Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate.

The double carbonate BaCa(CO3)2 holds potential as host compound for carbon in the Earth's crust and mantle. Here, we report the crystal structure determination of a high-pressure BaCa(CO3)2 phase characterized by single-crystal X-ray diffraction. This phase, named post-barytocalcite, was obtained at 5.7 GPa and can be described by a monoclinic Pm space group. The barytocalcite to post-baritocalcite phase transition involves a significant discontinuous 1.4% decrease of the unit-cell volume, and the increase of the coordination number of 1/4 and 1/2 of the Ba and Ca atoms, respectively. High-pressure powder X-ray diffraction measurements at room- and high-temperatures using synchrotron radiation and DFT calculations yield the thermal expansion of barytocalcite and, together with single-crystal data, the compressibility and anisotropy of both the low- and high-pressure phases. The calculated enthalpy differences between different BaCa(CO3)2 polymorphs confirm that barytocalcite is the thermodynamically stable phase at ambient conditions and that it undergoes the phase transition to the experimentally observed post-barytocalcite phase. The double carbonate is significantly less stable than a mixture of the CaCO3 and BaCO3 end-members above 10 GPa. The experimental observation of the high-pressure phase up to 15 GPa and 300 ºC suggests that the decomposition into its single carbonate components is kinetically hindered.

Multistimuli Response Micro- and Nanolayers of a Coordination Polymer Based on Cu2 I2 Chains Linked by 2-Aminopyrazine.

A nonporous laminar coordination polymer of formula [Cu2 I2 (2-aminopyrazine)]n is prepared by direct reaction between CuI and 2-aminopyrazine, two industrially available building blocks. The fine tuning of the reaction conditions allows obtaining [Cu2 I2 (2-aminopyrazine)]n in micrometric and nanometric sizes with same structure and composition. Interestingly, both materials show similar reversible thermo- and pressure-luminescent response as well as reversible electrical response to volatile organic solvents such as acetic acid. X-ray diffraction studies under different conditions, temperatures and pressures, in combination with theoretical calculations allow rationalizing the physical properties of this compound and its changes under physical stimuli. Thus, the emission dramatically increases when lowering the temperature, while an enhancement of the pressure produces a decrease in the emission intensity. These observations emerge as a direct consequence of the high structural flexibility of the Cu2 I2 chains which undergo a contraction in CuCu distances as far as temperature decreases or pressure increases. However, the strong structural changes observed under high pressure lead to an unexpected effect that produces a less effective CuCu orbital overlapping that justifies the decrease in the intensity emission. This work shows the high potential of materials based on Cu2 I2 chains for new applications.

Preparation and properties of the full series of cuboidal clusters [Mo(x)W4-xSe4(H2O)12]n+ (n = 4-6) and their derivatives.

Hydrothermal reactions between incomplete cuboidal cluster aqua complexes [M3Q4(H2O)9]4+ and M(CO)6 (M = Mo, W; Q = S, Se) offer easy access to the corresponding cuboidal clusters M4Q4. The complete series of homometal and mixed Mo/W clusters [Mo(x)W4-xQ4(H2O)12]n+ (x = 0-4, n = 4-6) has been prepared. Upon oxidation of the mixed-metal clusters, it is the W atom which is lost, allowing selective preparation of new trinuclear clusters [Mo2WSe4(H2O)9]4+ and [MoW2Se4(H2O)9]4+. The aqua complexes were converted by ligand exchange reactions into dithiophosphato and thiocyanato complexes, and crystal structures of [W4S4((EtO)2PS2)6], [MoW3S4((EtO)2PS2)6], [Mo4Se4((EtO)2PS2)6], [W4Se4((i-PrO)2PS2)6], and (NH4)6[W4Se4(NCS)12]-4H20 were determined. Cyclic voltammetry was performed on [Mo(x)W4-xCO4(H2O)12]n+, showing reversible redox waves 6+/5+ and 5+/4+. The lower oxidation states are more difficult to access as the number of W atoms increases. The [Mo2WSe4(H2O)9]4+ and [MoW2Se4(H2O)9]4+ species were derivatized into [Mo2WSe4(acac)3(py)3]+ and [MoW2Se4(acac)3(py)3]+, which were also studied by CV. When appropriate, the products were also characterized by FAB-MS and NMR (31P, 1H) data.

Synthesis, X-ray structure, polarized optical spectra and DFT theoretical calculations of two new organic-inorganic hybrid fluoromanganates(III): (bpaH2)[MnF4(H2O)2]2 and (bpeH2)[MnF4(H2O)2]2.

Two new fluoromanganates(III) of 1,2-bis(4-pyridyl)ethane (bpa) and trans-1,2-bis(4-pyridyl)ethylene (bpe), LH(2)[MnF(4)(H(2)O)(2)](2) (L = bpa or bpe), have been prepared and their structure have been solved by single-crystal X-ray diffraction. The [MnF(4)(H(2)O)(2)](-) anion displays an octahedral geometry with a strong Jahn-Teller tetragonal distortion along the H(2)O-Mn-OH(2) axis. The equatorial metal-ligand distances (Mn-F 1.827(1)-1.859(2) A) are shorter than the axial ones (Mn-O 2.203(2)-2.234(2) A). Three polarized absorption bands at 22,500, 18,300 and 14,500 cm(-1) are observed in the optical spectra of (bpaH(2))[MnF(4)(H(2)O)(2)](2). Finally, we present theoretical calculations on the equilibrium bond distances as well as the crystal-field electron structure using density functional methods. The calculated Mn-F bond distances (1.85 A) are in agreement with the experimental data but the obtained Mn-O distances (2.53-2.56 A) are higher than the experimental one as usually found in similar Jahn-Teller distorted systems. The calculated d-d transition energies are compared with experimental energies derived from the optical spectra. The variation of the HOMO energy and transition energies against the Mn-O distance is also shown.

Alkynoates as a source of reactive alkylinides for aldehyde addition reactions.

[see reaction]. The reaction of activated alkynes with carbonyl compounds in the presence of a catalytic amount of a nucleophile leads to enol-protected functionalized propargyl alcohols and 1,3-dioxolane compounds by way of a mild carbon-carbon bond formation reaction.

The ab initio crystal structure solution of proteins by direct methods. VI. Complete phasing up to derivative resolution.

The procedure described in the papers I-V of this series [Giacovazzo, Siliqi & Ralph (1994). Acta Cryst. A50, 503-505; Giacovazzo, Siliqi & Spagna (1994). Acta Cryst. A50, 609-621; Giacovazzo, Siliqi & Zanotti (1995). Acta Cryst. A51, 177-188; Giacovazzo & Gonzalez Platas (1995). Acta Cryst. A51, 398-404; Giacovazzo, Siliqi & Gonzalez Platas (1995). Acta Cryst. A51, 811-820], aiming at estimating protein phases via a single heavy-atom derivative, has been improved so as to extend phase determination to all the reflections up to derivative resolution. The quality of the resulting electron-density maps is checked for a number of test strutures. Some of the maps are immediately interpretable, and some can be interpreted after some cycles of solvent flattening and/or histogram matching. The correlation with classical SIR techniques is also discussed.