A theoretical study on the gas phase reactions of the anions NbO3-, NbO5-, and NbO2(0H)(2)(-) with H2O and O-2

The potential energy surfaces at the singlet (s) and the triplet (t) electronic states associated with the gas-phase ion/molecule reactions of NbO3-, NbO5-, and NbO2(OH)(2)(-) with H2O and O-2 have been investigated by means of DFT calculations at the B3LYP level. An analysis of the results points out that the most favorable reactive channel comprises s-NbO3- reacting with H2O to give an ion-molecule complex s-NbO3(H2O)without a barrier. From this minima, an intramolecular hydrogen transfer takes place between the incoming water molecule and an oxygen atom of the NbO3- fragment to render the most stable minimum, s-NbO2(OH)(2)(-). This oxyhydroxide system reacts with O-2 along a barrierless process to obtain the triplet t-NbO4(OH)(2)(-)-A intermediate, and the crossing point, CP1, between s and t electronic states has been characterized. The next step is the hydrogen-transfer process between the oxygen atom of a hydroxyl group and the one adjacent oxygen atom to render a minimum with the two OH groups near each other, t-NbO4(OH)(2)(-)-B. From this point, the last hydrogen migration takes place, to obtain the product complex, t-NbO5(H2O)(-), that can be connected with the singlet separated products, s-NbO5- and H2O. Therefore, a second crossing point, CP2, has been localized. The nature of the chemical bonding of the key minima (NbO3-, NbO2(OH)(2)(-), NbO4(OH)(2)(-)-B, and NbO5-) in both electronic states of the reaction and an interaction with O-2 has been studied by topological analysis of Becke-Edgecombe electron-localization function (ELF) and atoms-in-molecules (AIM) methodology. The niobium-oxygen interactions are characterized as unshared-electron (ionic) interactions and some oxygen-oxygen interactions as protocovalent bonds.

Mechanistic insights into the reaction between VO2+ and propene based on a DFT study

Calculations based on density functional theory have been carried out to investigate the free energy profiles at singlet and triplet electronic states associated with the gas-phase ion/molecule reactions of VO2++ ((1)A(1)/(3)A) with propene. The complex potential energy Surfaces, including Six reaction pathways (three dehydrogenation and three oxygen transfer processes), have been explored and analyzed. Along dehydrogenation reactive channels, three final products can be obtained: V(OH)(2)(+) ((1)Sigma(+)/(3)Sigma(-)) and allene (path Dehl), being the most kinetically and thermodynamically favorable reaction pathway, V(OH)(2)(+) ((1)Sigma(+)/(3)Sigma(-)) and propyne (path Deh2),and VO2+ ((1)A(1)/(3)A) and H-2 plus allene (path Deh3). The oxyoenation processes can yield its final products Vo(+) ((1)Delta/(3)Sigma) and acetone (path Ox1), VO+ ((1)Delta/(3)Sigma 2) and propanaldehyde (path Ox2), and VO+ ((1)Delta/(3)Sigma) and H-2 and propenaldehyde (path Ox3). Both paths Deh1 and Deh2 are associated with two consecutive hydrogen transfer processes from carbon atoms of the propene fragment to vanadyl oxygen atoms, while in path Deh3 the second hydrogen migration takes place to the vanadiurn atorn followed by the formation ola hydrogen molecule. Both paths Ox1 and Ox2 comprise an intramolecular hydrogen transfer between the ethylenic moiety of the propene fragment, while two consecutive hydrogen transfer processes take place from the propene fragment to oxygen and vanadium atoms of the vanadyl moiety along path Ox3. Three crossing points between both electronic states take place along path Deh1 (CP-Deh1) and path Deh2 (CP-Deh2) and in the entrance channel of oxidation processes (CP-Ox). A comparison with previous works on related reactions VO2+ + C2H4, VO2 + C2H6, and VO2+ + C3H8 allows us to rationalize the different reactivity patterns.

Efficient energy upconversion emission in Tm3+/Yb3+-codoped TeO2-based optical glasses excited at 1.064 mu m

Efficient energy upconversion of cw radiation at 1.064 mum into blue, red, and near infrared emission in Tm3+-doped Yb3+-sensitized 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) glasses is reported. Intense blue upconversion luminescence at 485 nm corresponding to the Tm3+ (1)G(4)--> H-3(6) transition with a measured absolute power of 0.1 muW for 800 mW excitation power at room temperature is observed. The experimental results also revealed a sevenfold enhancement in the upconversion efficiency when the sample was heated from room temperature to 235 degreesC yielding 0.7 muW of blue absolute fluorescence power for 800 mW pump power. High brightness emission around 800 nm (F-3(4)--> H-3(6)) in addition to a less intense 655 nm ((1)G(4)--> H-3(4) and F-3(2,3)--> H-3(6)) fluorescence is also recorded. The energy upconversion excitation mechanism for thulium emitting levels is assigned to multiphonon-assisted anti-Stokes excitation of the ytterbium-sensitizer followed by multiphonon-assisted sequential energy-transfer processes. (C) 2001 American Institute of Physics.

Two thermal methods to measure the energy fluence of a brief exposure of diagnostic x rays

This paper describes two simple thermal methods for measuring the energy fluence in J/cm 2 from a diagnostic x-ray exposure. Both detectors absorb essentially 100% of the radiation and give a signal that is directly proportional to the energy fluence of the x-ray beam. One detector measures the thermal effect when a pulse of x rays is totally absorbed in the pyroelectric detector of lead-zirconium-titanate (PZT). The other detector measures the expansion of a gas surrounding a lead disk detector in a photoacoustic chamber. The increased pressure of the gas is transmitted through a 1-mm duct to a sensitive microphone. Both detectors have previously been used to measure the energy fluence rate of continuous x-ray beams in the same energy region using a chopped beam and a lock-in amplifier. Measurement of the energy fluence of a pulse of radiation eliminates the need for the beam chopper and lock-in amplifier and results in a simple, rugged, and inexpensive dosimeter. Either method can be combined with the area of the beam to give an estimate of the imparted energy to the patient from a diagnostic x-ray exposure.

First crystal structures of lanthanide-hydrocinnamate complexes: Hydrothermal synthesis and photophysical studies

Five new lanthanide(III) complexes of hydrocinnamic acid (Hcin), [Ln(cin)3(H2O)3]·3Hcin (Ln = Tb(III) (1), Dy(III) (2), Er(III) (3), Eu(III) (4) and Gd(III) (5)) have been synthesized and characterized. The X-ray structures of 1-5 reveal that all compounds are isostructural and that each lanthanide ion is nine-coordinated by oxygen atoms in an overall distorted tricapped trigonal-prismatic geometry. Six oxygen atoms are provided by carboxylate moieties, and the other three by water molecules. The supramolecular architectures of 1-5 show the presence of uncoordinated hydrocinnamic acid molecules which induce the formation of numerous hydrogen bonds. The photophysical properties of these complexes in the solid state at room temperature were studied using diffuse reflectance (DR), fluorescence excitation and emission spectra. An energy level diagram was used to establish the most relevant channels involved in the ligand-to-metal energy transfer, indicating that cin- ligands can act as intramolecular energy donors for Tb(III), Dy(III) and Eu(III) ions. © 2012 Elsevier B.V.

Visible to infrared energy conversion in Pr3+-Yb3+ co-doped fluoroindate glasses

Processes involving visible to infrared energy conversion are presented for Pr3+-Yb3+ co-doped fluoroindate glasses. The emission in the visible and infrared regions, the luminescence decay time of the Pr 3+:3P0 → 3H4 (482 nm), Pr3+:1D2 → 3H6 (800 nm), Yb3+:2F5/2 → 2F 7/2 (1044 nm) transitions and the photoluminescence excitation spectra were measured in Pr3+ samples and in Pr3+-Yb 3+ samples as a function of the Yb3+ concentration. In addition, energy transfer efficiencies were estimated from Pr3+: 3P0 and Pr3+:1D2 levels to Yb3+:2F7/2 level. Down-Conversion (DC) emission is observed due to a combination of two different processes: 1-a one-step cross relaxation (Pr3+:3P0 → 1G4; Yb3+:2F7/2 → 2F5/2) resulting in one photon emitted by Pr3+ (1G4 → 3H5) and one photon emitted by Yb3+ (2F7/2 → 2F5/2); 2-a resonant two-step first order energy transfer, where the first part of energy is transferred to Yb3+ neighbor through cross relaxation (Pr3+:3P0 → 1G4; Yb3+:2F7/2 → 2F5/2) followed by a second energy transfer step (Pr 3+:1G4 → 3H4; Yb3+:2F7/2 → 2F5/2). A third process leading to one IR photon emission to each visible photon absorbed involves cross relaxation energy transfer (Pr3+: 1D2 → 3F4; Yb 3+:2F7/2 → 2F5/2). © 2013 Elsevier B.V. All rights reserved.

Synthesis, structural characterization and photophysical properties of highly photoluminescent crystals of Eu(III), Tb(III) and Dy(III) with 2,5-thiophenedicarboxylate

Lanthanide compounds of general formula [Ln2(2,5-tdc) 3(dmf)2(H2O)2] ·2dmf·H2O (Ln = Eu(III) (1), Tb(III) (2), Gd(III) (3) and Dy(III) (4), dmf = N,N′-dimethylformamide and 2,5-tdc2- = 2,5-thiophedicarboxylate anion) were synthesized and characterized by elemental analysis, X-ray powder diffraction patterns, thermogravimetric analysis and infrared spectroscopy. Phosphorescence data of Gd(III) complex showed that the triplet states (T1) of 2,5-tdc2- ligand have higher energy than the main emitting states of Eu(III), Tb(III) and Dy(III), indicating that 2,5-tdc2- ligand can act as intramolecular energy donor for these metal ions. An energy level diagram was used to establish the most relevant channels involved in the ligand-to-metal energy transfer. The high value of experimental intensity parameter Ω2 for the Eu(III) complex indicate that the europium ion is in a highly polarizable chemical environment. The emission quantum efficiency (η) of the 5D0 emitting level of Eu(III) was also determined. The complexes act as possible light conversion molecular devices (LCMDs). © 2013 Elsevier B.V. All rights reserved.

Photoluminescence of Eu(3+) ion in SnO(2) obtained by sol-gel

Photoluminescence data of Eu-doped SnO(2) xerogels are presented, yielding information on the symmetry of Eu(3+) luminescent centers, which can be related to their location in the matrix: at lattice sites, substituting to Sn(4+), or segregated at particles surface. Influence of doping concentration and/or particle size on the photoluminescence spectra obtained by energy transfer from the matrix to Eu(3+) sites is investigated. Results show that a better efficiency in the energy transfer processes is obtained for high symmetry Eu(3+) sites and low doping levels. Emission intensity from (5)D(0) -> (7)F(1) transition increases as the temperature is raised from 10 to 240 K, under excitation at 266 nm laser line, because in this transition the multiphonon emission becomes significant only above 240 K. As an extension of this result, we predict high effectiveness for room temperature operation of Eu-based optical communication devices. X-ray diffraction data show that the impurity excess inhibits particle growth, which may influence the asymmetry ratio of luminescence spectra.

DFT study of alpha-alanine as a function of the medium polarity

The zwitterionic (Z) form, neutral (N) form and transition structure (TS) connecting N to Z, have been studied at the B3LYP/6-31++G** level of calculation by using the SCRF methodology. The intramolecular proton transfer from oxygen to nitrogen atoms of alpha -alanine and vibrational spectrum were analyzed in the different environments employed: acetonitrile, ethanol, carbon tetrachloride and gas phase. The Z species is a stationary point in acetonitrile and ethanol, but not in carbon tetrachloride and gas phase media. The geometry of N, Z and TS was similar in acetonitrile and ethanol. The vibrational spectrum of Z was similar in the two solvents studied. (C) 2001 Elsevier B.V. B.V. All lights reserved.

Molecular assay optimized by Taguchi experimental design method for venous thromboembolism investigation

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Biochemical and Functional Characterization of a Metalloprotease from the Thermophilic Fungus Thermoascus aurantiacus

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Purification and characterization of a new alkaline serine protease from the thermophilic fungus Myceliophthora sp.

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Chaotic thermalization in Yang-Mills-Higgs theory on a spacial lattice

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Excitation mechanisms and effects of dopant concentration in Gd2O2S : Tb3+ phosphor

Gadolinium oxysulfide powders doped with different Tb3+ concentrations were prepared from sulfur vaporization on rare earths' basic carbonate precursors. Single-phase Gd2O2S samples were obtained, with Tb3+ doping up to 9 at%. The study of the excitation mechanisms revealed that the Tb3+ emission might occur after the direct Tb3+ excitation either by energy transfer from Gd3+ or from the phosphor host. The characteristic terbium emission lines were observed, resulting from the radiative decay from D-5(3) or D-5(4), to F-7(j) levels. The cross-relaxation phenomenon was observed and its effects on the materials emission color were discussed based on the CIE diagram. By using time-resolved spectroscopy, D-5(3) -> F-7(J) and D-5(4) -> F-7(J) transitions were separated. (c) 2007 Elsevier B.V. All rights reserved.

Pump excited state absorption in holmium-doped fluoride glass

The primary excited state absorption processes relating to the (5)I(6) -> (5)I(7) 3 mu m laser transition in singly Ho(3+)-doped fluoride glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the (5)I(6) and (5)I(7) energy levels established the occurrence of two excited state absorption transitions from these energy levels that compete with previously described energy transfer upconversion processes. The (5)I(7) -> (5)I(4) excited state absorption transition has peak cross sections at 1216 nm (sigma(esa)=2.8x10(-21) cm(2)), 1174 nm (sigma(esa)=1x10(-21) cm(2)), and 1134 nm (sigma(esa)=7.4x10(-22) cm(2)) which have a strong overlap with the (5)I(8) -> (5)I(6) ground state absorption. on the other hand, it was established that the excited state absorption transition (5)I(6) -> (5)S(2) had a weak overlap with ground state absorption. Using numerical solution of the rate equations, we show that Ho(3+)-doped fluoride fiber lasers employing pumping at 1100 nm rely on excited state absorption from the lowest excited state of Ho(3+) to maintain a population inversion and that energy transfer upconversion processes compete detrimentally with the excited state absorption processes in concentrated Ho(3+)-doped fluoride glass. (c) 2008 American Institute of Physics.