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Filipe A Almeida Paz

Publications and source records attributed to Filipe A Almeida Paz.

15 recordsLinked to original sources
PubMed2006

One-dimensional silver(I) chain of lacunary alpha-Keggin anions.

A novel chain-like silver polyoxotungstophosphate is formed when Ag(I) metal centres, exhibiting an unusual eight-coordination fashion, bridge a monolacunary [PW11O39]7- anion to four bridging mu2-oxygen atoms of a neighbouring lacunary alpha-Keggin anion.

Journal Article↗
PubMed2006

Characterization of microporous aluminophosphate IST-1 using (1)H Lee-Goldburg techniques.

The presence of two independent methylamine species in microporous aluminophosphate IST-1 (|(CH(3)NH(2))(4)(CH(3)NH(+)(3))(4)(OH(-))(4)|[Al(12)P(12)O(48)]) has been shown previously by synchrotron powder X-ray diffraction. One of these species, [N(1)-C(1)], links to a six-coordinated framework Al-atom [Al(1)], while the other methylamine [N(2)-C(2)] is protonated and hydrogen-bonded to three O-atoms [O(1), O(2) and O(12)]. We revisit the structure of IST-1 and report the complete assignment of the (1)H NMR spectra by combining X-ray data and high-resolution heteronuclear/homonuclear solid-state NMR techniques based on frequency-switched Lee-Goldburg homonuclear decoupling and (31)P-(31)P homonuclear recoupling. Careful analysis of the 2D (1)H-X homonuclear correlation (X=(1)H) and 2D heteronuclear correlation (X=(13)C, (31)P and (27)Al) spectra allowed the distinction of both methylamine species and the assignment of all (31)P and (13)C resonances. For the first time at a relatively high (9.4 T) magnetic field, symmetric doublet patterns have been observed in the (13)C spectra, caused by the influence of the (14)N second-order quadrupolar interaction.

Journal Article↗
PubMed2006

Spectroscopic study of a UV-photostable organic-inorganic hybrids incorporating an Eu3+ beta-diketonate complex.

New europium and gadolinium tris-beta-diketonate complexes have been prepared and incorporated in sol-gel-derived organic-inorganic hybrids, named di-ureasils. The general formula [Ln(btfa)3(4,4'-bpy)(EtOH)] (Ln=Eu, Gd; 4,4'-bpy=4,4'-bipyridine; btfa=4,4,4-trifluoro-1-phenyl-1,3-butanedione) for the complexes was confirmed by X-ray crystallography and elemental analysis. The ground-state geometry of the Eu3+ complex was calculated from the Sparkle/AM1 model. The calculated quantum yield obtained from the Sparkle model and from the crystal structure (both 46%) are in satisfactory agreement with the experimental value (38+/-4%). In the isolated complex the most efficient luminescence channel is S0-->S1-->T-->(5D1, 5D0)-->7F0-6, where the exchange mechanism dominates in the energy-transfer channel T-->(5D1, 5D0). For the Eu3+-based di-ureasils a 50% quantum yield enhancement compared to the Eu3+ complex is observed, which suggests an effective hybrid host-metal ion interaction and an active energy-transfer channel between the hybrid host and the Eu3+ complex. The Eu3+-based di-ureasils are photostable under UVA (360 nm) excitation, whereas under UVB (320 nm) and UVC (290 nm) photodegradation occurs.

Journal Article↗
PubMed2005

X-ray diffraction and solid-state NMR studies of a germanium binuclear complex.

A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).

Journal Article↗
PubMed2005

Hypothetical binodal zeolitic frameworks.

Hypothetical binodal zeolitic structures (structures containing two kinds of tetrahedral sites) were systematically enumerated using tiling theory and characterized by computational chemistry methods. Each of the 109 refineable topologies based on "simple tilings" was converted into a silica polymorph and its energy minimized using the GULP program with the Sanders-Catlow silica potential. Optimized structural parameters, framework energies relative to alpha-quartz and volumes accessible to sorption have been calculated. Eleven of the 30 known binodal topologies listed in the Atlas of Zeolite Framework Types were found, leaving 98 topologies that were unknown previously. The chemical feasibility of each structure as a zeolite was evaluated by means of a feasibility factor derived from the correlation between lattice energy and framework density. Structures are divided into 15 families, based on common structural features. Many "feasible" structures contain only small pores. Several very open structures were also enumerated, although they contain three-membered rings which are thermodynamically disfavoured and not found in conventional zeolites. We believe that such topologies may be realizable as framework materials, but with different elemental compositions to those normally associated with zeolites.

Computing Methodologies↗
PubMed2004

A novel microporous copper silicate: Na2Cu2Si4O11.2H2O.

The synthesis and structural characterisation of a thermally stable novel three-dimensional microporous copper silicate open-framework are described; the material is capable of undergoing reversible zeolitic water removal without destruction of the framework.

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PubMed2004

Photoluminescent layered lanthanide silicates.

The hydrothermal synthesis and structural characterization of layered lanthanide silicates, K(3)[M(1-a)Ln(a)Si(3)O(8)(OH)(2)] (M = Y(3+), Tb(3+); Ln = Eu(3+), Er(3+), Tb(3+), and Gd(3+)), named AV-22 materials, are reported. The structure of these solids was elucidated by single-crystal (180 K) and powder X-ray diffraction and further characterized by chemical analysis, thermogravimetry, scanning electron microscopy, (29)Si MAS NMR, and photoluminescence spectroscopy. The Er-AV-22 material is a room-temperature infrared phosphor, while Tb- and Eu-AV-22 are visible emitters with output efficiencies comparable to standards used in commercial lamps. The structure of these materials allows the inclusion of a second (or even a third) type of Ln(3+) ion in the framework and, therefore, the fine-tuning of their photoluminescent properties. For the mixed Tb(3+)/Eu(3+) materials, evidence has been found of the inclusion of Eu(3+) ions in the interlayer space by replacing K+ ions, further allowing the activation of Tb(3+)-to-Eu(3+) energy transfer mechanisms. The occurrence probability of such mechanisms ranges from 0.62 (a = 0.05) to 1.20 ms(-1) (a = 0.1) with a high energy transfer efficiency (0.73 and 0.84, respectively).

Journal Article↗
PubMed2004

Chemical evaluation of hypothetical uninodal zeolites.

Optimized structural parameters, framework energies relative to alpha-quartz, and volumes accessible to sorption have been calculated for the systematically enumerated hypothetical uninodal zeolitic structures (structures in which all tetrahedral sites are equivalent). The structures were treated as silica polymorphs, and their energies were minimized using the GULP program with the Sanders-Catlow silica potential. Results are given for 164 structures, which include all 21 known uninodal zeolites, two known minerals (tridymite and cristobalite), and 78 unknown zeolite topologies. Twenty-three hypothetical structures were identified as chemically feasible. Complete structural information is provided, and several structures are discussed in detail. The results will assist in the design of new synthetic routes and in the identification of newly synthesized materials.

Journal Article↗
PubMed2004

Two- and three-dimensional cadmium-organic frameworks with trimesic acid and 4,4'-trimethylenedipyridine.

Three novel cadmium-organic frameworks built-up from 1,3,5-benzenetricarboxylate anions (HXBTC(x-3)) and 4,4'-trimethylenedipyridine (TMD) have been hydrothermally synthesized, and characterized using single-crystal X-ray diffraction, thermoanalytical measurements, elemental analysis, and IR and Raman spectroscopies: [Cd(HBTC)(TMD)(2)].8.5H(2)O (I), [Cd(HBTC)(TMD)(H(2)O)].4.5H(2)O (II), and [Cd(2)(BTC)(TMD)(2)(NO(3))].3H(2)O (III), with structures I and II being isolated as a mixture of crystals. Structure I contains an undulating infinite two-dimensional [Cd(HBTC)(TMD)(2)] framework, with a (4,4) topology and rectangular pores, ca. 3.4 x 11.0 A in cross-section, distributed in a herringbone manner. The crystal structure of I is obtained by parallel packing of this 2D framework in an [ABAB.] fashion. Compound II has a porous 3D diamondoid framework with channels running in several directions of the unit cell, which allows 2-fold interpenetration to occur. The most prominent channels are distributed in a brick-wall fashion along the c axis and have a cross-section of ca. 3.2 x 13.2 A. Structure III can be seen as the three-dimensional assembly of binuclear secondary building units (SBU), which leads to a compact, neutral, and coordinatively bonded eight-connected framework, [Cd(2)(BTC)(TMD)(2)(NO(3))], exhibiting an unusual 3(6)4(22) topology. The increased flexibility of the TMD ligands (brought about by the three methylene groups between the two 4-pyridyl rings) can lead, for the same reactive system, to a large variety of crystal architectures.

Journal Article↗
PubMed2004

Synthesis and characterization of a novel cadmium-organic framework with trimesic acid and 1,2-bis(4-pyridyl)ethane.

The hydrothermal reaction between Cd(NO(3))(2), trimesic acid (H(3)BTC), 1,2-bis(4-pyridyl)ethane (BPE), and triethylamine under mild conditions yielded, after 3 days, a novel three-dimensional metal-organic framework, [Cd(1.5)(BTC)(BPE)(H(2)O)(2)].(H(2)O), which has been characterized structurally using single-crystal and powder X-ray diffraction, elemental analysis, infrared and Raman spectroscopies, thermogravimetry, and differential scanning calorimetry. The structure exhibits a 2-fold interpenetration of identical [Cd(1.5)(BTC)(BPE)(H(2)O)(2)] single frameworks, described as an unusual (9) net. Crystal data: Cd(1.5)C(21)H(21)N(2)O(9), monoclinic, space group C2/c, with a = 10.8264(5) A, b = 17.4563(5) A, c = 24.2605(11) A, beta = 91.978(2) degrees, V = 4582.2(3) A(3), and Z = 8.

Journal Article↗
PubMed2004

Chemically feasible hypothetical crystalline networks.

Our systematic enumeration of 4-connected crystalline networks (that is, networks in which each atom is connected to exactly four neighbours) used recent advances in tiling theory to evolve over 900 topologies. The results are relevant to the structures of zeolites and other silicates, aluminophosphates (AlPOs), oxides, nitrides, chalcogenides, halides, carbon networks, and even to polyhedral bubbles in foams. Given their importance as molecular sieves, ion exchangers, catalysts and catalyst supports, we have applied the results to microporous aluminosilicates and aluminophosphates (zeolites). Zeolite chemistry has to date produced 152 distinct types of structure. However, it was always clear that although many further structures can be synthesised, only a fraction of the mathematically generated networks would be chemically feasible (many are 'strained' frameworks requiring unrealistic bond lengths and bond angles), and that an effective 'filtering' process is needed to identify the most plausible frameworks. Here, we describe the use of computational chemistry methods to calculate optimized structural parameters, framework energies relative to alpha-quartz, volumes accessible to sorption, and X-ray diffraction patterns for systematically enumerated hypothetical 4-connected crystalline frameworks. Structures were treated as silica polymorphs with the empirical formula SiO(2), and their energies were minimized.

Crystallization↗
PubMed2002

A novel one-dimensional coordination polymer with Cd2+ and diethylenetriaminepentaacetic acid.

catena-Poly[[[bis[diaqua(4,4'-bipyridine)cadmium(II)]-bis[mu-(N"-carboxymethyldiethylenetriamine-N,N,N',N"-tetraacetato)cadmium(II)]]-mu-4,4'-bipyridine] tetradecahydrate], [Cd(4)(C(14)H(19)N(3)O(10))(2)(C(10)H(8)N(2))(3)(H(2)O)(4)].14H(2)O or [Cd(4)(HDTPA)(2)(BPY)(3)(H(2)O)(4)].14H(2)O, where BPY is 4,4'-bipyridine and HDTPA(4-) is N"-carboxymethyldiethylenetriamine-N,N,N',N"-tetraacetate, consists of a one-dimensional coordination polymer formed from a secondary building unit which comprises four Cd centres. The chain structure of the title compound was obtained by the use of a multidentate organic ligand, N,N,N',N",N"-diethylenetriaminepentaacetic acid (H(5)DTPA), which forms multiple chelate rings with the Cd metal centres. An extended network is formed via hydrogen bonds.

Journal Article↗