324 research outputs found
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Gas Phase Hydrolysis and Oxo-Exchange of Actinide Dioxide Cations: Elucidating Intrinsic Chemistry from Protactinium to Einsteinium.
Gas-phase bimolecular reactions of metal cations with water provide insights into intrinsic characteristics of hydrolysis. For the actinide dioxide cations, actinyl(V) AnO2 + , melding of experiment and computation provides insights into trends for hydrolysis, as well as for oxo-exchange between actinyls and water that proceeds by a hydrolysis pathway. Here this line of inquiry is further extended into the actinide series with CCSD(T) computations of potential energy surfaces, for the reaction pathway for oxo-exchange through hydrolysis of nine actinyl(V) ions, from PaO2 + to EsO2 + . The computed surfaces are in accord with previous experimental results for oxo-exchange, and furthermore predict spontaneous exchange for CmO2 + , BkO2 + , CfO2 + and EsO2 + , but not for AmO2 + . Natural Bond Order analysis of the species involved in both hydrolysis and oxo-exchange reveals an inverse correlation between the barrier to hydrolysis and the charge on the actinide centre, q(An). Based on this correlation, it can be concluded that hydrolysis, and related phenomena such as oxo-exchange, become less favourable as the charge on the metal centre decreases. The new results provide a straightforward rationalization of trends across a wide swathe of the actinide series
Spin dynamics of strongly-doped La_{1-x}Sr_xMnO_3
Cold neutron triple-axis measurements have been used to investigate the
nature of the long-wavelength spin dynamics in strongly-doped
LaSrMnO single crystals with =0.2 and 0.3. Both systems
behave like isotropic ferromagnets at low T, with a gapless ( meV)
quadratic dispersion relation . The values of the spin-wave
stiffness constant are large ( = 166.77 meV for =0.2
and D = 175.87 meV for =0.3), which directly shows that the
electron transfer energy for the band is large. exhibits a power law
behavior as a function of temperature, and appears to collapse as T -> T_C.
Nevertheless, an anomalously strong quasielastic central component develops and
dominates the fluctuation spectrum as T -> T_C. Bragg scattering indicates that
the magnetization near exhibits power law behavior, with for both systems, as expected for a three-dimensional ferromagnet.Comment: 4 pages (RevTex), 3 figures (encapsulated postscript
Structure and Spin Dynamics of LaSrMnO
Neutron scattering has been used to study the structure and spin dynamics of
LaSrMnO. The magnetic structure of this system is
ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities
and new superlattice peaks consistent with the onset of a spin-canted phase
below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0,
0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a
concomitant lattice distortion. The long-wavelength magnetic excitations are
found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic
dispersion relation . The spin stiffness constant D has a
dependence at low T, and the damping at small q follows . An
anomalously strong quasielastic component, however, develops at small wave
vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At
larger q, on the other hand, the magnetic excitations become heavily damped at
low temperatures, indicating that spin waves in this regime are not eigenstates
of the system, while raising the temperature dramatically increases the
damping. The strength of the spin-wave damping also depends strongly on the
symmetry direction in the crystal. These anomalous damping effects are likely
due to the itinerant character of the electrons.Comment: 8 pages (RevTex), 9 figures (encapsulated postscript
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Activation of Water by Pentavalent Actinide Dioxide Cations: Characteristic Curium Revealed by a Reactivity Turn after Americium.
Swapping of an oxygen atom of water with that of a pentavalent actinide dioxide cation, AnO2+ also called an "actinyl", requires activation of an An-O bond. It was previously found that such oxo exchange in the gas phase occurs for the first two actinyls, PaO2+ and UO2+, but not the next two, NpO2+ and PuO2+. The An-O bond dissociation energies (BDEs) decrease from PaO2+ to PuO2+, such that the observation of a parallel decrease in the An-O bond reactivity is intriguing. To elucidate oxo exchange, we here extend experimental studies to AmO2+, americyl(V), and CmO2+, curyl(V), which were produced in remarkable abundance by electrospray ionization of Am3+ and Cm3+ solutions. Like other AnO2+, americyl(V) and curyl(V) adsorb up to four H2O molecules to form tetrahydrates AnO2(H2O)4+ with the actinide hexacoordinated by oxygen atoms. It was found that AmO2+ does not oxo-exchange, whereas CmO2+ does, establishing a "turn" to increasing the reactivity from americyl to curyl, which validates computational predictions. Because oxo exchange occurs via conversion of an actinyl(V) hydrate, AnO2(H2O)+, to an actinide(V) hydroxide, AnO(OH)2+, it reflects the propensity for actinyl(V) hydrolysis: PaO2+ hydrolyzes and oxo-exchanges most easily, despite the fact that it has the highest BDE of all AnO2+. A reexamination of the computational results for actinyl(V) oxo exchange reveals distinctive properties and chemistry of curyl(V) species, particularly CmO(OH)2+
Electrochemically driven efficient enzymatic conversion of CO2 to formic acid with artificial cofactors
Enzymatic reduction of CO2 to formic acid with the enzyme formate dehydrogenase (FDH) and a cofactor is a promising method for CO2 conversion and utilization. However, the natural cofactor nicotinamide adenine dinucleotide (NADH) shows some drawbacks such as a low reduction efficiency and forms isomers or dimers (1,6 - NADH or NAD dimer) in the regeneration reaction. To overcome them and to improve the production of formic acid, in this work, the artificial cofactors, i.e., the bipyridinium-based salts of methyl viologen (MV2+), 1,1’-dicarboxymethyl-4,4’-bipyridinium bromine (DC2+), and 1,1’-diaminoethyl-4,4’-bipyridinium bromine (DA2+), were used to replace NADH, and the effect of different functional groups on the electrochemical regeneration and catalytic performance in the enzymatic reaction was studied systematically. Also, studies using the natural cofactor NADH were carried out for comparison. It was found that the cofactor with amino groups showed the highest catalytic efficiency (kcat/Km) of 0.161 mM-1min-1, which is 536 times higher than that of the natural cofactor NADH. Molecular Dynamics simulations were conducted to give further molecular insight into the behavior of the cofactors. Analyzing the free energy profiles of the complexes between CO2 in the FDH active site with different artificial cofactors indicated that the artificial cofactor with the amino groups had the highest affinity for CO2, being consistent with the experimental observations
Remarkably High Stability of Late Actinide Dioxide Cations: Extending Chemistry to Pentavalent Berkelium and Californium.
Actinyl chemistry is extended beyond Cm to BkO2+ and CfO2+ through transfer of an O atom from NO2 to BkO+ or CfO+ , establishing a surprisingly high lower limit of 73 kcal mol-1 for the dissociation energies, D[O-(BkO+ )] and D[O-(CfO+ )]. CCSD(T) computations are in accord with the observed reactions, and characterize the newly observed dioxide ions as linear pentavalent actinyls; these being the first Bk and Cf species with oxidation states above IV. Computations of actinide dioxide cations AnO2+ for An=Pa to Lr reveal an unexpected minimum for D[O-(CmO+ )]. For CmO2+ , and AnO2+ beyond EsO2+ , the most stable structure has side-on bonded η2 -(O2 ), as AnIII peroxides for An=Cm and Lr, and as AnII superoxides for An=Fm, Md, and No. It is predicted that the most stable structure of EsO2+ is linear [O=EsV =O]+ , einsteinyl, and that FmO2+ and MdO2+ , like CmO2+ , also have actinyl(V) structures as local energy minima. The results expand actinide oxidation state chemistry, the realm of the distinctive actinyl moiety, and the non-periodic character towards the end of the periodic table
Magnon Broadening Effect by Magnon-Phonon Interaction in Colossal Magnetoresistance Manganites
In order to study the magnetic excitation behaviors in colossal
magnetoresistance manganites, a magnon-phonon interacting system is
investigated. Sudden broadening of magnon linewidth is obtained when a magnon
branch crosses over an optical phonon branch. Onset of the broadening is
approximately determined by the magnon density of states. Anomalous magnon
damping at the brillouine zone boundary observed in low Curie temperature
manganites is explained.Comment: 4 pages incl. 4 figs. New e-mail: [email protected]
SOLVENT EFFECTS ON THE ABSORPTION AND FLUORESCENCE SPECTRA OF Er(III) (AZO-DYES AND SCHIFF BASES) COMPLEXES: DETERMINATION OF GROUND AND EXCITED STATE DIPOLE MOMENT
Absorption and fluorescence emission spectra of some thermally stable Er(III) mononuclear complexes of azo dyes, namely o, o` -dihydroxy azobenzene (DAB), o, o` -dihydroxy 4-Cl-phenylazonaphatalene (P1) and Schiff bases o, o` -dihydroxy phenylazomethinnaphatalene (D2), o-hidroxy -o` -carboxy (D3), were recorded in solvents with different solvent dielectric constant ε and refractive index n. The solvatochromic shift method was used to determine the experimental ground and excited state dipole moments of the complexes. All the complexes were proved to be more polar in the excited state as compared with the ground state
The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7
A system of strongly-interacting electron-lattice polarons can exhibit charge
and orbital order at sufficiently high polaron concentrations. In this study,
the structure of short-range polaron correlations in the layered colossal
magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by
a crystallographic analysis of broad satellite maxima observed in diffuse X-ray
and neutron scattering data. The resulting q=(0.3,0,1) modulation is a
longitudinal octahedral-stretch mode, consistent with an incommensurate
Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual
orbital-stripe pattern parallel to the directions.Comment: Reformatted with RevTe
Safety of drotrecogin alfa (activated) treatment in patients with severe sepsis on renal replacement therapy without additional anticoagulation
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