Porous materials displaying tailor-made pore sizes and shapes are particularly interesting in a great variety of real and potential applications where molecular recognition is needed, such as shape-selective catalysis, molecular sieving, and selective adsorption.[1±4] Classically, apart from silica, materials most commonly used for catalysis and catalyst supports have been those based on high surface aluminas, owing to their thermal, chemical, and mechanical stability and their low cost.[5] Earlier aluminas with high surface areas (~500 m 2 /g) had been prepared using structure-directing agents. However, they were X-ray amorphous materials and their porosity was purely textural, characterized by wide pore size distributions.[5] More recently, the discovery by researchers at Mobil of the M41S family of mesoporous silicas synthesized by using micellar aggregates as templates, [6,7] has promoted considerable development in the synthesis of materials with uniform pores in the mesoporous range.[8±14] However, in the case of mesoporous aluminum oxide, the usual strategies used in the synthesis of mesoporous silica have not always yielded satisfactory results and only a few papers have reported on surfactant-assisted synthesis of mesoporous alumina. Davis and co-workers [15] have reported the preparation of aluminas with narrow pore size distributions by the use of anionic surfactants but their solids always have an approximately constant pore size (ca. 20 ) that cannot be tailored by changing the surfactant length. Conversely, Pinnavaia and co-workers [16,17] report the use of neutral polyethylene oxides as directing agents for the synthesis of mesoporous solids for which both the d spacing and the pore diameters increase as the surfactant size does. In both cases, the synthetic pathway is based on typical procedures originally used for mesoporous silicas: the variation of the micelle diameter is achieved by increasing the surfactant chain length and/or addition of hydrophobic organic molecules. However, the scarcity and diversity of the reported results suggest that there is still a long way to go to obtain real control of the synthetic procedures for the preparation of mesoporous aluminas.In this context, we show that self-assembling processes leading to the formation of mesoporous aluminas can be controlled by adequately balancing such processes and the hydrolysis and condensation reactions occurring at the inorganic phase. This method has allowed us to isolate for the first time mesoporous aluminum oxides using cationic surfactants and, what is more important, to tune their pore size by the sole adjustment of the molar ratio of the reactants.Thermally stable aluminas with different pore diameters, henceforth denoted as ICMUV-1, were synthesized using CTABr (cetyltrimethylammonium bromide) as surfactantdirecting agent in a water/TEA (triethanolamine) medium. A constant 2/1 Al/CTABr molar ratio was always used, and the pore size adjustment was achieved by changing the Al (or surfactant)/water/TEA molar ratio.A typica...

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The chemical bond in complexes of the M2(formamidinate)4 type with different nominal bond orders has been investigated within the framework of the present topological theories. The atoms-in-molecules (AIM) analysis of the theoretically calculated electron density shows low ρ(r) values at the metal−metal bond critical point (r c), which makes difficult a topological description of the interaction using the electron density as the scalar function. When the electron localization function (ELF) is used instead, four disynaptic metal−metal valence basins, V(M,M), are found for the Mo and Nb dimers, one for each the Ru and Rh complexes, while no disynaptic basins are obtained for the Tc and Pd systems. The V(M,M) basins are not the dominant features of the interaction due to their low population values with the main contribution arising from the “4d” metal electrons. However, the molecular orbitals involving the “4d” function of the metal essentially contribute to the metal core basins, C(M). The most important characteristic of the metal−metal bond is the abnormally high values for the metal−metal core covariance, B(M,M), and the AIM atomic basins covariances, λc(ρ). This large electron fluctuation which occurs between the two metallic cores is interpreted in terms of simple resonance arguments. Except for Rh, there is an excellent correlation between the core covariances, B(M,M), and the metal−metal distances.

Pure mesoporous aluminum phosphonates and diphosphonates have been synthesized through an S + Isurfactant-assisted cooperative mechanism by means of a one-pot preparative procedure from aqueous solution and starting from aluminum atrane complexes and phosphonic and/or diphosphonic acids. A soft chemical extraction procedure allows opening the pore system of the parent mesostructured materials by exchanging the surfactant without mesostructure collapse. The hybrid nature of the pore wall can be modulated continuously from organic-free mesoporous aluminum phosphates (ALPOs) up to total incorporation of organophosphorus entities (mesoporous phosphonates and diphosphonates). The organic functional groups become basically attached to the pore surface or inserted into the ALPO framework (homogeneously distributed along the surface and inner pore walls) depending on the use of phosphonic or diphosphonic acids, respectively. X-ray powder diffraction, transmission electron microscopy, and surface analysis techniques show that these new hybrid materials present regular unimodal pore systems whose order decreases as the organophosphorus moiety content increases. NMR spectroscopic results not only confirm the incorporation of organophosphorus entities into the framework of these materials but also provide us useful information to elucidate the mechanism through which they are formed.

In this paper, we report the angle-dispersive x-ray diffraction data of barite, BaSO 4 , measured in a diamond-anvil cell up to a pressure of 48 GPa, using three different fluid pressure-transmitting media (methanol-ethanol mixture, silicone oil, and He). Our results show that BaSO 4 exhibits a phase transition at pressures that range from 15 to 27 GPa, depending on the pressure media used. This indicates that nonhydrostatic stresses have a crucial role in the high-pressure behavior of this compound. The new high-pressure (HP) phase has been solved and refined from powder data, having an orthorhombic P2 1 2 1 2 1 structure. The pressure dependence of the structural parameters of both room-and HP phases of BaSO 4 is also discussed in light of our theoretical first-principles total-energy calculations. Finally, a comparison between the different equations of state obtained in our experiments is reported.

The complex reaction between VO 2 + ( 1 A 1 / 3 A′′) and C 2 H 4 ( 1 A g / 3 A 1 ) to yield VO + ( 1 ∆/ 3 Σ) and CH 3 CHO ( 1 A′/ 3 A′′) has been studied by means of B3LYP/6-31G* and B3LYP/6-311G(2d,p) calculations. The structures of all reactants, products, intermediates, and transition structures of this reaction have been optimized and characterized at the fundamental singlet and first excited triplet electronic states. Crossing points are localized, and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach. Relevant stationary points along the most favorable reaction pathways have been studied at the CCSD/6-311G(2d,p)//B3LYP/6-311G(2d,p) calculation level. The theoretical results allow the development of thermodynamic and kinetic arguments about the reaction pathways of the title process. In the singlet state, the first step is the barrierless obtention of a reactant complex associated with the formation of a V-C bond, while in the triplet state a three-membered ring addition complex with the V bonded to the two C atoms is obtained. Similar behavior is found in the exit channels: the product complexes can be formed from isolated products without barriers. The reactant and product complexes are the most stable stationary points in the singlet and triplet electronic states. From the singlet state reactant complex, two reaction pathways are posssible to reach the triplet state product complex. (i) A mechanism in which a hydrogen transfer process is the first and rate limiting step and the second step is an oxygen transfer between vanadium and carbon atoms with a concomitant change in the spin state. The crossing point between singlet and triplet spin states is not kinetically relevant because it takes place at a later stage occurring in the exit channel. (ii) A mechanism in which the first stage renders a four-membered ring between vanadyl cation and the ethylene fragment and an oxygencarbon bond is formed; on going from this minimum to the second transition structure, associated with a carbon-vanadium bond breaking process, the crossing point between singlet and triplet spin states is reached. The final step is the hydrogen transfer between both carbon atoms to yield the product complex. In this case the spin change opens a lower barrier pathway. The transition structures with larger values of relative energies for both reactive channels of VO 2 + ( 1 A 1 ) + C 2 H 4 ( 1 A g ) f VO + ( 3 Σ) + CH 3 CHO ( 1 A′) present similar energies, and the two reaction pathways can be considered as competitive.

First-principles quantum-mechanical techniques, based on density functional theory ͑B3LYP level͒ were employed to study the electronic structure of ordered and deformed asymmetric models for Ba 0.5 Sr 0.5 TiO 3 . Electronic properties are analyzed and the relevance of the present theoretical and experimental results on the photoluminescence behavior is discussed. The presence of localized electronic levels in the band gap, due to the symmetry break, would be responsible for the visible photoluminescence of the amorphous state at room temperature. Thin films were synthesized following a soft chemical processing. Their structure was confirmed by x-ray data and the corresponding photoluminescence properties measured.

Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --> CaCl2-type, Pnnm (12 GPa) --> alpha-PbO2-type, Pbcn (17 GPa) --> pyrite-type, Pa (17 GPa) --> ZrO2-type orthorhombic phase I, Pbca (18 GPa) --> fluorite-type, Fmm (24 GPa) --> cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.