The steps of thermal treatment of Na-magadiite: a computational study using ab initio DFT calculations.

We investigated each of the successive transformations of this material using ab initio calculations based on DFT. Possible structures produced from three reaction steps of the thermal treatment were simulated. Thermodynamic analysis was performed to assess the energy stability of each reaction. The dehydration of the interlamellar region confirmed the selective loss of water molecules, with axial H2O being responsible for the first part of the mass loss experimentally observed in TG-DTA while the loss of equatorial H2O molecules is observed above 150 °C. The reactions of the proposed intermediates after dehydration indicated that the formation of a zeolite Si14O28 is thermodynamically unfavorable in relation to zeolite sodium silicate. Kinetic effects and new heat treatment protocols should be studied to improve the understanding of these materials. The final steps indicated that after the condensation of the layers, sodium silicate was formed together with quartz.

Structural investigation of three distinct amorphous forms of Ar hydrate.

Three amorphous forms of Ar hydrate were produced using the crystalline clathrate hydrate Ar·6.5H2O (structure II, Fd3̄m, a ≈ 17.1 Å) as a precursor and structurally characterized by a combination of isotope substitution (36Ar) neutron diffraction and molecular dynamics (MD) simulations. The first form followed from the pressure-induced amorphization of the precursor at 1.5 GPa at 95 K and the second from isobaric annealing at 2 GPa and subsequent cooling back to 95 K. In analogy to amorphous ice, these amorphs are termed high-density amorphous (HDA) and very-high-density amorphous (VHDA), respectively. The third amorph (recovered amorphous, RA) was obtained when recovering VHDA to ambient pressure (at 95 K). The three amorphs have distinctly different structures. In HDA the distinction of the original two crystallographically different Ar guests is maintained as differently dense Ar-water hydration structures, which expresses itself in a split first diffraction peak in the neutron structure factor function. Relaxation of the local water structure during annealing produces a homogeneous hydration environment around Ar, which is accompanied with a densification by about 3%. Upon pressure release the homogeneous amorphous structure undergoes expansion by about 21%. Both VHDA and RA can be considered frozen solutions of immiscible Ar and water in which in average 15 and 11 water molecules, respectively, coordinate Ar out to 4 Å. The local water structures of HDA and VHDA Ar hydrates show some analogy to those of the corresponding amorphous ices, featuring H2O molecules in 5- and 6-fold coordination with neighboring molecules. However, they are considerably less dense. Most similarity is seen between RA and low density amorphous ice (LDA), which both feature strictly 4-coordinated H2O networks. It is inferred that, depending on the kind of clathrate structure and occupancy of cages, amorphous states produced from clathrate hydrates display variable local water structures.

Synthesis of glycerol carbonate over a 2D coordination polymer built with Nd3+ ions and organic ligands.

In this work, a two-dimensional coordination polymer was synthesized and the structure was determined by single-crystal X-ray diffraction. The crystal structure belongs to the space group Pna21 and was characterized by Raman and FT-infrared spectroscopy, powder X-ray diffraction and Brunauer-Emmett-Teller surface area analysis. Catalyst activities were evaluated through the synthesis of glycerol carbonate from glycerol and urea using a batch reactor. After the optimization of both reaction and reaction conditions, the activity results showed that the coordination polymer used as a heterogeneous catalyst has good values of conversions and selectivity for the manufacturing of glycerol carbonate in a fine-chemical process. The analysis of powder X-ray diffraction and spectroscopy for the coordination polymer employed, before and after the reaction, shows that some changes have taken place in the crystal structure during the process, in spite of a recovery at the end of the reaction. The advantages and limitations of the coordination polymer were discussed and compared with those of the previous heterogeneous catalysts in the literature.

DFT calculations for structural prediction and applications of intercalated lamellar compounds.

The intercalated layered materials are commonly built from structures complex enough to have large unit cells and, because of this, calculations of their electronic structures are very demanding in terms of memory, processing and time. Also, the versatility of these compounds enables the synthesis of a large number of derived materials difficult to characterize. Only in the last two decades, a combination of theoretical methodologies and advances in processing made density-functional theory (DFT) calculations quite interesting as an investigation tool for this family of materials. Since the intercalated layered or lamellar compounds correspond to a large group of important classes of materials and their experimental data were, and are still being, generated, only a small part of the data comes from electronic structure simulations. In this review, we have listed some relevant types of intercalated lamellar materials, the useful methodologies implemented in the standard suit of codes for DFT calculations and examples of the many applications of the calculations to the understanding of physical and chemical properties, to the planning of novel materials with desirable properties, and even to assist the structural characterization, by simulating complex results from nuclear magnetic resonance, vibrational spectroscopy and powder X-ray diffraction. In addition to the properties simulated directly as observables, other quantities such as density of states, partial charges and electronic density difference, provide relevant information about the materials and their behavior under diverse physical and chemical conditions. The combination of the geometric, electronic and vibrational structures also leads to the simulations of thermodynamic potentials, entropy and phase diagrams in the solid state. This significant ensemble of research tools makes DFT calculations very compelling and useful to gain new insights into innovation developments for intercalated lamellar materials.

Combining Nuclear Magnetic Resonance Spectroscopy and Density Functional Theory Calculations to Characterize Carvedilol Polymorphs.

The experiments of carvedilol form II, form III, and hydrate by (13)C and (15)N cross-polarization magic-angle spinning (CP MAS) are reported. The GIPAW (gauge-including projector-augmented wave) method from DFT (density functional theory) calculations was used to simulate (13)C and (15)N chemical shifts. A very good agreement was found for the comparison between the global results of experimental and calculated nuclear magnetic resonance (NMR) chemical shifts for carvedilol polymorphs. This work aims a comprehensive understanding of carvedilol crystalline forms employing solution and solid-state NMR as well as DFT calculations.

Thermally activated surface oxygen defects at the perimeter of Au/TiO2: a DFT+U study.

Density functional theory calculations were performed to examine the formation of oxygen atom vacancies on three model surfaces namely, clean anatase TiO2(001) and, Au3 and Au10 clusters supported on anatase TiO2(001). On the Au/TiO2 systems, three different types of lattice oxygen atoms can be identified: the Ti-O-Au bridge, the Ti-O-Ti bridge in the perimeter of the Au cluster and the Ti-O-Ti bridge away from the Au cluster, the oxygen atoms on the clean surface. The variation in ΔG° with temperature for surface O vacancy formation was calculated for these three situations using total-energy, vibrational structure and optimized geometries of the material surfaces and the O2 molecule. The calculations reveal that the O defect formation on the clean anatase TiO2(001) surface seems very difficult due to the large positive value of ΔG° (290 kJ mol(-1)) from 0 to 650 K. However, the presence of the Au cluster on the TiO2 surface changes the surface chemistry of the TiO2 significantly. We observed that the trend in ΔG° variation for the vacancy formation from the Ti-O-Au bridge is the same as on Au3/TiO2 and Au10/TiO2 systems, almost constant with large positive values of ΔG° around 250 and 350 kJ mol(-1), respectively. The ΔG° for the perimeter defect formation (Ti-O-Ti bridge in the perimeter of the Au cluster) is smaller for Aun/TiO2 systems than the clean TiO2 surface, however, the vacancy formation is possible only for the Au10/TiO2 system (close to 506 K). Finally, extended calculations for other oxygen atoms on the Au10/TiO2 model reveal that the trend in ΔG° variation is similar for all the interface or perimeter O atoms around the Au cluster with marginal differences in the numerical value of ΔG°. Since, the surface O atoms are activated only in the presence of a particular sized Au, we propose that a Au catalyzed Mars-van Krevelen mechanism could be a possible reaction mechanism for CO oxidation on Au/TiO2 catalysts at slightly elevated temperatures.

Ab initio simulation of changes in geometry, electronic structure, and Gibbs free energy caused by dehydration of hydrotalcites containing Cl⁻ and CO3²â» counteranions.

This ab initio study was performed to better understand the correlation between intercalated water molecules and layered double hydroxides (LDH), as well as the changes that occur by the dehydration process of Zn-Al hydrotalcite-like compounds containing Cl⁻ and CO3²â» counterions. We have verified that the strong interaction among intercalated water molecules, cointercalated anions, and OH groups from hydroxyl layers is reflected in the thermal stability of these compounds. The Zn(2/3)Al(1/3)(OH)2Cl(1/3)·2/3H2O hydrotalcite loses all the intercalated water molecules around 125 °C, while the Zn(2/3)Al(1/3)(OH)2(CO3)(1/6)·4/6H2O compound dehydrates at about 175 °C. These values are in good agreement with experimental data. The interlayer interactions were discussed on the basis of electron density difference analyses. Our calculation shows that the electron density in the interlayer region decreases during the dehydration process, inducing the migration of the Cl⁻ anion and the displacement of the hydroxyl layer from adjacent layers. Changes in these compound structures occur to recover part of the hydrogen bonds broken due to the removal of water molecules. It was observed that the chloride ion had initially a lower Löwdin charge (Cl(-0.43)), which has increased its absolute value (Cl(-0.58)) after the water molecules removal, while the charges on carbonate ions remain invariant, leading to the conclusion that the Cl⁻ anion can be more influenced by the amount of water molecules in the interlayer space than the CO3²â» anion in hydrotalcite-like compounds.

Solid-state experimental and theoretical investigation of the ammonium salt of croconate violet, a pseudo-oxocarbon ion.

The present work describes the crystal structure, vibrational spectra, and theoretical calculations of ammonium salts of 3,5-bis-(dicyanomethylene)cyclopentane-1,2,4-trionate, (NH(4))(2)(C(11)N(4)O(3)) [(NH(4))(2)CV], also known as ammonium croconate violet. This compound crystallizes in triclinic P1 and contains two water molecules per unit formula. The crystal packing is stabilized by hydrogen bonds involving water molecules and ammonium cations, giving rise to a 3D polymeric arrangement. In this structure, a pi-stacking interaction is not observed, as the smaller centroid-centroid distance is 4.35 A. Ab initio electronic structure calculations under periodic boundary conditions were performed to predict vibrational and electronic properties. The vibrational analysis was used to assist the assignments of the Raman and infrared bands. The solid structure was optimized and characterized as a minimum in the potential-energy surface. The stabilizing intermolecular hydrogen bonds in the crystal structure were characterized by difference charge-density analysis. The analysis of the density of states of (NH(4))(2)CV gives an energy gap of 1.4 eV with a significant contribution of carbon and nitrogen 2p states for valence and conduction bands.

Theoretical study of the reaction between HF molecules and hydroxyl layers of Mg(OH)2.

The reaction of HF molecules with brucite, Mg(OH)(2), leading to the formation of Mg(OH)(2-x)F(x), was theoretically studied by ab initio density functional theory (DFT) with periodic boundary conditions. We proposed as mechanism for this reaction four elementary steps: adsorption of the HF molecule, OH(-) liberation from brucite as a water molecule, desorption of the newly formed H(2)O, and rearrangement of the F(-) anion into a hydroxyl position. For the Mg(OH)(2-x)F(x) formation, with x = 1/9, the final product, outcome from an initially adsorbed HF molecule, we computed the Helmholtz free energy variation DeltaF = -23 kcal/mol. The calculated frequency for the most intense infrared band, a Mg-F stretching mode, was 342 cm(-1). Two transition states, corresponding to the hydroxyl reacting with a proton forming a water molecule and migration of a fluoride anion into a hydroxyl vacancy, were computed. The calculated reaction barriers indicate that the reaction between Mg(OH)(2) layers and HF molecules is slow and irreversible.

DFT calculations of EPR parameters in an ionic lattice of [M(CN)4](3-) (M = Ni, Pd, Fe, Ru, Os) complexes.

The electronic g-tensor and hyperfine coupling constants were calculated for cyanide coordination complexes [M(CN)4]3- (M = Ni, Pd, Fe, Ru, Os) in KCl or NaCl host lattices through an embedded calculation approach using the Density Functional Theory and compared with previous experiments. For all tested complexes, the B3LYP functional is in good agreement with the experiments for the hyperfine coupling constants. For the electronic g-tensor calculations, performed using the coupled perturbed SCF theory, some discrepancies were found, and the best agreements with the experimental values were achieved by the B3LYP functional.