studied in the temperature range 20–50°C. The solutions are in methanol, with the molar fraction of polymer units x ₂ = (3 × 10^?3)?(1.5 × 10^?2), and in water, with x ₂ = (4 × 10^?5)?(4 × 10^?3). The permittivity ε₁₂ of the polyacid solutions in methanol is shown to be lower than the permittivity of the pure solvent ε₁; the permittivity of the polyacid solutions in water exceeds ε₁ of water in the concentration range x ₂ = (4 × 10^?5)?(2.13 × 10^?4) and becomes lower than ε₁ as the polymer concentration in the solution increases further. A procedure for estimating the dipole moment μ₂ per monomer unit of the polymer macromolecule in solution is proposed. The estimation is based on Buckingham’s statistical polarization theory for a two-component mixture of polar molecules under the conditions of infinite dilution. The μ₂ values amount to 2.76–2.14 D (x ₂ < 1.5 × 10^?2) in methanol at 20–50°C and to 11.4?3.8 D (x ₂ < 2.13 × 10^?4) in water at 20–40°C. The difference in the dipole moments of the polyacid and in the patterns of their temperature dependences in methanol and in water is due to the effects of the polyacid-solvent hydrogen bonding, to intramacromolecular hydrogen bonds, and to specificity of the local structure of the solvent. It is shown that the μ₂ value corresponds to the dipole moment of the solvates and decreases with temperature owing to changes in the stoichiometry of the solvates, to the formation of cyclic associates in the macromolecule, and to conformational changes in the chain.     

Structure and bonding between an aryl group and metal surfaces

Modifying solid surfaces with aryl groups has many potential applications. Using first principles density functional theory methods, we investigated the trend of the structure and bonding of the phenyl group (C6H5, the simplest aryl group) on selected transition metals across the periodic table. We found that the bond between C6H5 and metal surfaces is chemical in nature. Decreasing bond strength is found from left to right, concurrent with a switching of the preferred orientation for C6H5 from the flat-lying configuration to the upright configuration. This switching is attributed to the increasing of d-electrons; that is, early transition metals, lacking d-electrons, favor the carbon-metal pi-bond and therefore the flat-lying configuration, while late transition metals rich in d-electrons prefer the carbon-metal sigma-bond and thus the upright fashion. C6H5 is also found to undergo beta-dehydrogenation on early transition metals. This work invites further theoretical and experimental research on the aryl-solid interface.     

Covalent bonding of alkene and alkyne reagents to graphitic carbon surfaces

Various aromatic and aliphatic alkynes and one alkene were covalently bonded to sp(2)-hybridized carbon surfaces by heat treatment in an argon atmosphere. X-ray photoelectron spectroscopy, Raman, and FTIR spectra of the modified surfaces showed that the molecules were intact after the 400 degrees C heat treatment but that the alkyne group had reacted with the surface to form a covalent bond. Alkynes with ferrocene and porphyrin centers exhibited chemically reversible voltammetric waves that could be cycled many times. Atomic force microscopy of the modified surfaces indicated a thickness of the molecular layer consistent with monolayer coverage, and surface coverage determined by voltammetry was also in the monolayer range. Raman spectroscopy of the porphyrin monolayers formed from a porphyrin alkyne showed no evidence for dimer formation, although multilayer formation may occur at undetected levels. FTIR spectra of the porphyrin-modified carbon surfaces were well-defined, similar to the parent molecule, and indicative of an average tilt angle between the porphyrin plane and the surface normal of 37 degrees . The bond between the molecular monolayer and the carbon surface was quite stable, withstanding sonication in tetrahydrofuran, mild aqueous acid and base, and repeated voltammetric cycling in propylene carbonate electrolyte. Heat treatment of alkynes and alkenes appears to be a generally useful method for modifying carbon surfaces, which can be applied to both aromatic and aliphatic molecules.     

Fabrication of a molecular-level multilayer film on organic polymer surfaces via chemical bonding assembly

A fresh multilayer film was fabricated on a molecular level and successfully tethered to the surface of a hydroxylated organic substrate via chemical bonding assembly (CBA). Sulfate anion groups (SO4-) were preintroduced onto the surface of biaxially oriented polypropylene (BOPP) films via a reference method. Upon hydrolysis of the SO4- groups, hydroxyl groups (--OH) were formed that subsequently acted as initial reagents for a series of alternate reactions with terephthalyl chloride (TPC) and bisphenol A (BPA). A stable and well-defined multilayer film was thus fabricated via the CBA method. As a result of the nanoscale multilayer fresh film being abundant with reactive groups, it is believed that the film and its fabrication method should provide a fundamental platform for further surface functionalization and direct the design of advanced materials with desired properties.     

Assembly of oriented zeolite monolayers and thin films on polymeric surfaces via hydrogen bonding

The b-oriented monolayers of microsized silicalite-1 crystals have been manually assembled on glass plate supported poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), chitosan, and poly(methyl methacrylate) (PMMA) thin films via hydrogen bonding with much enhanced binding strength and satisfactory degrees of coverage and close packing. The exerted pressure and rubbing time in the manual assembly do not affect the binding strength of the silicalite-1 monolayer on the glass plate supported polymeric film. This manual assembly has been further applied to fabricate zeolite monolayers on commercially available Plexiglas surfaces and b-oriented multilayered films of silicalite-1 crystals on glass plates. The assembly method established in this study provides a feasible way to produce zeolite monolayers on polymer-modified solid substrates and Plexiglas and to fabricate zeolite-polymer composite membranes by means of the layer-by-layer technique.