Structure-property relationships in protic ionic liquids : a study of solvent-solvent and solvent-solute Interactions

The ionic nature of a functionalized protic ionic liquid cannot be rationalized simply through the differences in aqueous proton dissociation constants between the acid precursor and the conjugate acid of the base precursor. The extent of proton transfer, i.e. the equilibrium ionicity, of a tertiary ammonium acetate protic ionic liquid can be significantly increased by introducing an hydroxyl functional group on the cation, compared to the alkyl or amino-functionalized analogues. This increase in apparent ionic nature correlates well with variations in solvent-solute and solvent-solvent interaction parameters, as well as with physicochemical properties such as viscosity

Structure-property relationships in protic ionic liquids : A thermochemical study

How does cation functionality influence the strength of intermolecular interactions in protic ionic liquids (PILs)? Quantifying the energetics of PILs can be an invaluable tool to answer this fundamental question. With this in view, we have determined the standard molar enthalpy of vaporization, Delta H_vap , and the standard molar enthalpy of formation, Delta H_f, of three tertiary ammonium acetate PILs with varying cation functionality, and of their corresponding precursor amines, through a combination of Calvet-drop microcalorimetry, solution calorimetry, and ab-initio calculations. The obtained results suggest that these PILs vaporize as their neutral acid and base precursors. We also found a strong correlation between Delta H_vap of the PILs and of their corresponding amines. This suggests that, within this series of PILs, the influence of cation modification on their cohesive energies follows a group additivity rule. Finally, no correlation between the Delta H_vap of PILs and the extent of proton transfer, as estimated from the difference in aqueous pKa between the precursor acid and the conjugate acid of the precursor base, was observed

Crystallization of 4′-Hydroxyacetophenone from Water: Control of Polymorphism via Phase Diagram Studies

The preparation of polymorphs and solvates and the characterization of their stability domains have received considerable attention in recent years, due to the importance of these studies for fundamental research and for the production of new materials for task-specific applications. In this work, the selective and reproducible crystallization of different solid forms of 4′-hydroxyacetophenone (HAP) from water was investigated, through the determination of a temperature–concentration (<i>T</i>–<i>c</i>_HAP) phase diagram. This determination was mainly based on gravimetric solubility measurements, slurry tests, and metastable zone width (MZW) studies with thermometric and turbidity detection. The experimental conditions for the formation of five different HAP phases by cooling crystallization could be established: the previously characterized anhydrous forms I and II and the hydrate HAP·1.5H₂O (H1), and two new hydrates, one of stoichiometry HAP·3H₂O (H2) and another (H3) which proved too unstable for a stoichiometry determination. The crystallization precedence of the various phases, their approximate lifetimes, and transformation sequences could also be elucidated. It was finally found that for a specific <i>T</i>–<i>c</i>_HAP domain the crystallization of HAP solid phases was mediated by a colloidal dispersion. Preliminary dynamic light scattering experiments indicated that this dispersion consisted of particles with diameters in the range of 100–800 nm

A new polymorph of 4′-hydroxyvalerophenone revealed by thermoanalytical and X-ray diffraction studies

A new polymorph of 1-(4-hydroxyphenyl)pentan-1-one (4′-hydroxyvalerophenone, HVP) was identified by using differential scanning calorimetry, hot stage microscopy, and X-ray powder diffraction. This novel crystal form (form II) was obtained by crystallization from melt. It has a fusion temperature of Tfus = 324.3 ± 0.2 K and an enthalpy of fusion ΔfusHmo = 18.14±0.18 kJ·mol−1. These values are significantly lower than those observed for the previously known phase (form I, monoclinic, space group P21/c, Tfus = 335.6 ± 0.7 K; ΔfusHmo = 26.67±0.04 kJ·mol−1), which can be prepared by crystallization from ethanol. The results here obtained, therefore, suggest that form I is thermodynamically more stable than the newly identified form II and, furthermore, that the two polymorphs are monotropically related

Polymorphism in 4‑Hydroxybenzaldehyde: A Crystal Packing and Thermodynamic Study

A procedure for the selective and reproducible preparation of the two known 4-hydroxybezaldehyde polymorphs was developed, based on the investigation of their relative stabilities by differential scanning calorimetry and solubility studies. From the obtained results, the stability domains of the two forms could be quantitatively represented in a Δ_f<i>G</i>_m^°–<i>T</i> phase diagram. The system was found to be enantiotropic: form II is more stable than form I up to 277 ± 1 K; above this temperature, the stability order is reversed, and the fusion of form I subsequently occurs at 389.9 ± 0.2 K. Analysis of the crystal structures revealed that in both polymorphs the 4-hydroxybezaldehyde molecule exhibits the OH and C­(O)H substituents in a <i>Z</i> conformation, which, according to B3LYP/6-31G­(d,p) calculations, is more stable than the <i>E</i> conformation by only 0.4 kJ·mol^–1. The two forms are monoclinic, space group <i>P</i>2₁/<i>c</i>, <i>Z</i>′/<i>Z</i> = 1/4, and have essentially identical densities at ambient temperature (1.358 g·cm^–3 for form I; 1.357 g·cm^–3 for form II), but differ in their packing. These differences are discussed, and the dissimilarities in the interactions sustaining the packing are highlighted using Hirshfeld surfaces. Finally, the relative stability and volumetric properties of both forms are analyzed by molecular dynamics simulations

All-Atom Force Field for Molecular Dynamics Simulations on Organotransition Metal Solids and Liquids. Application to M(CO)_<i>n</i> (M = Cr, Fe, Ni, Mo, Ru, or W) Compounds

A previously developed OPLS-based all-atom force field for organometallic compounds was extended to a series of first-, second-, and third-row transition metals based on the study of M­(CO)<i>_n</i> (M = Cr, Fe, Ni, Mo, Ru, or W) complexes. For materials that are solid at ambient temperature and pressure (M = Cr, Mo, W) the validation of the force field was based on reported structural data and on the standard molar enthalpies of sublimation at 298.15 K, experimentally determined by Calvet-drop microcalorimetry using samples corresponding to a specific and well-characterized crystalline phase: Δ_sub<i>H</i>_m^° = 72.6 ± 0.3 kJ·mol^–1 for Cr­(CO)₆, 73.4 ± 0.3 kJ·mol^–1 for Mo­(CO)₆, and 77.8 ± 0.3 kJ·mol^–1 for W­(CO)₆. For liquids, where problems of polymorphism or phase mixtures are absent, critically analyzed literature data were used. The force field was able to reproduce the volumetric properties of the test set (density and unit cell volume) with an average deviations smaller than 2% and the experimentally determined enthalpies of sublimation and vaporization with an accuracy better than 2.3 kJ·mol^–1. The Lennard-Jones (12-6) potential function parameters used to calculate the repulsive and dispersion contributions of the metals within the framework of the force field were found to be transferable between chromium, iron, and nickel (first row) and between molybdenum and ruthenium (second row)

Kinetics and Mechanism of the Thermal Dehydration of a Robust and Yet Metastable Hemihydrate of 4‑Hydroxynicotinic Acid

Hydrates are the most common type of solvates and certainly the most important ones for industries such as pharmaceuticals which strongly rely on the development, production, and marketing of organic molecular solids. A recent study indicated that, in contrast with thermodynamic predictions, a new hemihydrate of 4-hydroxynicotinic acid (4HNA·0.5H₂O) did not undergo facile spontaneous dehydration at ambient temperature and pressure. The origin of this robustness and the mechanism of dehydration were investigated in this work, through a combined approach which involved kinetic studies by thermogravimetry (TGA), crystal packing analysis based on X-ray diffraction data, and microscopic observations by hot stage microscopy (HSM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The TGA results indicated that the resilience of 4HNA·0.5H₂O to water loss is indeed of kinetic origin, c.f., due to a significant activation energy, <i>E</i>_a, which increased from 85 kJ·mol^–1 to 133 kJ·mol^–1 with the increase in particle size. This <i>E</i>_a range is compatible with the fact that four moderately strong hydrogen bonds (typically 20–30 kJ·mol^–1 each) must be broken to remove water from the crystal lattice. The dehydration kinetics conforms to the Avrami-Erofeev A2 model, which assumes a nucleation and growth mechanism. Support for a nucleation and growth mechanism was also provided by the HSM, SEM, and AFM observations. These observations further suggested that the reaction involves one-dimensional nucleation, which is rarely observed. Finally, a statistical analysis of Arrhenius plots for samples with different particle sizes revealed an isokinetic relationship between the activation parameters. This is consistent with the fact that the dehydration mechanism is independent of the sample particle size

From Molecules to Crystals: The Solvent Plays an Active Role Throughout the Nucleation Pathway of Molecular Organic Crystals

Crystallization is indisputably one of the oldest and most widely used purification methods. Despite this fact, our current understanding of the early stages of crystallization is still in its infancy. In this work dynamic light scattering and proton nuclear magnetic resonance were used to investigate the changes occurring in 4′-hydroxyacetophenone colloidal particles, as they form in a supersaturated aqueous solution and evolve toward anhydrous or hydrate materials during a cooling crystallization process. In the concentration range probed, the particles are initially composed by both solute and water. If the outcome of crystallization is an anhydrous phase, a complete loss of solvent from the particles is progressively observed up to the onset of crystal precipitation. These findings provide unique experimental evidence that the role of solvent in the formation of crystals can go well beyond influencing the self-assembly and clustering of solute molecules prior to nucleation