Boron–π interactions in two 3-(dihydroxyboryl)anilinium salts analyzed by crystallographic studies and supported by the noncovalent interactions (NCI) index theoretical approach

In the title compounds, 3-(dihydroxyboryl)anilinium bisulfate monohydrate, C₆H₉BNO₂⁺·HSO₄⁻·H₂O ( I ), and 3-(dihydroxyboryl)anilinium methyl sulfate, C₆H₉BNO₂⁺·CH₃SO₄⁻ ( II ), the almost planar boronic acid molecules are linked by pairs of O—H…O hydrogen bonds, forming centrosymmetric motifs that can be described by the graph-set R₂²(8) motif. In both crystals, the B(OH)₂ group acquires a syn–anti conformation (with respect to the H atoms). The presence of the hydrogen-bonding functional groups B(OH)₂, NH₃⁺, HSO₄⁻, CH₃SO₄⁻ and H₂O generates three-dimensional hydrogen-bonded networks, in which the bisulfate (HSO₄⁻) and methyl sulfate (CH₃SO₄⁻) counter-ions act as the central building blocks within the crystal structures. Furthermore, in both structures, the packing is stabilized by weak boron–π interactions, as shown by noncovalent interactions (NCI) index calculations.     

Viscosities for Ionic Liquid Binary Mixtures with a Common Ion

At present, there is a considerable amount of work devoted to the study of the thermophysical properties of pure ionic liquids, which contrasts with the few data available for their mixtures. One of the most appealing characteristics of ionic liquids is the capability of subtly changing the chemical structure of the cation and anion in order to design appropriate solvents for specific applications. Mixtures of ionic liquids increase enormously this specificity, due to the unlimited combinations that arise from mixing two or more ionic liquids. In this context, the study of the thermophysical properties of these mixtures is revealed as a fundamental task. In this work the viscosities of the ionic liquid binary mixtures with a common ion ([C₆mim] + [C₂mim])[BF₄], ([C₆mim] + [C₄mim])[BF₄], [C₄mim]([BF₄] + [MeSO₄]) and [C₄mim]([PF₆] + [BF₄]) were determined within the temperature range (298.15–308.15) K. The temperature dependence of the viscosity for pure liquids is analyzed by means of the Vogel-Tammann-Fulcher equation and several mixing rules are applied for the mixtures.     

New calibration methodology for calorimetric determination of isobaric thermal expansivity of liquids as a function of temperature and pressure

A new method for determining isobaric thermal expansivity of liquids as a function of temperature and pressure through calorimetric measurements against pressure is described. It is based on a previously reported measurement technique, but due to the different kind of calorimeter and experimental set up, a new calibration procedure was developed. Two isobaric thermal expansivity standards are needed; in this work, with a view on the quality of the available literature data, hexane and water are chosen. The measurements were carried out in the temperature and pressure intervals (278.15 to 348.15) K and (0.5 to 55) MPa for a set of liquids, and experimental values are compared with the available literature data in order to evaluate the precision of the experimental procedure. The analysis of the results reveals that the proposed methodology is highly accurate for isobaric thermal expansivity determination, and it allows obtaining a precise characterisation of the temperature and pressure dependence of this thermodynamic coefficient.     

Heat capacity of associated systems. Experimental data and application of a two-state model to pure liquids and mixtures

The predictions from a recently reported (J. Chem. Phys. 2004, 120, 6648) two-state association model (TSAM) have been tested against experimental data. The temperature, T, and pressure, p, dependence of the isobaric heat capacity, C(p), for three pure alcohols and the temperature dependence at atmospheric pressure of the excess heat capacity, C(p)(E), for four alcohol + ester mixtures have been measured. The branched alcohols were 3-pentanol, 3-methyl-3-pentanol, and 3-ethyl-3-pentanol, and the mixtures were 1-butanol and 3-methyl-3-pentanol mixed with propyl acetate and with butyl formate. These data, together with literature data for alcohol + n-alkane and alcohol + toluene mixtures, have been analyzed using the TSAM. The model, originally formulated for the C(p) of pure liquids, has been extended here to account for the C(p)(E) of mixtures. To evaluate its performance, quantum mechanical ab initio calculations for the H-bond energy, which is one of the model parameters, were performed. The effect of pressure on C(p) for pure liquids was elucidated, and the variety of C(p)(E)(T) behaviors was rationalized. Furthermore, from the C(p) data at various pressures, the behavior of the volume temperature derivative, (deltaV/deltaT)(p), was inferred, with the existence of a (deltaV/deltaT)(p) versus T maximum for pure associated liquids such as the branched alcohols being predicted. It is concluded that the TSAM captures the essential elements determining the behavior of the heat capacity for pure liquids and mixtures, providing insight into the macroscopic manifestation of the association phenomena occurring at the molecular level.     

Synthetic and natural silica‐aluminates as inorganic acidic catalysts in ring opening polymerization of cyclosiloxanes

Ring opening polymerization (ROP) of hexamethylcyclotrisiloxane (D₃) and octamethylcyclotetrasiloxane (D₄) was promoted by acid‐treated synthetic and natural silica‐aluminates. Silica‐alumina (1:3 Si/Al molar ratio) was obtained using a simple and economic route from precipitation of aluminum sulfate solutions. The material was treated in an acidic medium to improve the content of acid sites and successfully tested as inorganic acidic catalyst for ROP of D₃ or D₄ cyclosiloxanes. Natural bentonite was treated and used in a similar manner. Once the ROP reaction completed, the catalyst was easily removed and it was found that the recovered synthetic silica‐alumina was active in a second ROP reaction. The effect of the concentration and type of catalyst in respect to the molecular weight and polydispersity of polydimethylsiloxanes was analyzed: increasing the amount of silica‐alumina in ROP of D₄ from 0.05 to 0.1 g decreased the average molecular weight (M_n = 13–1.8 kDa) associated with an increase in the polydispersity (2.95 vs. 1.81). Analogous results were found with bentonite. These values suggest that an increase in the catalyst concentration led to a lower M_n, with a more homogeneous molecular chain dimension. Copyright © 2012 John Wiley & Sons, Ltd.     

Isobaric thermal expansivity of the binary system 1-hexanol + n-hexane as a function of temperature and pressure

The isobaric thermal expansivity against temperature and pressure for the system 1-hexanol + n-hexane was directly determined by means of a calorimetric method. From these data, the excess isobaric thermal expansivity is calculated at representative temperatures and pressures. The obtained results for this excess quantity are qualitatively discussed by applying well-known arguments often used for explaining the thermodynamic behavior of alcohol + alkane mixtures. In order to check the consistency of these data with those of literature, the derivative of excess molar volume against temperature and that of excess isobaric molar heat capacity against pressure are calculated and compared with those obtained from literature data. Very good coherence between both data sources is obtained.     

Bifurcation of homoclinics of Hamiltonian systems

We obtain sufficient conditions for bifurcation of homoclinic trajectories of nonautonomous Hamiltonian vector fields parametrized by a circle, together with estimates for the number of bifurcation points in terms of the Maslov index of the asymptotic stable and unstable bundles of the linearization at the stationary branch.