Stability Of Anions Research Articles

Role of Amino Anion in Metal Amides/Imides for Hydrogen Storage: A First Principle Study

For favorable thermodynamics and the considerable kinetics of the hydrogen reaction, Li−Mg−N−H compounds hold promise as novel hydrogen storage materials for on-board usage in metal amide hydrogen storage systems. To improve their performance, much effort has been devoted to the fundamental properties of metal amides, such as electronic structure, and the energetics of their hydrogen reactions. Though understanding the amino anion transition in the hydrogen reaction is essential for further development, the role of the amino anion in metal amides/imides for hydrogen storage is still ambiguous. In this study, we investigated the electronic structures and chemical bonds of Mg(NH2)2, LiNH2, and Li2MgN2H2 by way of a first principle approach. Then, the H vacancy formation energies in LiNH2 and Li2MgN2H2 were estimated to comprehend the stability of the amino anion in metal amides/imides and its effect on the thermodynamics of the hydrogen reaction. Also, the transformation between the highest occupied molecul...

Electrogenerated Chemiluminescence of 9,10-Diphenylanthracene, Rubrene, and Anthracene in Fluorinated Aromatic Solvents

The electrogenerated chemiluminescence (ECL) of 9,10-diphenylanthracene (DPA), rubrene, and anthracene has been studied in fluorinated aromatic solvents. Mixed annihilation ECL between aromatic luminophores and quinones was observed in solvent systems containing acetonitrile and either benzene, benzotrifluoride, 3-fluorobenzotrifluoride, or 1,3-bis(trifluoromethyl)benzene. Increases in ECL efficiency (phi ecl, photons generated per redox event) correlated with decreasing solvent polarity when 1,4-benzoquinone was used as a nonemitting ECL partner. However, opposite results were observed using 1,4-naphthaoquinone (NQ) as a nonemitting partner. phi ecl also correlated with radical anion stability of NQ in these solvent systems, as indicated by reverse/forward current ratios ( I r/ I f), suggesting noncovalent interactions between the solvent and the nonemitting ECL partner. Specifically, the reaction of an aromatic luminophore with 1,4-naphthoquinone in acetonitrile/benzotrifluoride showed a 1.03-1.63-fold increases in ECL efficiency over that of acetonitrile/benzene. Slight blue shifts ( approximately 3 nm) in photoluminescence and ECL emissions were seen as solvent polarity increased. Reaction enthalpies of each system were estimated using half-wave potentials of oxidation and reduction and were found to correlate well with emission energy.

Reactivity of 13,13-Dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward Organolithiums: Remarkable Resistance to the DMS Rearrangement

Reactions of dibromocyclopropane 2a, containing two spiro-fused 1,3-dioxane rings, with MeLi gave only the methylation products 8 and 9 even at elevated temperatures. In contrast, the cyclohexane analogue 2b treated with MeLi underwent a smooth rearrangement to bicyclo[1.1.0]butane 11b at -78, -10, or +35 degrees C. Treatment of 2a with PhLi gave the alpha-Ph anion 13 as the only product, which underwent smooth methylation with MeI to give 14. Under the same conditions, 2b with PhLi gave bicyclo[1.1.0]butane 11b accompanied by bromophenyl derivative 8b. Treatment of either dibromide with t-BuLi gave a mixture of products including debrominated cyclopropanes 12. Experimental results were augmented with DFT calculations for salts 23 and MP2//DFT-level calculations for carbenes 22. They demonstrated a higher stability of the dioxane alpha-bromo anion with respect to alpha-elimination by 4.8 kcal/mol and also a lower tendency of the carbene 22a to undergo rearrangement by 4.0 kcal/mol than the cyclohexane analogues. These differences have been attributed to the inductive effect of the four oxygen atoms, which results in lower LUMO energy, the higher positive charge at the carbenic center, and the overall more electrophilic character of carbene 22a as compared to the cyclohexane derivative 22b. The rearrangement of carbenes 22 to the corresponding allenes 1, the thermodynamic products, requires a higher activation energy DeltaG(double dagger)(298) by 4.2 kcal/mol for dioxane and 6.4 kcal/mol for cyclohexane derivatives than for the formation of the bicyclo[1.1.0]butanes 11. The DeltaG(double dagger)(298) for intramolecular insertions to the CH bond is low and calculated as 6.0 kcal/mol for dioxane 22a and 2.0 kcal/mol for the formation of cyclohexane 22b.

A Computational Study on the Kinetic Stability of Cyclic Boryl Anions~!2007-12-13~!2008-05-13~!2008-06-11~!

A Computational Study on the Kinetic Stability of Cyclic Boryl Anions~!2007-12-13~!2008-05-13~!2008-06-11~!

Tetravalent Metal Complexation by Keggin and Lacunary Phosphomolybdate Anions

We report the synthesis, spectroscopic and structural characterization, and computational analysis of a series of phosphomolybdate complexes with tetravalent metal cations. The reaction between Ce (IV) and Th (IV) with phosphomolybdate at the optimum pH for the stabilization of the lacunary heteropolyoxometalate anion, [PMo 11O 39] (7-), results in the formation of compounds containing the anions [Ce(PMo 11O 39) 2] (10-) and [Th(PMo 11O 39) 2] (10-), respectively. Single crystal X-ray diffraction analysis was performed on salts of both species, Cs 10[Ce(PMo 11O 39) 2].20H 2O and (NH 4) 10[Th(PMo 11O 39) 2].22H 2O. In both anionic complexes the f-block metal cation is coordinated to the four unsaturated terminal lacunary site oxygens of each [PMo 11O 39] (7-) anion, yielding 8 coordinate sandwich complexes, analogous to previously prepared related complexes. Spectroscopic characterization points to the stability of these complexes in solution over a reasonably wide pH range. Density functional analysis suggests that the Ce-O bond strength in [Ce(PMo 11O 39) 2] (10-) is greater than the Th-O bond strength in [Th(PMo 11O 39) 2] (10-), with the dominant bonding interaction being ionic in both cases. In contrast, under similar reaction conditions, the dominant solid state Zr (IV) and Hf (IV) complexes formed contain the anions [Zr(PMo 12O 40)(PMo 11O 39)] (6-) and [Hf(PMo 12O 40)(PMo 11O 39)] (6-), respectively. In these complexes the central Group 4 d-block metal cations are coordinated to the four unsaturated terminal lacunary site oxygens of the [PMo 11O 39] (7-) ligand and to four bridging oxygens of a plenary Keggin anion, [PMo 12O 40] (3-). In addition, (NH 4) 5{Hf[PMo 12O 40][(NH 4)PMo 11O 39]}.23.5H 2O can be crystallized as a minor product. The structure of the anion, {Hf[PMo 12O 40][(NH 4)PMo 11O 39]} (5-), reveals coordination of the central Hf (IV) cation via four bridging oxygens on both the coordinated [PMo 11O 39] (7-) and [PMo 12O 40] (3-) anions. Unusually, the highly charged lacunary site remains uncoordinated to the Hf metal center but instead interacts with an ammonium cation. (31)P NMR indicates that complexation of the Keggin anion, [PMo 12O 40] (3-), to Hf (IV) and Zr (IV) will stabilize the Keggin anion to a much higher pH than usually observed.

Correlation of Hydrolysis and Desilylation of 2‐[(Trimethylsilyl)methyl]acrylate Derivatives in Aqueous Alkali Solutions

AbstractHydrolysis and desilylation reaction of 2‐[(trimethylsilyl)methyl]acrylate (=2‐[(trimethylsilyl)methyl]prop‐2‐enoate) derivatives were studied to evaluate the effect of the presence/absence of a further conjugating substituent (Schemes 3 and 4 and Tables 1 and 2). The substrates having a nonconjugating substituent at the acrylate moiety were stable to dilute alkali conditions, and afforded simple hydrolysis products under concentrated alkali conditions. In contrast, both hydrolysis and desilylation occurred from the substrates bearing conjugated substituents at the acrylate skeleton. The difference in reactivity can be explained in terms of the stabilization of the intermediate anion.

Exaltation effects in the metal electrodeposition from acid electrolytes

The exaltation of mass transfer of discharging cations, caused by the concurrent hydrogen evolution from acidified solutions, results in a sharp increase in the metal deposition rate. In this case the limiting process rate depends linearly on the hydrogen evolution current density; it depends but weakly on the solution agitation and temperature. Under the electrolysis of solutions containing weak acids as a supporting electrolyte, the higher the acid formation constant, the more pronounced is the dependence of the electrodeposition limiting rate on the hydrogen current density. When microelectrodes are used, the varying of the background acid nature may affect the hydrogen evolution rate markedly, while the metal electrodeposition rate mainly depends on the cell voltage. When metals are electrodeposited from complex anions, the parallel hydrogen evolution hinders the mass transfer; this process depends on the anion stability constant and charge, all other conditions being the same. The found peculiarities can be used in the inversion voltammetry for the lowering of the metal detection limit, improving of the analysis selectivity, and time saving.

A microscopic view of substitution reactions solvated by ionic liquids

The solvation effect of the ionic liquid 1-N-butyl-3-methylimidazolium hexafluorophosphate on nucleophilic substitution reactions of halides toward the aliphatic carbon of methyl p-nitrobenzenesulfonate (pNBS) was investigated by computer simulations. The calculations were performed by using a hybrid quantum-mechanical/molecular-mechanical (QM/MM) methodology. A semiempirical Hamiltonian was first parametrized on the basis of comparison with ab initio calculations for Cl(-) and Br(-) reaction with pNBS at gas phase. In condensed phase, free energy profiles were obtained for both reactions. The calculated reaction barriers are in agreement with experiment. The structure of species solvated by the ionic liquid was followed along the reaction progress from the reagents, through the transition state, to the final products. The simulations indicate that this substitution reaction in the ionic liquid is slower than in nonpolar molecular solvents proper to significant stabilization of the halide anion by the ionic liquid in comparison with the transition state with delocalized charge. Solute-solvent interactions in the first solvation shell contain several hydrogen bonds that are formed or broken in response to charge density variation along the reaction coordinate. The detailed structural analysis can be used to rationalize the design of new ionic liquids with tailored solvation properties.

Microhydration of NO3-: A Theoretical Study on Structure, Stability and IR Spectra

A systematic study on the structure and stability of nitrate anion hydrated clusters, NO3(-) x n H2O (n = 1-8) are carried out by applying first principle electronic structure methods. Several possible initial structures are considered for each size cluster to locate equilibrium geometry by applying a correlated hybrid density functional with 6-311++G(d,p) basis function. Three different types of arrangements, namely, symmetrical double hydrogen bonding, single hydrogen bonding and inter-water hydrogen bonding are obtained in these hydrated clusters. A structure having inter-water hydrogen bonding is more stable compared to other arrangements. Surface structures are predicted to be more stable over interior structures. Up to five solvent H2O molecules can stay around solute NO3(-) anion in structures having an inter-water hydrogen-bonded cyclic network. A linear correlation is obtained for weighted average solvent stabilization energy with the size (n) of the hydrated cluster. Distinctly different shifts of IR bands are observed in these hydrated clusters for different kinds of bonding environments of O-H and N=O stretching modes compared to isolated H2O and NO3(-) anion. Weighted average IR spectra are calculated on the basis of statistical population of individual configurations of each size cluster at 150 K.

Structure and stability of thioureate anions with water, DFT calculations

Structure and stability of thioureate anion with the water clusters CSNH 2NH −-(H 2O) n = 1–7 , are studied, using density functional theory. Molecular structures and the stability of the clusters are discussed based on the calculation results of the stable conformers and their relative energies. All the clusters are stable thermodynamically in gas phase with respect to separate monomers. The clusters are stabilized progressively with an increasing number of water molecules, as indicated by the increasing of the binding energies. The binding energies of CSNH 2NH − and a water molecule are 14.34 and 16.36 kcal/mol for cis CSNH 2NH − and trans CSNH 2NH −, respectively. As the reaction in aqueous solution progresses, the C S bond distance increases monotonically, indicating that the C S bond of the thioureate anion unit in the clusters is de-stabilized with an increasing number of water molecules.

Electrochemical oxidation and reduction of osmium and iridium complexes with fullerene C60

The electrochemical oxidation and reduction of a series of complexes of platinum-group metals (Os, Ir, and Rh) with fullerene C60 with common formula C60[MLn] (MLn is the metal and the metal-bonded ligands) in the tetrahydrofuran solution are studied using the method of cyclic voltammetry. The effect of metal and ligands on the variations in the redox properties of complexes and stability of anions, which are formed in the reduction, is considered. It is shown that all complexes studied are reduced more difficultly than free fullerene, and the reduced forms of metallofullerenes are less stable as compared with free fullerene anions.

Gas-Phase Observation of the Heterolytic Dissociation of Negative Ions into Counter Ions: Dissociation of [Cu phthalocyanine (SO 3) 4Na] 3−

Ion pair formation proceeding in solution at room-temperature, where it is driven by energies of solvation, has been observed in the gas-phase in the collision-induced dissociation of [Cu phthalocyanine (SO3)4Na]3- and [Cu phthalocyanine (SO3)4Na2]2- at low energies. The gas-phase observation of these charge separations into positive and negative ions, seemingly counter-intuitive, is favored sufficiently by thermodynamics to proceed at low collision energies. The combination of a high stability of the negative product anion against electron detachment and a low recombination energy of the departing positive counter ion appears to be the key requirement for such dissociations to proceed at low energies in the gas-phase.

Solvation of excess electrons trapped in charge pockets on hydrated molecular surfaces

AbstractWe have previously computed a set of hypothetical molecular surfaces, which formed charge pockets that were capable of excess electron entrapment. These charge pockets arose due to the fact that the molecular surfaces possessed an extended network of OH groups on one side of the surface and hydrogen atoms on the opposite side. The uneven distribution of the OH groups coupled to the partial positive charge of the hydrogen atoms caused electrons to be attracted to the surface. In the present investigation we will consider the ability of the hydrogen cyanide (HCN)‐water complex in stabilizing excess electrons on molecular surfaces. The computed vertical detachment energy (VDE) values are high, suggesting anion stability. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Dimensional scaling treatment of stability of atomic anions induced by superintense, high-frequency laser fields

We show that dimensional scaling, combined with the high-frequency Floquet theory, provides useful means to evaluate the stability of gas phase atomic anions in a superintense laser field. At the large-dimension limit (D-->infinity), in a suitably scaled space, electrons become localized along the polarization direction of the laser field. We find that calculations at large D are much simpler than D=3, yet yield similar results for the field strengths needed to bind an "extra" one or two electrons to H and He atoms. For both linearly and circularly polarized laser fields, the amplitude of quiver motion of the electrons correlates with the detachment energy. Despite large differences in scale, this correlation is qualitatively like that found between internuclear distances and dissociation energies of chemical bonds.

Synthesis, In Vitro Degradation, and Mechanical Properties of Two-Component Poly(Ester Urethane)Urea Scaffolds: Effects of Water and Polyol Composition

The development of minimally invasive therapeutics for orthopedic clinical conditions has substantial benefits, especially for osteoporotic fragility fractures and vertebral compression fractures. Poly(ester urethane)urea (PEUUR) foams are potentially useful for addressing these conditions because they cure in situ upon injection to form porous scaffolds. In this study, the effects of water concentration and polyester triol composition on the physicochemical, mechanical, and biological properties of PEUUR foams were investigated. A liquid resin (lysine diisocyanate) and hardener (poly(epsilon-caprolactone-co-glycolide-co-DL-lactide) triol, tertiary amine catalyst, anionic stabilizer, and fatty acid-derived pore opener) were mixed, and the resulting reactive liquid mixture was injected into a mold to harden. By varying the water content over the range of 0.5 to 2.75 parts per hundred parts polyol, materials with porosities ranging from 89.1 to 95.8 vol-% were prepared. Cells permeated the PEUUR foams after 21 days post-seeding, implying that the pores are open and interconnected. In vitro, the materials yielded non-cytotoxic decomposition products, and differences in the half-life of the polyester triol component translated to differences in the PEUUR foam degradation rates. We anticipate that PEUUR foams will present compelling opportunities for the design of new tissue-engineered scaffolds and delivery systems because of their favorable biological and physical properties.

Diameter Selection of Single-Walled Carbon Nanotubes through Programmable Solvation in Binary Sulfonic Acid Mixtures

Pristine carbon nanotubes (CNTs) dissolve as polycarbocations in superacids through direct protonation. The solvating power of a superacid is determined by the stability of the conjugate base anion that competes with the CNTs for the dissociated proton. We have demonstrated that this equilibrium can be controlled in a predictable fashion, thus rendering the solvating power of a superacid system tunable. In this article, we show that the solvating power of chlorosulfonic acid can be changed in a desired fashion by forming binary mixtures in different proportions with a non-superacid such as methane sulfonic acid. Thus, the successive extraction of carbon nanotubes with binary acid mixtures of increasing solvating power leads to the differentiation of CNTs by their molecular geometry. We show that solvation by direct protonation is sensitive to the geometric strain at the carbon atom and, hence, to the nanotube diameter. In this respect, the direct protonation method is distinct from surfactant-based or electrical-field-based methods that distinguish metallic CNTs from semiconducting types mainly on the existence of finite density of states or not at the Fermi level. We have employed solid-state Raman spectroscopic analysis of the CNT radial breathing modes and UV−vis absorption spectroscopy and a systematic mapping method to support our conclusions. We believe the concept demonstrated in this paper holds the potential to be developed into a chemical tool kit useful in the scaleable separation of CNTs by their (n, m) types, thus paving the way for molecular carbon nanotechnology.

Nanocluster Formation and Stabilization Fundamental Studies: Ranking the Nanocluster Stabilizing Ability of Halides

Following a brief introduction to the nanocluster stabilization literature and DLVO (Derjaugin-Landau-Verwey-Overbeek) theory of colloidal stability, F-, CI-, Br-, and I- are evaluated for their efficacy in the formation and stabilization of prototype Ir(O)n nanoclusters prepared from a [(1,5-COD)lr(CH3CN)2][BF4] precursor in both acetone and propylene carbonate solvent. First, under conditions utilized previously for establishing an anion stabilization series ("Standard Conditions," 1.2 mM Ir precursor concentration at 22 degrees C in acetone solvent), the 5 criteria developed in 2002 for ranking nanocluster stabilizers are evaluated for each halide (each with 1 equiv BF4 present from the Ir precursor). Under Standard Conditions, bulk metal is the final product (i.e., no stable nanoclusters) in the presence of each of the four halides, as well as for BF4 in the absence of any halide. Next, each halide, again in the presence of 1 equiv BF4, is evaluated under "Improved Conditions" (0.24 mM Ir precursor concentration at 60 degrees C in propylene carbonate solvent), propylene carbonate being known to be a preferred nanocluster solvent in the presence of anionic (electrostatic) stabilizers. Nanocluster syntheses under the Improved Conditions did, as expected, yield Ir(O)n nanoclusters for each of the four halide plus BF4 systems as well as BF4 alone, although none of these nanoclusters are isolable from solution. Importantly, even the traditionally weakly coordinating BF4 is shown to contribute significantly to nanocluster stability in the high dielectric constant solvent propylene carbonate. Hence, the importance of anions in conjunction with a high dielectric constant solvent for nanocluster formation and stabilization is illustrated.

A Short and Efficient Synthesis of (R,R)-2-Methylcyclopropanecarboxylic Acid

We report herein a short and efficient synthesis of (R,R)-2-methylcyclopropanecarboxylic acid via a Horner−Wadsworth−Emmons reaction involving commercially available (S)-propylene oxide and triethylphosphonoacetate (TEPA). The TEPA/base/propylene oxide stoichiometry was found critical to achieve high yields. We therefore studied the TEPA anion formation and stability using in situ IR spectroscopy. The reaction yield is strongly influenced by the counterion and solvent, whereas high diastereoselectivities are always obtained. Under the best experimental conditions (HexLi/MeTHF/150 °C), crude (R,R)-2-methylcyclopropanecarboxylic acid is obtained in 85−90% yield with >98% trans selectivity.

Industrial Yogurt Manufacture: Monitoring of Fermentation Process and Improvement of Final Product Quality

Lactic acid fermentation during the production of skim milk and whole fat set-style yogurt was continuously monitored by measuring pH. The modified Gompertz model was successfully applied to describe the pH decline and viscosity development during the fermentation process. The viscosity and incubation time data were also fitted to linear models against ln(pH). The investigation of the yogurt quality improvement practices included 2 different heat treatments (80°C for 30min and 95°C for 10min), 3 milk protein fortifying agents (skim milk powder, whey powder, and milk protein concentrate) added at 2.0%, and 4 hydrocolloids (κ-carrageenan, xanthan, guar gum, and pectin) added at 0.01% to whole fat and skim yogurts. Heat treatment significantly affected viscosity and acetaldehyde development without influencing incubation time and acidity. The addition of whey powder shortened the incubation time but had a detrimental effect on consistency, firmness, and overall acceptance of yogurts. On the other hand, addition of skim milk powder improved the textural quality and decreased the vulnerability of yogurts to syneresis. Anionic stabilizers (κ-carrageenan and pectin) had a poor effect on the texture and palatability of yogurts. However, neutral gums (xanthan and guar gum) improved texture and prevented the wheying-off defect. Skim milk yogurts exhibited longer incubation times and higher viscosities, whereas they were rated higher during sensory evaluation than whole fat yogurts.

Substituent Effects of the Alkyl Groups: Polarity vs. Polarizability

AbstractSubstituents effects of the alkyl groups, both straight‐chain and branched, were evaluated by means of model reaction series, each comparing the acidities and basicities of the same or very similar compounds. Four such models wereexamined, one based on known gas‐phase experimental data (acidity and basicity of alcohols), the three others on density‐functional calculations at the levels B3LYP/6‐311++G(2df,2pd)//B3LYP/6‐311++G(2df,2pd) or B3LYP/6‐311+G(d,p)//B3LYP/6‐311+G(d,p): acidity and basicity of 4‐substituted bicyclo[2.2.2]octan‐1‐ols, stability of borate anions compared to the stability of ammonium cations, and acidity of imines compared to the basicity of ketones. In all cases the alkyl groups stabilize both the anions and cations; with straight‐chain alkyls the effect is proportional and can be called polarizability effect. However, simple mathematical expression of the polarizability is not correct because the effect on the cations is always stronger than on the anions. Effects of secondary and tertiary alkyls, in some cases also of branched primary alkyls, are different and deviate from the proportionality. They can be interpreted in some cases as steric effects or hyperconjugation. In any case they are different from the effects of straight‐chain alkyls and they are also fundamentally different from the effects of dipolar substituents; both groups should not be correlated together in one reaction series.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)