Showing papers on "Zwitterion published in 2016"

The Mechanism of CO2 Adsorption under Dry and Humid Conditions in Mesoporous Silica-Supported Amine Sorbents

Abstract: The anomalous behavior of CO2 adsorption in anhydrous and humid conditions in silica-supported, polyethylenimine (PEI)-impregnated sorbents has been elucidated in a new chemical model. New quantum chemical calculations show that the zwitterion, whose existence was called into question by earlier theoretical and spectroscopic studies, can be stabilized by water as well as amines, to an extent that zwitterions can serve as diffusive intermediates for CO2 transport through PEI. Water-stabilized zwitterions are more numerous and have a lower activation energy for deprotonation compared with amine-stabilized zwitterions, leading to the formation of carbamate. Calculations also show that bicarbonate is disfavored in the system compared with hydronium carbamate, in accord with spectroscopic evidence. A reaction–diffusion model based on this new chemistry and quantitatively linked to quantum calculations reproduces the anomalous experimentally observed macroscopic behavior of the sorbents in a quantitative fashion.

Oxidative Cleavage of C=S and P=S Bonds at an AlI Center: Preparation of Terminally Bound Aluminum Sulfides.

Abstract: The treatment of cyclic thioureas with the aluminum(I) compound NacNacAl (1; NacNac=[ArNC(Me)CHC(Me)NAr]−, Ar=2,6-Pri2C6H3) resulted in oxidative cleavage of the C=S bond and the formation of 3 and 5, the first monomeric aluminum complexes with an Al=S double bond stabilized by N-heterocyclic carbenes. Compound 1 also reacted with triphenylphosphine sulfide in a similar manner, which resulted in cleavage of the P=S bond and production of the adduct [NacNacAl=S(S=PPh3)] (8). The Al=S double bond in 3 can react with phenyl isothiocyanate to furnish the cycloaddition product 9 and zwitterion 10 as a result of coupling between the liberated carbene and PhN=C=S. All novel complexes were characterized by multinuclear NMR spectroscopy, and the structures of 5, 9, and 10 were confirmed by X-ray diffraction analysis. The nature of the Al=S bond in 5 was also probed by DFT calculations.

Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity.

Abstract: The deployment of transformational nonaqueous CO2-capture solvent systems is encumbered by high viscosities even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we present key molecular features that control bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of internal hydrogen bonds between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecu...

Importance of Intermolecular Hydrogen Bonding for the Stereochemical Control of Allene–Enone (3+2) Annulations Catalyzed by a Bifunctional, Amino Acid Derived Phosphine Catalyst

Abstract: The origin of stereoselectivity in the (3+2) annulation of allenes and enones catalyzed by an amino acid derived phosphine catalyst has been investigated by the use of dispersion-corrected density functional theory. An intermolecular hydrogen bond between the intermediate zwitterion and the enone was found to be the key interaction in the two enantiomeric transition states. Additional stabilization is provided by intermolecular hydrogen-bonding interactions between acidic positions on the catalyst backbone and the substrate. Enantioselectivity occurs because the intermolecular hydrogen bond in the transition state leading to the minor enantiomer is only possible at the expense of reactant distortion.

N-Heterocyclic Olefin-Carbon Dioxide and -Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns.

Abstract: Depending on the amount of methanol present in solution, CO2 adducts of N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO2Me] and [EMMIm][OCO2Me]. The reactivity pattern of representative 1-ethyl-3-methyl-NHO–CO2 adduct 4 has been investigated and compared with the corresponding NHC–CO2 zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO2H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO2 zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]2 cyanomalonate salts formed exemplify the first C−C bond-forming carboxylation reactions with NHO-activated CO2. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO2Me][X]. The first NHO-SO2 adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.

Control of CO2 Adsorption and Desorption Using Polyethylene Glycol in a Tetraethylenepentamine Thin Film: An In Situ ATR and Theoretical Study

Abstract: The reversible adsorption of CO2 on tetraethylenepentamine (TEPA) and the polyethylene glycol (PEG)-modified amine sites were investigated using attenuated total reflection infrared (ATR-IR) spectroscopy, mass spectrometry (MS), and density functional theory (DFT). The presence of PEG at the amine site increased the rate of formation of adsorbed CO2, enhanced the formation of weakly adsorbed CO2 which can be removed from flowing inert gas at room temperature, and decreased CO2 desorption peak (i.e., sorbent regeneration) temperature. The calculated CO2 binding energy (BE) and optimized structures suggest CO2 adsorbed on TEPA primarily in the form of ammonium carbamate. The presence of PEG promoted the formation of a species which exhibited an experimental IR spectrum resembling the simulated spectrum of a low BE zwitterion species. The observation suggests PEG controlled the formation of the adsorbed intermediate species. Modeling of the transient CO2 adsorption profiles further showed PEG accelerated the...

The hydration properties of carboxybetaine zwitterion brushes

Abstract: Combined quantum mechanical calculations and classical molecular dynamics simulations were conducted to investigate the hydration properties of carboxybetaine zwitterion brushes with varying separation distances between the quaternary ammonium cation and carboxylic anion. The brushes consist of zwitterion trimers and are investigated to mimic interacting zwitterion chains grafted on a substrate as well as polymers with interacting zwitterion side chains. Our results show that the values of both positive and negative charges, their separation distances as well as chain interactions appear to play a critical role in the hydration properties of the zwitterions. The overall hydration property of these zwitterions is dictated by the competition between the strong hydration of the charged groups and the dehydration of the hydrocarbon chains. The strongest hydration occurs when the -CH2- unit in the hydrocarbon chain reaches 6-8 for these trimers. Further increase in the hydrocarbon chain length to 10-14 leads to significant and sudden dehydration of the trimers. The water structure and the water residence time surrounding the zwitterions also demonstrate substantial alteration at this length scale. This hydrophilic-to-hydrophobic transition is induced by the hydrophobic interactions of the trimer chains. Our hydration results could explain the observed trend of the superiority of the methylated carbohydrates and poly(ethylene glycol) as antifouling materials compared to corresponding hydroxyl-terminated compounds.

Bio-functionalized hybrid nanocomposite membranes for direct methanol fuel cells

Abstract: Bio-molecules are responsible for rapid proton conduction in biological systems. In view of this, hybrid nanocomposite membranes are prepared by incorporating amino acid functionalized titaninum dioxide (AA–TiO2) bio-hybrid nanoparticles into a polyvinylalcohol (PVA) matrix. Sequential binary cross-linking of PVA with sulfosuccinic acid (SSA) and glutaraldehyde (GA) provides adequate mechanical strength and thermal stability. TiO2 nanoparticles are functionalized with three different amino acids (AAs) namely glycine, phenyl alanine and lysine. AA functionalization to TiO2 improves the interaction and degree of adhesion between organic–inorganic phases leading to a homogenous dispersion when used as an additive in PVA. Interfacial interactions between AA–TiO2 and PVA are confirmed by Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), thermogravimetry (TG) and UV-Visible spectral analyses. Elemental mapping of the PVA–LY–TiO2 hybrid nanocomposite membrane confirms the incorporation of AA–TiO2 bio-hybrid nanoparticles within the PVA matrix. The suitability of these membranes as electrolytes for direct methanol fuel cells (DMFCs) has been demonstrated through assessment of proton conductivity and methanol permeability. The zwitterion (dipolar ion) architecture created by the AA–TiO2 bio-hybrid nanoparticles substantially facilitates proton conduction through acid–base pairs analogous to biological systems and simultaneously eases methanol permeation. The optimized membrane PVA–lysine–TiO2 is used as an electrolyte in a DMFC which delivers nearly a two-fold higher peak power density of 58 mW cm−2, compared to the pristine PVA membrane.

Photoacid behaviour in a fluorinated green fluorescent protein chromophore:Ultrafast formation of anion and zwitterion states

Abstract: The photophysics of the chromophore of the green fluorescent protein in Aequorea victoria (avGFP) are dominated by an excited state proton transfer reaction. In contrast the photophysics of the same chromophore in solution are dominated by radiationless decay, and photoacid behaviour is not observed. Here we show that modification of the pKa of the chromophore by fluorination leads to an excited state proton transfer on an extremely fast (50 fs) time scale. Such a fast rate suggests a barrierless proton transfer and the existence of a pre-formed acceptor site in the aqueous solution, which is supported by solvent and deuterium isotope effects. In addition, at lower pH, photochemical formation of the elusive zwitterion of the GFP chromophore is observed by means of an equally fast excited state proton transfer from the cation. The significance of these results for understanding and modifying the properties of fluorescent proteins are discussed.

Highly efficient polymer solar cells using a non-conjugated small-molecule zwitterion with enhancement of electron transfer and collection

Abstract: A novel non-conjugated small-molecule zwitterion is developed as a cathode interfacial material to enhance the electron transfer and collection properties of high-performance PSCs. The devices show significantly increased performance with power conversion efficiencies up to 9.51%. It is noteworthy that the results here provide significant scientific insights into further improvement of interfacial modification and performance of polymer solar cells.

Positron Binding Properties of Glycine and Its Aqueous Complexes

Abstract: We investigate positron binding to glycine and its aqueous complexes by first-principles calculation. We show that while glycine in its ground state (Gly) does not bind positrons, several of its strongly polar conformers do, and in particular, its zwitterion form (GlyZI) binds positrons strongly. Aqueous complexes Gly·nH2O and GlyZI·nH2O also bind positrons, if their dipole moment μ > μcr. However, μ is not a sufficient quantity to describe positron binding to these complexes. We show that in addition to μ, positron binding strongly depends on the intramolecular bonding of glycine. In Gly·nH2O, positrons are weakly bound to the nitrogen in Gly, whereas in GlyZI·nH2O, the ionic oxygen in GlyZI is a strong "positron attractor".

Bipyridinium-bis(carboxylate) Radical Based Materials: X-ray, EPR and Paramagnetic Solid-State NMR Investigations

Abstract: The zwitterionic 1,1′-bis(4-carboxyphenyl)-4,4′-bipyridinium (bp4pc) has been synthesized and crystals of its hydrated form bp4pc·2H2O and of its protonated reduced form H-bp4pc have been obtained. Upon heating, bp4pc·2H2O undergoes partial dehydration, leading to bp4pc·H2O at 160 °C, together with a color change from yellow (room temperature) to green (140 °C) and finally to brown (160–180 °C). Analysis of bond lengths in the solid state reveals the expected short (d = 1.425 A) and long (d = 1.485 A) C–C central bond lengths in the all-radical salt H-bp4pc and bp4pc·2H2O, respectively, whereas the distance of 1.475 A in bp4pc·H2O does not allow a conclusion to be drawn regarding the presence of radicals in this compound. EPR and solid-state paramagnetic NMR experiments of H-bp4pc and the hydrated zwitterion bp4pc·2H2O at different temperatures, however, show that the color change of the latter upon heating is due to the presence of bipyridinium radicals, the concentration of which, although low, increases with increasing temperature. The nature of the electron donor involved in this thermal-induced electron transfer is not fully understood. Most plausible is the possibility that it is the carboxylate group with an intramolecular electron-transfer process; on the other hand it, cannot be excluded that the electron stems from the water molecule, which decomposes into O2, H+, and e– giving H-bp4pc entities.

A comparison of the experimental and theoretical charge density distributions in two polymorphic modifications of piroxicam

Abstract: Experimental charge density distribution studies of two polymorphic forms of piroxicam, β-piroxicam (1) and piroxicam monohydrate (2), were carried out via high-resolution single crystal X-ray diffraction experiments and multipole refinement. The asymmetric unit of (2) consists of two discrete piroxicam molecules, (2a) and (2b), and two water molecules. Geometry differs between (1) and (2) due to the zwitterionic nature of (2) which results in the rotation of the pyridine ring around the C(10)–N(2) bond by approximately 180°. Consequently, the pyridine and amide are no longer co-planar and (2) forms two exclusive, strong hydrogen bonds, H(3)⋯O(4) and H(2)⋯O(3), with bond energies of 66.14 kJ mol−1 and 112.82 kJ mol−1 for (2a), and 58.35 kJ mol−1 and 159.51 kJ mol−1 for (2b), respectively. Proton transfer between O(3) and N(3) in (2) results in significant differences in surface electrostatic potentials. This is clarified by the calculation of atomic charges in the zwitterion that shows the formally positive charge of the pyridyl nitrogen which is redistributed over the whole of the pyridine ring instead of concentrating at N–H. Similarly, the negative charge of the oxygen is distributed across the benzothiazine carboxamide moiety. The multipole derived lattice energy for (1) is −304 kJ mol−1 and that for (2) is −571 kJ mol−1, which is in agreement with the experimentally determined observations of higher solubility and dissolution rates of (1) compared to (2).

Preparation method and application of zwitterion water gel dressing

Abstract: The invention relates to a preparation method and application of zwitterion water gel dressing. According to the preparation method, zwitterion monomers, cross linking agents, initiators, sodium chloride and distilled water are uniformly mixed according to the ingredient content; a glass slide is used for preparing templates; a polyfluortetraethylene pad is clamped between two glass slides, and a gap is then formed there; gauze is cut into the proper size to be clamped between the glass slides; then, a uniformly mixed reaction solution is dripped into the template gap; through crosslinking reaction, the zwitterion water gel modified gauze dressing can be obtained. Identically, during the preparation of a zwitterion water gel woundplast, the zwitterion water gel modified woundplast dressing can be obtained through the crosslinking reaction. The water gel dressing provided by the invention has the advantages that the water content is high, so that the wound is in a moist environment; meanwhile, the water absorption capability is high, and the secreted body fluid can be absorbed at any time, so that no excessive pyema exists in the wound position; high biocompatibility is realized; the problems of skin allergy inflammation and the like cannot be caused; the fast heeling of the wound can be promoted.

Reaction dynamics of $$\hbox {CO}_2$$ CO 2 in aqueous amines from ab initio molecular dynamics: 2-amino-2-methyl-1,3-propanediol (AMPD) compared to monoethanolamine (MEA)

Abstract: We present Car–Parrinello molecular dynamics simulations—combined with metadynamics—of the reactions accompanying the capture of $$\hbox {CO}_2$$ in an aqueous amine solution. The selected amine is 2-amino-2-methyl-1,3-propanediol, which has been investigated experimentally in the search for optimum absorbents. In analogy with the empirical search, we use as reference aqueous monoethanolamine (MEA) at 30 wt%, namely the chemical absorbent most frequently used for the removal of $$\hbox {CO}_2$$ from combustion gases. In particular, we refer to our own results which have led to a detailed characterization of several reaction paths for the absorption of $$\hbox {CO}_2$$ and the amine regeneration (Ma et al. in J Chem Theory Comput 11:3189, 2015). Our simulations refer to the zwitterion mechanism leading to the formation of a carbamate or carbamic acid and to the release of the molecule in solution. The scenario established for the reactions in MEA is confirmed, and in particular the role of the zwitterion mechanism also for $$\hbox {CO}_2$$ release. We find that the difference in the structure of the two molecules does not influence the dynamics of either the formation or the dissociation of the zwitterion. However, it affects its interaction with water in a significant way, thus reducing the probability of carbamate formation and its stability in solution.

Induction of an Infinite Periodic Minimal Surface by Endowing An Amphiphilic Zwitterion with Halogen-Bond Ability.

Abstract: We have designed an amphiphilic zwitterion with an iodine-substituted imidazolium cation. Although it forms a layered assembly with flat interfaces, the addition of an equimolar amount of bis(trifluoromethane)sulfonimide results in the formation of a bicontinuous cubic liquid-crystalline assembly with a primitive-type infinite periodic minimal surface, where its zwitterionic headgroup sits regularly. IR measurements revealed that halogen bond between the iodine atoms on the imidazolium cation and the anions is involved in its molecular-assembly behavior. The present results clearly indicate the potential utility of halogen bonding to control the dimensionality and continuity of the ionic/nonionic interface of amphiphiles in bulk and consequent mesophase patterns, which may be a significant new molecular technology for precisely arranging functional molecules on a 3D continuous interfaces.

A zwitterionic triphosphenium compound as a tunable multifunctional donor

Abstract: The preparation of a novel triphosphenium zwitterion featuring di-, tri-, and tetra-coordinate phosphorus centres derived from a 1,2,4-tris-(diphenylphoshinyl)cyclopentadienyl framework is described. The reactivity of this potentially multidentate donor with protons, elemental sulfur and gold(I) chloride is examined, and the preferential reactivity of the pendant phosphine group over the P(I) centre is rationalized on the basis of density functional theory investigations.