Solute - solvent repulsion effects on the absorption spectra of anthracene in n-hexane investigated under high pressure.

The band positions in the UV-VIS absorption spectra of compressed solution of anthracene in n-hexane significantly depend not only on the dispersive but also on the repulsive solute-solvent interactions, what has so far been omitted. Their strength is determined not only by the solvent polarity but also by Onsager cavity radius changing with pressure. The results obtained for anthracene show that repulsive interactions should be included in the interpretation of barochromic and solvatochromic results of aromatic compounds. We show that the barochromic studies in the liquid solvent can be an alternative to solvatochromic studies, e.g. to determine the polarizability of organic molecules in the electronic excited state. The pressure-induced polarity change in n-hexane exceeds that induced by the exchange of n-alkane solvents between n-pentane and n-hexadecane.

Low-density preference of the ambient and high-pressure polymorphs of DL-menthol.

Lower-density polymorphs of DL-menthol were nucleated and crystallized in their high-pressure stability regions. Up to 0.30 GPa, the triclinic DL-menthol polymorph α, which is stable at atmospheric pressure, is less dense than a new ß polymorph, which becomes stable above 0.40 GPa, but is less dense than the α polymorph at this pressure. The compression of polymorph α to at least 3.37 GPa is monotonic, with no signs of phase transitions. However, recrystallizations of DL-menthol above 0.40 GPa yield the ß polymorph, which is less compressible and becomes less dense than α-DL-menthol. At 0.10 MPa, the melting point of the ß polymorph is 14°C, much lower compared with those of α-DL-menthol (42-43°C) and L-menthol (36-38°C). The structures of both DL-menthol polymorphs α and ß are very similar with respect to the lattice dimensions, the aggregation of OH...O molecules bonded into Ci symmetric chains, the presence of three symmetry-independent molecules (Z' = 3), their sequence ABCC'B'A', the disorder of the hydroxyl protons and the parallel arrangement of the chains. However, the different symmetries relating the chains constitute a high kinetic barrier for the solid-solid transition between polymorphs α and ß, hence their crystallizations below or above 0.40 GPa, respectively, are required. In the structure of polymorph α, the directional OH...O bonds are shorter and the voids are larger compared with those in polymorph ß, which leads to the reverse density relation of the polymorphs in their stability regions. This low-density preference reduces the Gibbs free-energy difference between the polymorphs: when polymorph α is compressed to above 0.40 GPa, the work component pΔV counteracts the transition to the less dense polymorph ß, and on reducing the pressure of polymorph ß to below 0.40 GPa, its transition to the less dense polymorph α is also hampered by the work contribution.

Competition of interactions and a new high-temperature phase of selenourea.

The aggregation of molecules is usually associated with a specific type of interaction, which can be altered by thermodynamic conditions. Under normal conditions, the crystal structure of selenourea, SeC(NH2)2, phase α is trigonal, space group P31, Z = 27. Its large number of independent molecules (Zα' = 9) can be associated with the formation of an NH...N hydrogen bond substituting one of 36 independent NH...Se hydrogen bonds, which prevail among intermolecular interactions. Phase α approximates the trigonal structure with a threefold smaller unit cell (Z = 9), which in turn approximates another still threefold smaller unit cell (Z = 3). The temperature-induced transformations of selenourea have been characterized by calorimetry and by performing 21 single-crystal X-ray diffraction structural determinations as a function of temperature. At 381.0 K, phase α undergoes a first-order displacive transition to phase γ, with space group P3121 and Z reduced to 9, when the NH...N bond is broken and an NH...Se bond is formed in its place. Previously, an analogous competition was observed between NH...N and NH...O hydrogen bonds in high-pressure phase III of urea. The lattice vectors along the (001) plane in low- and high-temperature phases of selenourea are related by a similarity rule, while the lattice dimensions along direction c are not affected. This similarity rule also applies to the structures of phase γ and hypothetical phase δ (Z = 3). The thermally controlled transition between enantiomorphic phases of selenourea contrasts with its high-pressure transition at 0.21 GPa to a centrosymmetric phase ß, where both the NH...Se and NH...N bonds are present. The compression and heating reduce the number of independent molecules from Z' = 9 in phase α, to Z' = 2 in phase ß and to Z' = 1.5 in phase γ.

High-Pressure Crystallization and Thermodynamic Stability Study on the Resolution of High-Density Enantiomers from Low-Density Racemates.

High-pressure recrystallization could be the cheapest clean method of resolving enantiomers from the racemates defying Wallach's rule. We have investigated the effect of pressure on sodium tartrate monohydrate (NaTa·H2O), a notorious exception from Wallach's rule: both racemic polymorphs α-dl-NaTa·H2O and ß-dl-NaTa·H2O are less dense than the enantiomers. According to the mobile-equilibrium principle, such high-density enantiomorphs should spontaneously separate under high pressures. The pressure dependence of the Gibbs free energy explains the preferential crystallization of mixed enantiomers of NaTa·H2O.

Exchanged Metal-Hydrogen Anagostic Bonds and Resonance of Dithiocarbamate and Thioureide Mesomers.

The pressure-induced transformation of plane-square complex nickel(II) bis(N,N-diethyldithiocarbamate) between its soft dithiocarbamate (form I) and thioureide (form II) mesomeres is coupled to the interchange of anagostic Ni⋅⋅⋅H-C interactions from methylene to the methyl group, respectively. At 1.23 GPa, the clearly visible giant anomalous compressibility of the crystal reveals a potential-energy difference of 5.4 kJ mol-1 between the two complex forms. The structural and spectroscopic results, which are supported by quantum-mechanical calculations, connect this solid-state phase transition with the mesomeric transition, and this is accompanied by the conformational transformation of anagostic Ni⋅⋅⋅H-C rearrangement and formation of the charge-assisted S- ⋅⋅⋅H-C bond under pressure.

High-pressure and environment effects in selenourea and its labile crystal field around molecules.

Ambient-pressure trigonal phase α of selenourea SeC(NH2)2 is noncentrosymmetric, with high Z' = 9. Under high pressure it undergoes several intriguing transformations, depending on the pressure-transmitting medium and the compression or recrystallization process. In glycerine or oil, α-SeC(NH2)2 transforms into phase ß at 0.21 GPa; however in water, phase α initially increases its volume and can be compressed to 0.30 GPa due to the formation of α-SeC(NH2)2·xH2O. The single crystals of α-SeC(NH2)2 and of its partial hydrate α-SeC(NH2)2·xH2O are shattered by pressure-induced transitions. Single crystals of phase ß-SeC(NH2)2 were in situ grown in a diamond-anvil cell and studied by X-ray diffraction. The monoclinic phase ß is centrosymmetric, with Z' = 2. It is stable to 3.20 GPa at least, but it cannot be recovered at ambient conditions due to strongly strained NH...Se hydrogen bonds. No hydrogen-bonding motifs present in the urea structures have been found in selenourea phases α and ß.

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.

High-pressure and temperature dependence of the spontaneous resolution of 1,1'-binaphthyl enantiomers.

High pressure increases the temperature of the spontaneous resolution of 1,1'-binaphthyl conformational enantiomers in the crystalline state, which confirms that the enantiomers and racemates are stabilized in the molecular environments in compressed structures. The established pressure-temperature (p-T) preference diagram for the racemate-enantiomer spontaneous crystallization corresponds to a boundary between solid phases, as it is consistent with the Clausius-Clapeyron equation, however, the hysteresis of such a solid-state transformation extends to very high pressure, to 3 GPa, at least according to this study. High-pressure X-ray diffraction study on single crystals of 1,1'-binaphthyl racemate and enantiomer reveals their monotonic compression and structural changes up to 3 GPa. It also reveals the increasing role of intermolecular interactions for stabilizing the structures, despite the exceptionally large density difference between the racemate (1.277 g cm-1) and enantiomers (1.183 g cm-1).

Double helix quinine-based supergelator.

10,11-Didehydroquinine is a simple, low molecular weight supergelator which forms, in nonpolar media, stable chiral organogels composed of unique double-helix nano-sized fibers. A novel gelation mechanism involves a hydrogen bonding network formed by an acidic alkyne proton of the Cinchona gelator and the carbonyl group of ethyl acetate used as a solvent.