The effective volumes of waters of crystallization: non-ionic pharmaceutical systems

Solvatomorphism in a series of copper(II) complexes with the 5-phenylimidazole/perchlorate system as ligands

In the course of an investigation of the supramolecular behaviour of copper(II) complexes with the 5-phenylimidazole/perchlorate ligand system (`blend') remarkable solvatomorphism has been observed. By employing a variety of crystallization solvents (polar protic, polar/non-polar aprotic), a series of 12 crystalline solvatomorphs with the general formula [Cu(ClO4)2(LH)4]·x(solvent) have been obtained [LH = 5-phenylimidazole, x(solvent) = 3.3(H2O) (1), 2(methanol) (2), 2(ethanol) (3), 2(1-propanol) (4), 2(2-propanol) (5), 2(2-butanol) (6), 2(dimethylformamide) (7), 2(acetone) (8), 2(tetrahydrofurane) (9), 2(1,4-dioxane) (10), 2(ethyl acetate) (11) and 1(diethyl ether) (12)]. The structures have been solved using single-crystal X-ray diffraction and the complexes were characterized by thermal analysis and infrared spectroscopy. The solvatomorphs are isostructural (triclinic, P1), with the exception of compound 9 (monoclinic, P21/n). The supramolecular structures and the role of the various solvents is discussed. All potential hydrogen-bond functionalities, both of the [Cu(ClO4)2(LH)4] units and of the solvents, are utilized in the course of the crystallization process. The supramolecular assembly in all structures is directed by strong recurring Nimidazole-H...Operchlorate motifs leading to robust scaffolds composed of the [Cu(ClO4)2(LH)4] host complexes. The solvents are located in channels and, with the exception of the disordered waters in 1 and the diethyl ether in 12, participate in hydrogen-bonding formation with the [Cu(ClO4)2(LH)4] complexes, serving as both hydrogen-bond acceptors and donors (for the polar protic solvents in 2-6), or solely as hydrogen-bond acceptors (for the polar/non-polar aprotic solvents in 7-11), linking the complexes and contributing to the stability of the crystalline compounds.

Stochastic hydration of a high-nitro-gen-content molecular compound recrystallized under pressure

Partial hydration of organic compounds can be achieved by high-pressure crystallization. This has been demonstrated for the high-nitro-gen-content compound 6-chloro-1,2,3,4-tetrazolo[1,5-b]pyridazine (C4H2N5Cl), which becomes partly hydrated by isochoric crystallizations below 0.15 GPa. This hydrate, C4H2N5Cl·xH2O, is isostructural with the ambient-pressure phase α of C4H2N5Cl, but the crystal volume is somewhat larger than that of the anhydrate. At 0.20 GPa, the α-C4H2N5Cl anhydrate phase transforms abruptly into a new higher-symmetry phase, α'; the transformation is clearly visible due to a strong contraction of the crystals. The hydrate α-C4H2N5Cl·xH2O can also be isothermally compressed up to 0.30 GPa before transforming to the α'-C4H2N5Cl·xH2O phase. The isochoric recrystallization of C4H2N5Cl above 0.18 GPa yields a new anhydrous phase β, which, on releasing pressure, transforms back to the α phase below 0.15 GPa. The structural transition from the α to the β phase is destructive for the single crystal and involves a large volume drop and significant elongation of all the shortest intermolecular distances which are the CH⋯N and CH⋯Cl hydrogen bonds, as well as the N⋯N contacts. The α-to-α' phase transition increases the crystal symmetry in the subgroup relation; however, there are no structural nor symmetry relations between phases α and β.

Water evaporation from solute-containing aerosol droplets: Effects of internal concentration and diffusivity profiles and onset of crust formation

The evaporation of droplets is an important process not only in industrial and scientific applications, but also in the airborne transmission of viruses and other infectious agents. We derive analytical and semi-analytical solutions of the coupled heat and mass diffusion equations within a spherical droplet and in the ambient vapor phase that describe the evaporation process of aqueous free droplets containing nonvolatile solutes. Our results demonstrate that the solute-induced water vapor-pressure reduction considerably slows down the evaporation process and dominates the solute-concentration dependence of the droplet evaporation time. The evaporation-induced enhanced solute concentration near the droplet surface, which is accounted for using a two-stage evaporation description, is found to further slow-down the drying process. On the other hand, the presence of solutes is found to produce a lower limit for the droplet size that can be reached by evaporation and, also, to reduce evaporation cooling of the droplet, which tend to decrease the evaporation time. Overall, the first two effects are dominant, meaning that the droplet evaporation time increases in the presence of solutes. Local variation of the water diffusivity inside the droplet near its surface, which is a consequence of the solute-concentration dependence of the diffusion coefficient, does not significantly change the evaporation time. Crust formation on the droplet surface increases the final equilibrium size of the droplet by producing a hollow spherical particle, the outer radius of which is determined as well.

Crystal design by CH...N and N...N interactions: high-pressure structures of high-nitrogen-content azido-triazolopyridazines compounds

High-nitrogen-content compounds 6-azido-1,2,4-triazolo[4,3-b]pyridazine (C₅H₃N₇) and its 3-methyl derivative (C₆H₅N₇) have been in situ crystallized in a diamond-anvil cell and their structures determined by single-crystal X-ray diffraction. Under ambient and high-pressure conditions the crystallizations yield the same phases: the C₅H₃N₇ anhydrate and C₆H₅N₇ hydrated clathrate. In both the structures there are clearly distinguished regions of short CH...N and N...N intermolecular contacts, the latter involving exclusively the azide groups. High pressure initially increases the contents of water in the channel pores of the clathrate.

High-pressure preference for reduced water content in porous zinc aspartate hydrates

The zinc aspartate (ZnAsp2) complex, a common dietary supplement, preferentially crystallizes as the dihydrate (ZnAsp2·2H2O) from aqueous solution. Under normal conditions the dihydrate easily transforms into the sesquihydrate (ZnAsp2·1.5H2O). The dihydrate crystal structure is triclinic, space group P1, and the sesquihydrate is monoclinic, space group C2/c. However, their structures are closely related and similarly consist of zinc aspartate ribbons parallel to pores accommodating water molecules. These porous structures can breathe water molecules in and out depending on the temperature and air humidity. High pressure above 50 MPa favours the sesquihydrate, as shown by recrystallizations under pressure and compressibility measured by single-crystal X-ray diffraction up to 4 GPa. This preference is explained by the reduced volume of the sesquihydrate and water compressed separately, compared with the dihydrate. The sesquihydrate undergoes an isostructural phase transition when the voids collapse at 0.8 GPa, whereas no phase transitions occur in the dihydrate, because its pores are supported by increased water content.

The effective volumes of waters of crystallization: general organic solids

Using a list of compatible hydrate/anhydrate pairs prepared by van de Streek and Motherwell [CrystEngComm (2007), 9, 55-64], we have examined the effective volume per water of crystallization for 179 pairs of organic solids using current data from the Cambridge Crystallographic Structural Database (CSD). The effective volume is the difference per water molecule between the asymmetric unit volumes of the hydrate and parent anhydrate, and has the mean value 24 ų. The conformational changes in the reference molecule between the hydrate and its anhydrate are shown in two figures: one for a relatively rigid standard organic molecule and (in the supplementary file) one for a more flexible linear molecule. Using data from Nyman and Day [Phys. Chem. Chem. Phys. (2016), 18, 31132-31143], we have also established a generic volumetric coefficient of thermal expansion of organic solids with a value of 147 ± 56 × 10^-6 K^-1. There is a significant number of outliers to the data, negative, near zero, and large and positive. Some explanation for the existence of these outliers is attempted.