Fractional power dependence of rate on activation energy for reactions with very low internal barriers

The dynamics of reactions with low internal barriers are studied both analytically and numerically for two different models. Exact expressions for the average rate,kI, are obtained by solving the associated first passage time problems. Both the average rate constant, kI, and the numerically calculated long-time rate constant, kL, show a fractional power law dependence on the barrier height for very low barriers. The crossover of the reaction dynamics from low to high barrier is investigated.

Mordenite-incorporated PVA-PSSA membranes as electrolytes for DMFCs

Composite membranes with mordenite (MOR) incorporated in poly vinyl alcohol (PVA)–polystyrene sulfonic acid (PSSA) blend tailored with varying degree of sulfonation are reported. Such a membrane comprises a dispersed phase of mordenite and a continuous phase of the polymer that help tuning the flow of methanol and water across it. The membranes on prolonged testing in a direct methanol fuel cell (DMFC) exhibit mitigated methanol cross-over from anode to the cathode. The membranes have been tested for their sorption behaviour, ion-exchange capacity, electrochemical selectivity and mechanical strength as also characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. Water release kinetics has been measured by magnetic resonance imaging (NMR imaging) and is found to be in agreement with the sorption data. Similarly, methanol release kinetics studied by volume-localized NMR spectroscopy (point resolved spectroscopy, PRESS) clearly demonstrates that the dispersion of mordenite in PVA–PSSA retards the methanol release kinetics considerably. A peak power-density of 74 mW/cm2 is achieved for the DMFC using a PVA–PSSA membrane electrolyte with 50% degree of sulfonation and 10 wt.% dispersed mordenite phase. A methanol cross-over current as low as 7.5 mA/cm2 with 2 M methanol feed at the DMFC anode is observed while using the optimized composite membrane as electrolyte in the DMFC, which is about 60% and 46% lower than Nafion-117 and PVA–PSSA membranes, respectively, when tested under identical conditions.

Self-assembly of three new coordination complexes: Formation of 2-D square grid, 1-D chain and tape structures

Three distinct coordination complexes, viz., [Co(imi)(2)(tmb)(2)] (1) [where imi = imidazole], {[Ni(tmb)(2)(H2O)(3)]center dot 2H(2)O}(n) (2) and [Cu-2(mu-tmb)(4)(CH3OH)(2)] (3), have been synthesized hydrothermally by the reactions of metal acetates,2,4,6-trimethylbenzoic acid (Htmb) and with or without appropriate amine. The Ni analogue of 1 and the Co analogue of 2 have also been synthesized. X-ray single-crystal diffraction suggests that complex 1 represents discrete mononuclear species and complex 2 represents a 1D chain coordination polymer in which the Ni(H) ions are connected by the bridging water molecules. Complex 3 represents a neutral dinuclear complex. In 1, the central metal ions are associated by the carboxylate moiety and imidazole ligands, whereas the central metal atom is coordinated to the carboxylate moiety and the respective solvent molecules in 2 and 3. In 3, the four 2,4,6-trimethylbenzoate moieties act as a bridge connecting two copper (11) ions and the 0 atoms of methanol coord geometry, with the methanol molecule at the apical position. In all the three structures the central metal atom sits on a crystallographic inversion centre. In all the cases, the coordination entities are further organized via hydrogen bonding interactions to generate multifarious supramolecular networks. Complexes 1, 2 and 3 have also been characterized by spectroscopic (UV/Vis and IR) and thermal analysis (TGA). In addition, the complexes were found to exhibit antimicrobial activity. The magnetic susceptibility measurements, measured from 8 to 300 K, revealed antiferromagnetic interactions between the Co(II) ions in compound 1 and the Ni(II) ions in la, respectively.

Coexistence curve of acetonitrile and cyclohexane liquid system

The paper reports a detailed determination of the coexistence curve for the binary liquid system acetonitrile+cyclohexane, which have very closely matched densities and the data points get affected by gravity only for t=(Tc−T)/ Tc[approximately-equal-to]10−6. About 100 samples were measured over the range 10−6crossover expansion. The use of the new variables proposed by Johnston et al. and Vnuk in making the coexistence curve appear symmetric in the new variables is pointed out. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.

Hydrophobic channels in crystals of an alpha-aminoisobutyric acid pentapeptide

The crystal structure of the pentapeptide p-toluene-sulfonyl-(α-aminoisobutyryl)5-methyl ester (Tosyl-(Aib)5-OMe) has been determined in the space group PImage . Pentapeptide molecules are folded in the 310 helical conformation and packed together, so as to yield a hydrophobic channel with a minimim diameter of 5.2 �. The channel contains crystallographically disordered material. This structure provides a model for channel formation by hydrophobic peptide aggregates and should prove useful in studies of alamethicin, suzukacillin and related Aib containing membrane channels. Triclinic (PImage ) crystals of the pentapeptide are obtained in the presence of LiClO4 in aqueous methanol, whereas crystallization from methanol alone yields crystals in the space group Pbca. The conformations of the peptide in the two crystal forms are very similar and only the molecular packing is dramatically different.

Phase transitions in (NH4)2SO4 and (NH4)2SO4---K2SO4 mixed crystals as studied by EPR of CrO3- radicals

A study of the phase transitions in (NH4)2SO4 and (NH4)2SO4---K2SO4 mixed crystals by EPR of the CrO3- ion is reported. The results indicate a bilinear coupling of the order parameter with spontaneous polarization and a crossover from a discontinuous to a continuous nature of the phase transition in mixed crystals.

Photo-Electron Spectroscopic Studies Of The Adsorption Of Organic-Molecules With Lone Pair Orbitals On Transition-Metal Surfaces

Ultraviolet and x-ray photoelectron spectroscopy have been employed to investigate the adsorption of methanol, ethanol, diethylether, acetaldehyde, acetone, methyl acetate and methylamine on surfaces of Fe, Ni and Cu. All these molecules adsorb molecularly at low temperatures (≤100 K). Lone pair orbitals of these molecules are stabilized on these metal surfaces (by 0·4–1·0eV) due to molecular chemisorption. The molecules generally undergo transformations as the temperature is raised to 120 K or above. The new species produced seems to depend on the metal surface. Some of the product species identified are methoxy species, formaldehyde and carbon monoxide in the case of methanol and methyl acetate, ethoxy species in the case of ethanol and 2-propanol in the case of acetone.

Resistivity noise in crystalline magnetic nanowires and its implications to domain formation and kinetics

We have investigated the time-dependent fluctuations in electrical resistance, or noise, in high quality crystalline magnetic nanowires within nanoporous templates. The noise increases exponentially with increasing temperature and magnetic field, and has been analyzed in terms of domain wall depinning within the Neel-Brown framework. The frequency-dependence of noise also indicates a crossover from nondiffusive kinetics to long-range diffusion at higher temperatures, as well as a strong collective depinning, which need to be considered when implementing these nanowires in magnetoelectronic devices.

Vapor phase oxidation of ethanol over thorium molybdate catalyst

Ethanol oxidation in the vapor phase was studied in an isothermal flow reactor using thorium molybdate catalyst in the temperature range 220–280 °C. Under these conditions the catalyst was highly selective to acetaldehyde formation. The rate data were well represented by a steady state two-stage redox model given by the equation: View the MathML source The parameters of the above model were estimated by linear and nonlinear least squares methods. In the case of nonlinear estimation the sum of the squares of residuals decreased. The activation energies and preexponential factors for the reduction and oxidation steps of the model, estimated by nonlinear least squares technique are: 9.47 kcal/mole, 9.31 g mole/ (sec) (g cat) (atm) and 9.85 kcal/mole, 0.17 g mole/(sec) (g cat) (atm)0.5, respectively. Oxidations of ethanol and methanol over thorium molybdate catalyst were compared under similar conditions.

The influence of esterification of carboxyl groups of ribonuclease-a on its structure and immunological activity

The effect of modification of carboxyl groups of Ribonuclease-Aa on the enzymatic activity and the antigenic structure of the protein has been studied. Modification of four of the eleven free carboxyl groups of the protein by esterification in anhydrous methanol/0.1 M hydrochloric acid resulted in nearly 80% loss in enzymatic activity but had very little influence on the antigenic structure of the protein. Further increases in the modification of the carboxyl groups caused a progressive loss in immunological activity, and the fully methylated RNase-A exhibited nearly 30% immunological activity. Concomitant with this change in the antigenic structure of the protein, the ability of the molecule to complement with RNase-S-protein increased, clearly indicating the unfolding of the peptide "tail" from the remainder of the molecule. The susceptibility to proteolysis, accessibility of methionine residues for orthobenzoquinone reaction and the loss in immunological activity of the more extensively esterified derivatives of RNase-A are suggestive of the more flexible conformation of these derivatives as compared with the compact native conformation. The fact that even the fully methylated RNase-A retains nearly 30% of its immunological activity suggested that the modified protein contained antibody recognizable residual native structure, which presumably accommodates some antigenic determinants.

An NAD(P)(+)-dependent secondary-alcohol dehydrogenase of Alcaligenes eutrophus: Purification, characterization and its application for the production of chiral alcohols

A soil micro-organism identified as Alcaligenes eutrophus capable of utilizing nerolidol, a sesquiterpene alcohol as the sole source of carbon, contains an inducible NAD(P)(+)-linked secondary-alcohol dehydrogenase (SADH), The enzyme was purified 252-fold from crude cell-free extract by a combination of salt precipitation, ion-exchange and affinity-matrix chromatography, Native and SDS/PAGE PAGE of the purified enzyme showed a single protein band and the enzyme appears to be a homotetramer having an apparent molecular mass of 139 kDa comprising four identical subunits of 38.5 kDa, The isoelectric point (pi) of SADH was determined to be 6.2, Depending on pH of the reaction media, the enzyme carried out both oxidation and reductions of various terpenoids and steroids, At pH 5.5, the enzyme catalysed the stereospecific reduction of prochiral ketones to optically active (S)-alcohols and the oxidation reaction was predominated over the former at pH 9.5, NADP(+) and NADPH were respectively preferred over NAD(+) and NADH for oxidation and reduction reactions, The K-m values for testosterone, NADP(+) and NAD(+) were 11.8, 55.6, and 122 mu M respectively, Neither enzyme was significantly inhibited by metal-binding agents, but some thiol-blocking compounds inhibited it, SADH tolerates moderate concentrations of water-miscible organic solvents such as ethanol, methanol, acetone and dioxan, Some of the properties of this enzyme were found to be significantly different from those thus far described.

Palladium(II) and platinum(II) complexes of beta-functionalized ethyl selenolates: Effect of substitution on synthesis, reactivity, spectroscopy, structures and thermal behavior

Sodium ethylselenolates with functional groups X (where X = -OH, -COOH, -COOMe and -COOEt) at beta-carbon were prepared in situ by reductive cleavage of corresponding diselenide with NaBH4 either in methanol or aqueous ammonia. Treatment of these selenolates with [M2Cl2(mu-Cl)(2)(PR'(3))(2)] (M = Pd or Pt; PR'(3) = PMePh2, PnPr(3)) in different stoichiometry yielded various bi- and tri-nuclear complexes. The homoleptic hexanuclear complexes [Pd(mu-SeCH2CH2X)(2)](6) (X = OH, COOH, COOEt), were obtained by reacting Na2PdCl4 with NaSeCH2CH2X. All these complexes have been fully characterized. Molecular structures of ethylselenolates containing hydroxyl and carboxylic acid groups revealed solid state associated structures through inter-molecular hydrogen bond interactions. Trinuclear complex, [Pd3Cl2(mu-SeCH2CH2COOH)(4)(PnPr(3))(2)] (3a), was disposed in a boat form unlike chair conformation observed for the corresponding methylester complex. The effect of beta-functionality in ethylselenolate ligands towards reactivity, structures and thermal properties of palladium and platinum complexes has been extensively Studied.

A new mixed-matrix membrane for DMFCs

A new mixed-matrix membrane based on stabilized phosphotungstic acid (PTA) incorporated to chitosan (CS)-hydroxy ethyl cellulose (HEC) for application in direct methanol fuel cells (DMFCs) is reported. Membranes are characterised using Fourier Transform Spectroscopy (FTIR), Thermo-Gravimetric Analysis (TGA), Scanning Electron Microscopy (SEM) and their mechanical properties are evaluated. The PTA content in the CS-HEC blend and its influence on proton conductivity, water/methanol sorption, and methanol cross-over in the DMFC is studied. The DMFC with 3 wt. % stabilized PTA-CS-HEC mixed-matrix membrane delivers peak power-density of 58 mW/cm(2) at a load current-density of 210 mA/cm(2) with a lower methanol cross-over than that observed for a DMFC operating with a Nafion membrane electrolyte.

Formation of an unusual copper(II) complex from the degradation of a novel tricopper(II) carbohydrazone complex

An unusual copper(II) complex [Cu(L-1a)(2)Cl-2] CH3OH center dot H2O center dot H3O+Cl- (1a) was isolated from a solution of a novel tricopper(II) complex [Cu-3(HL1)Cl-2]Cl-3 center dot 2H(2)O (1) in methanol. where L-1a is 3-(2-pyridyl)triazolo [1,5-a]-pyridine, and characterized with single crystal X-ray diffraction study. The tricopper(II) complex of potential ligand 1,5-bis(di-2-pyridyl ketone) carbohydrazone (H2L1) was synthesized and physicochemically characterized, while the formation of the complex la was followed by time-dependant monitoring of the UV-visible spectra. which reveals degradation of ligand backbone as intensity loss of bands corresponding to O -> Cu(II) charge transfer.

Structure and phase transition of the (1-x)PbTiO3-(x)BiAlO3 system

The system (1 - x)PbTiO3 - (x)BiAlO3 has been studied with regard to its structure and phase transition behaviour using x-ray, neutron and dielectric measurements. The structure is tetragonal within the solid solubility limit (x < 0.25). Interatomic distance analysis revealed that the Pb-O bond lengths remain unaffected and the only the Ti-O lengths are altered by BiAlO3 substitution. The results are suggestive of a crossover from and A-site & B site driven ferroelectric system for x = 0 to a dominant A-site driven ferroelectric system for higher x. This cross-over is brought about by (i) a reduction in the contribution to the ferroelectric stability from B-site cations due to dilution of the Ti-sublattice by Al and (ii) a reinforcement to the stability of the ferroelectric state by the A-site cations by the Bi+3 cations..