Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) experiments showed that when aprotic ionic liquids vaporize under pressure and temperature conditions similar to those of a reduced-pressure distillation, the gas phase is composed of discrete anion-cation pairs. The evolution of the mass spectrometric signals recorded during fractional distillations of binary ionic liquid mixtures allowed us to monitor the changes of the gas-phase composition and the relative volatility of the components. In addition, we have studied a protic ionic liquid, and demonstrated that it exists as separated neutral molecules in the gas phase.

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).

The energetics of intramolecular hydrogen bonds (H-bonds) is a subject of fundamental importance in chemistry and biochemistry. In contrast with intermolecular H-bonds, whose enthalpy can be determined by experiment or accurately evaluated through a supermolecular approach, there is no general accepted procedure to determine the enthalpy of an intramolecular H-bond. In this work, different ways for assessing the energetics of intramolecular H-bonds of selected aromatic systems were applied and compared. They include the widely used conformational analysis approach (cis−trans method), a recently proposed isodesmic reaction method, and a new procedure that we designate as the ortho−para method. Energy calculations were carried out at several theory levels, including a modified complete basis set extrapolation method (CBS-QMPW1), in which the geometries are based on MPW1PW91/aug-cc-pVDZ density functional theory optimizations. The obtained results, together with a simple dipole−dipole interaction model, help to explain why the enthalpies of intramolecular H-bonds are often overestimated by the cis−trans method. The results also show that intramolecular H-bond enthalpies based on the isodesmic reaction method may be unreliable. The ortho−para method, which can be applied when accurate theoretical or experimental standard enthalpies of formation are available, is probably the best way of estimating the enthalpies of intramolecular hydrogen bonds. Finally, our results illustrate the important role played by intramolecular H-bonds in the energetics of homolytic dissociation reactions involving di-substituted benzenes.

A new polymorph of 4′-hydroxyacetophenone (form I, monoclinic, P21/c, Z′ = 1) was isolated and characterized. The structural differences between this phase and the previously known one (form II, orthorhombic, P212121, Z′ = 2) were investigated by X-ray diffraction. The two polymorphs exhibit distinct packing features and, at the molecular level, they seem to differ by the relative conformations of the OH and C(O)CH3 groups. The stability domains of the two phases from 298.15 K to the fusion temperature were also studied by a variety of thermodynamic methods and by density functional theory calculations. On the basis of the obtained results, p−T and Δf G m o−T phase diagrams for 4′-hydroxyacetophenone were defined. Differential scanning calorimetry experiments indicated that the system is enantiotropic, with form II first transforming into form I at 351.2 ± 2.7 K, followed by fusion of form I at 381.9 ± 0.1 K. Solution calorimetry demonstrated that form II is more stable than form I at 298.15 K, with Δtrs H m o(II→I) = 0.49 ± 0.13 kJ mol−1. Despite this small enthalpy difference compared to the thermal energy at 298.15 K (RT = 2.5 kJ mol−1), a sample of form I could be stored at ambient temperature, for at least 1 year, without change. Results of B3LYP/6–31G(d,p) calculations indicated that the most stable conformation of the isolated molecule is also that corresponding to the most stable polymorph of 4′-hydroxyacetophenone at ambient temperature (form II). The computations further suggest that the occurrence of the II → I transition through a simple rotation of the OH group is unlikely. Finally, the fact that the more stable form II has a greater Z′ than the less stable form I contrasts with the recent proposal that high Z′ polymorphs are metastable precursors of lower Z′ forms along the crystallization pathway.

Dynamic spin interchange where crystals explode with preservation of magnetic memory is observed for a mononuclear hysteretic Fe(iii) Schiff-base compound.