Manuel E. Minas da Piedade

The Structure of Aqueous Solutions of a Hydrophilic Ionic Liquid: The Full Concentration Range of 1-Ethyl-3-methylimidazolium Ethylsulfate and Water

Several structural features of aqueous solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate were analyzed in the entire concentration range using molecular dynamics simulation results. Different analysis tools developed in-house were applied to describe the size and connectivity of different water and ion aggregates as a function of the solution concentration. Four concentration ranges-x(H(2)O)<0.5, 0.50.95-with four distinct structural regimes--isolated water molecules, chain-like water aggregates, bicontinuous system, and isolated ions or small ion clusters--were identified and discussed, including two different percolation limits: that of water in the ionic liquid network (around x(H(2)O)=0.8) and that of the ionic liquid in water (around x(H(2)O)=0.95).

Enthalpies of formation of buckminsterfullerene (C60) and of the parent ions C60+, C602+, C603+ and C60–

The standard enthalpy of formation of buckminsterfullerene (C60) in the crystalline state has been determined as 2278.1 ± 14.4 kJ mol–1 using a microcombustion calorimeter. Statistical mechanical calculations of the heat capacity of gaseous C60 as a function of the temperature and a critical survey of the literature data for the enthalpy of sublimation, the heat capacity in the crystalline state, the first, second and third ionization energies and the electron affinity of C60, enabled the enthalpy of formation of gaseous C60 and the enthalpies of formation of the ions C60+, C602+, C603+ and C60– to be derived.

Direct experimental observation of the aggregation of α-amino acids into 100–200 nm clusters in aqueous solution

Spherical supramolecular aggregates of α-amino acids with a typical diameter of 100–200 nm are formed spontaneously after dissolution in water at a concentration of a few mM, i.e. well below the solubility limit. Their presence was shown by nanoparticle tracking analysis (NTA), atomic force microscopy (AFM), and ESI mass spectrometry (ESI-MS). There is a dynamic equilibrium between the aggregates and dissolved individual molecules which allows them to penetrate through dialysis membranes and filters. The same phenomenon was observed for para-amino salicylic acid and two dipeptides. Thermodynamic considerations suggest an entropy-controlled process.

Cationic benzyl nickel complexes as homogeneous catalysts for styrene oligomerization. X-ray crystal structure of [Ni(η3-CH2C6H5)(PPh3)2]PF6 · CH2CI2 *

Abstract New cationic complexes [Ni(η3-CH2C6H5)(PPh3)2]PF6 (3) and [Ni(η3-CH2C6H5)((+)-DIOP)]PF6 (4) were synthesized from the corresponding neutral compounds [Ni(η3-CH2C6H5)X(PR3)2] (X = Cl, Br), by metathetical halide abstraction with TIPF6. Complex 3 was characterized by X-ray crystallography and shown to have a distorted square-planar geometry with the benzylic group coordinated in a quasi-allylic fashion. Crystals are monoclinic, space group P21/n, a = 11.798(5), b = 19.251(2), c = 18.561(3) A, β = 93.03°, V = 4208.6 A3. Solution NMR studies showed benzyl fluxionality and phosphine lability in compound 3, but not in 4. The latter one is catalytically inactive towards styrene oligomerization, whereas 3 produces styrene oligomers ( M ¯ n ≈ 1000 ) with 67% isotactic content. Although [Ni(η3-CH2C6H5)CI(PCy3)] and trans[Ni(Cl)(H)(PCy3)2] are inactive, addition of TlPF6 induced catalytic activity with formation of highly isotactic (90%) styrene oligomers with similar M ¯ n and a different terminal group.

Energetics and structure of nicotinic acid (niacin).

The standard molar enthalpies of formation and sublimation of crystalline (monoclinic, space group P2(1)/c) nicotinic acid (NA), at 298.15 K, were determined as Delta(f)H(m)(o)(NA, cr) = -344.7 +/- 1.2 kJ x mol(-1) and Delta(sub)H(m)(o)(NA) = 112.1 +/- 0.5 kJ x mol(-1) by using combustion calorimetry, drop-sublimation Calvet microcalorimetry, and the Knudsen effusion method. The experimental determinations were all based on a sample of NIST Standard Reference Material 2151, which was characterized in terms of chemical purity, phase purity, and morphology. From the above results, Delta(f)H(m)(o)(NA, g) = -232.6 +/- 1.3 kJ x mol(-1) could be derived. On the basis of this value and on published experimental data, the enthalpy of the isodesmic reaction nicotinic acid(g) + benzene(g) --> benzoic acid(g) + pyridine(g) was calculated as -3.6 +/- 2.7 kJ x mol(-1) and compared with the corresponding predictions by the B3LYP/cc-pVTZ (-3.6 kJ x mol(-1)), B3LYP/aug-cc-pVTZ (-3.7 kJ x mol(-1)), B3LYP/6-311++G(d,p) (-4.2 kJ x mol(-1)), G3MP2 (-4.3 kJ x mol(-1)), and CBS-QB3 (-4.0 kJ x mol(-1)) quantum chemistry models. The excellent agreement between the experimental and theoretical results supports the reliability of the Delta(f)H(m)(o) (NA, cr), Delta(sub)H(m)(o)(NA), and Delta(f)H(m)(o)(NA, g) recommended in this work. These data can therefore be used as benchmarks for discussing the energetics of nicotinic acid in the gaseous and crystalline states and, in particular, to evaluate differences imparted to solid forms by the production and processing methods. Such differences are perhaps at the root of the significant inconsistencies found between the published enthalpies of sublimation of this important active pharmaceutical ingredient and thermochemical standard. The molecular packing in the crystalline phase studied in this work was also discussed and its influence on the molecular structure of nicotinic acid was analyzed by comparing bond distances and angles published for the solid state with those predicted by the B3LYP/cc-pVTZ method. No advantage in terms of accuracy of the structural predictions was found by the use of the larger aug-cc-pVTZ or 6-311++G(d,p) basis sets.

The Nature of Protic Ionic Liquids in the Gas Phase Revisited: Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Study of 1,1,3,3-Tetramethylguanidinium Chloride

The sublimation/vaporization of the protic ionic liquid 1,1,3,3-tetramethylguanidinium chloride, [Htmg]Cl, was studied by Fourier transform ion cyclotron resonance mass spectrometry in the temperature range 298-488 K and under a reduced pressure of 3.2 x 10(-6) to 1.5 x 10(-5) Pa. The results show that no charged species are present in the vapor over the condensed phase. Furthermore, ion-molecule reaction studies found no evidence of neutral ion pairs in the gas phase. This indicates that the sublimation/vaporization of [Htmg]Cl conforms to the general mechanism postulated for the distillation of protic ionic liquids, which involves a proton transfer leading to the formation of the neutral acid and base precursors, in this case hydrogen chloride and 1,1,3,3-tetramethylguanidine.

Vaporisation of a Dicationic Ionic Liquid Revisited

The vaporization of a dicationic ionic liquid at moderate temperatures and under reduced pressures--recently studied by line-of-sight mass spectrometry--was further analyzed using an ion-cyclotron resonance mass spectroscopy technique that allows the monitoring of the different species present in the gas phase through the implementation of controlled ion-molecule reactions. The results support the view that the vapour phase of an aprotic dicationic ionic liquid is composed of neutral ion triplets (one dication attached to two anions). Molecular dynamics simulations were also performed in order to explain the magnitude of the vaporization enthalpies of dicationic ionic liquids vis-à-vis their monocationic counterparts.

Bridging the gap between ionic liquids and molten salts: group 1 metal salts of the bistriflamide anion in the gas phase.

Fourier transform ion cyclotron resonance mass spectrometry experiments showed that liquid Group 1 metal salts of the bistriflamide anion undergoing reduced-pressure distillation exhibit a remarkable behavior that is in transition between that of the vapor-liquid equilibrium characteristics of aprotic ionic liquids and that of the Group 1 metal halides: the unperturbed vapors resemble those of aprotic ionic liquids, in the sense that they are essentially composed of discrete ion pairs. However, the formation of large aggregates through a succession of ion-molecule reactions is closer to what might be expected for Group 1 metal halides. Similar experiments were also carried out with bis{(trifluoromethyl)sulfonyl}amine to investigate the effect of H(+), which despite being the smallest Group 1 cation, is generally regarded as a nonmetal species. In this case, instead of the complex ion-molecule reaction pattern found for the vapors of Group 1 metal salts, an equilibrium similar to those observed for aprotic ionic liquids was observed.

A new calorimetric system to measure heat capacities of solids by the drop method

A new electrically calibrated calorimetric system to measure heat capacities of solids has been designed and tested by determining the heat capacity of sapphire and benzoic acid in the temperature range 298–393 K. Comparison of the obtained results with recommended adiabatic calorimetry data showed that the maximum and average absolute deviations were 3.3% and 1.6%, respectively. The technique provides a convenient alternative to routine heat capacity determinations by DSC, since the experiments are faster and, in general, the results have better accuracy. In addition, the sample cells can be kept under argon or nitrogen atmosphere thus enabling the study of air sensitive compounds.

The enthalpy of sublimation of diphenylacetylene from Knudsen effusion studies

The enthalpy of sublimation of diphenylacetylene at 298.15 K, ΔgcrH⊖m(C2(C6H5)2] = 95.1 ± 1.1 kJmol−1, was derived from vapour pressure-temperature data, obtained with two different Knudsen effusion apparatus, and from heat capacity measurements obtained by differential scanning calorimetry. The molybdenum-diphenylacetylene bond dissociation enthalpy in Mo(η5-C5H5)2[C2(C6H5)2] was reevaluated as 115 ± 26 kJ mol−1, on the basis of the new value for ΔcrgH⊖m[C2(C6H5)2].