An ab initio study of microsolvation of LiF in water: structures and properties of LiF-Wn,n = 1-9 complexes

Geometries of clusters of water molecules (W(n)) and those of the LiF-W(n) (n = 1-9) complexes were optimized using the B3LYP/6-31+G** method. Geometries of the complexes up to n = 7 were also optimized using the MP2/6-31+G** approach. Only one structure of each of W(n), n = 1-5 was considered to generate the complexes with LiF while two structures, one of a cage type and the other of a prism type, were considered for n = 6-9. The LiF-W(2) complex is found to be most stable among the various complexes. The LiF-W(6) complex, where W(6) is of a cage type, is predicted to be substantially less stable than that where W(6) is of a prism type. Certain existing ambiguities regarding the most stable structures of the LiF-W(n) (n = 1-3) complexes have been resolved. The LiF molecule seems to divide the W(n) clusters in the LiF-W(n) (n = 3-6) complexes into different fragments where at least one W(2)-like fragment is present. In LiF-W(6) (cage), there is one W(2)-like fragment while in LiF-W(6) (prism), there are three W(2)-like fragments. The LiF bond length is substantially increased in going from the gas phase to the different complexes, this increase being most prominent in LiF-W(6), where W(6) is of the cage or prism type. The LiF molecule, however, does not acquire the ionic structure Li(+)F(-) in any of the complexes studied here. An appreciable amount of electronic charge is transferred from LiF to the water molecules involved in the different complexes. In this process, the Li atom gains electronic charge in some cases, while the F atom considered separately, as well as the Li and F atoms taken together, lose the same in most cases.     

Kinetics and mechanism of osmium(VIII)-catalysed and uncatalysed oxidation of aminoalcohols by chloramine-T in alkaline medium

Summary The kinetics of osmium(VIII)-catalysed oxidation of aminoalcohols,viz., 2-aminoethanol, 3-aminopropanol, diethanolamine and triethanolamine, by chloramine-T in alkaline medium have been investigated. The reactions follow a first-order-rate dependence with respect to oxidant. The order in [aminoalcohol] decreases from unity at higher concentrations of aminoalcohols while that in [OH^–] is nearly –1. The oxidation rate is directly proportional to [osmium(VIII)] in primary amino-alcohols, while that is proportional to {k+k [osmium(VIII)]} (where k and k are rate constants) in diethanolamine and triethanolamine. The uncatalysed oxidation of diethanolamine and triethanolamine follows a first-order-rate dependence in each of the oxidant and the sustrate. Suitable mechanisms consistent with the observed kinetic results are proposed.     

Hydrogen effects on the catalytic activity and selectivity of the zeolites H?NaY and PtH?NaY

The decomposition of methylcyclohexane on H–NaY and PtH–NaY zeolites has been studied. It has been found that the presence or abscence of H₂ affects not only the activity of these catalysts but also their selectivity to dehydrogenation and isomerization products.

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H–NaY PtH–NaY. , H₂ , .

     

Kinetics and mechanism of the ruthenium(III)-catalysed oxidation of aminoalcohols by chloramine-T in perchloric acid

Summary The kinetics of the ruthenium(III) catalysed oxidation of 2-aminoethanol, 3-aminopropanol, diethanolamine and triethanolamine by chloramine-T in HClO₄ medium have been studied. The reactions exhibits first order rate dependence with respect to the oxidant and to the catalyst, and are zeroth order with respect to the substrate. The rate of oxidation is proportional to k[HClO₄]/{k+k'[HClO₄]} where k, k and k' are constants (a mixture of elementary rate constants and equilibrium constants). A suitable mechanism consistent with the observed kinetic data is proposed.     

Synthetic thermally reversible gel systems. VIII. Poly(vinyl chloride)–dioxane

When interpreted by network theory, equilibrium swelling measurements on poly-(vinyl chloride) (PVC) film in dioxane and moduli measurements on the equilibrium swelled films yield values for the approximate molecular weight between thermally reversible crosslinks and for the number of these crosslinks per polymer chain. These values are in reasonable agreement with the thermodynamic analysis of PVC–dioxane gels by Takahashi, Nakamura, and Kagawa and with the premise that three-dimensional network formation in these gels occurs by crystallization of a very limited number of syndiotactic sequences per chain having a sequence length of between 8 and 10. Failure to observe fusion endotherms by DTA on PVC–dibutyl phthalate gels supports the view that PVC gels have a low crystalline crosslink density and a low heat of crosslinking. The heat of crosslinking obtained by the method of Eldridge and Ferry shows only moderate agreement with expectations based on the heat of fusion of PVC and the number of repeating units per PVC chain passing through a crystalline crosslink in a PVC–dioxane gel.     

Chemistry of the phenylglyoxal-p-diethylaminoanil and/or thiourea substituted ammine complexes

Summary Mixed ligand complexes of chromium(III), cobalt(II), cobalt(III), copper(II) and zinc(II) involving either the phenylglyoxal-p-diethylaminoanil and/or thiourea, and ammonia have been obtained by the partial or complete replacement of the strongly coordinated ammonia of ammine complexes. All the products were characterized by elemental analysis, molar conductance, magnetic susceptibility and i.r. and electronic spectral measurements for their bonding and structures.     

Differential selectivity in electrochemical oxidation of ascorbic acid and hydrogen peroxide at the surface of functionalized ormosil-modified electrodes

Functionalized ormosil-modified electrodes have been developed for electroanalytical applications. The functionalized ormosil-modified electrodes are made by encapsulating potassium ferricyanide/potassium ferrocyanide within ormosil film derived from an optimum composition of 3-aminopropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl trimethoxysilane and phenyltrimethoxy silane in acidic medium in absence of Nafion/crown ether (system 1), in the presence of Nafion (system 2) and in the presence of dibenzo-18-crown-6 (system 3). Another modified electrode (system 4) is also developed using the reaction product of potassium ferricyanide, 3-aminopropyltrimethoxysilane and either tetrahydrofuran (THF) or cyclohexanone followed by ormosil formation in the presence of 2-(3,4-epoxycyclohexyl)ethyl trimethoxysilane and phenyltrimethoxy silane in acidic medium. The electrochemical oxidation of hydrogen peroxide and ascorbic acid conducted at the surface of these four types of functionalized electrodes shows very interesting observations on the selective sensing of ascorbic acid and peroxide. The results based on cyclic voltammetry justify the relative performances on the kinetics of hydrogen peroxide oxidation and reduction. System 3 shows relatively much better oxidation kinetics of hydrogen peroxide as compared to other three systems with relatively weak reduction kinetics whereas system 4 shows relatively faster reduction kinetics of hydrogen peroxide as compared to other three systems. Similarly system 4 shows excellent response to ascorbic acid whereas system 3 shows insensitivity to ascorbic acid under similar experimental conditions. Typical response curve for the analysis of hydrogen peroxide and ascorbic acid using system 3 and system 4 respectively are reported. The results show that system 3 is the best for probing hydrogen peroxide with lowest detection limit of 0.5 μM without any interference from ascorbic acid as commonly encountered using many conventional and chemically modified electrodes.     

Synthetic thermally reversible gel systems. VI

Forming and conditioning thermally reversible aqueous gels of polyacrylyglycinamide (PAG) at various temperatures has little effect on either the melting point (T_m) of the gels or the heat of crosslinking (ΔH_c) except at temperatures where partial hydrolysis can occur. This is added evidence that unlike with gelatin, crystallite formation does not play a role in gel formation. For unfractioned PAG, the linear relationship between the logarithm of molecular weight and 1/T_m predicted and observed for gelatin gels, does not hold. With mixed gelatin-PAG gels, a gelatin/PAG ratio of ≥4 completely inhibits the formation of a PAG gel network. At lower gelatin/PAG ratios, the PAG network forms, and if gelatin is considered as an inert diluent, normal values for the melting points and ΔH_c for PAG gels are observed. At a gelatin/PAG ratio of 4, the presence of PAG reduces the ΔH_c for the gelatin gel by inhibiting the formation of as large or as ordered crystallite crosslinks. To reconcile the problem of aggregation preceding gelation one can assume that M?_w of an aggregate is a linear function of C². If this is done, the same relationship which normally relates C with T_m is obtained. The equilibrium swelling of PAG films in water at 25°C is markedly molecular weight-de-pendent and can vary from below 5 to about 40 wt-% polymer at equilibrium. It has been found that long-term dark storage of dry samples of PAG under ambient temperature conditions results in pronounced decreases in the intrinsic viscosities of their aqueous solutions. It is speculated that this results from weak links, perhaps peroxy, in the polymer backbone. The possible relationship of this phenomenon to the slow stage of the viscosity deterioration of aqueous polyacrylamide solutions is pointed out. The higher viscosity of low DP PAG in 2M NaCNS compared to H₂O and the larger percentage increse of [η] with increasing temperature in the latter, verify the greater solvent power of 2M aqueous thiocyanate for PAG. At a concentration level of 3%, aqueous PAG solutions are almost Newtonian whereas at higher concentrations (5%), the viscosity decreases appreciably with increasing rates of shear. The copolymerization of AG with isopropylacrylamide has been studied and the somewhat unusual results discussed. Copolymers containing an AG mole fraction greater than 0.40 do not exhibit a cloud point up to 100°C. If the isopropylacrylamide mole fraction approaches 0.60, the solutions do not gel down to 0°C. This ability to prepare copolymers over a narrow composition range that neither gel or undergo phase separation in the temperature range 0–100°C is probably related to the random distribution of monomer units in the copolymer backbone.     

Hydrotalcite catalysis in ionic liquid medium: a recyclable reaction system for heterogeneous Knoevenagel and nitroaldol condensation

Knoevenagel condensation proceeds efficiently in recyclable [bmim]PF₆ and [bmim]BF₄ without any catalyst, and hydrotalcites in ionic liquid serve as a safe and recyclable reaction system for both Knoevenagel as well as nitroaldol condensations.