9 research outputs found
Production and Characterization of CarbonâSilica Nanocomposites from the Pyrolysis of Polyacrylonitrile in a Porous Silica Matrix
Ca<sub>12</sub>InC<sub>13â<i>x</i></sub> and Ba<sub>12</sub>InC<sub>18</sub>H<sub>4</sub>: Alkaline-Earth Indium Allenylides Synthesized in AE/Li Flux (AE = Ca, Ba)
Two new complex main-group metal
carbides were synthesized from
reactions of indium, carbon, and a metal hydride in metal flux mixtures
of an alkaline earth (AE = Ca, Ba) and lithium. Ca<sub>12</sub>InC<sub>13â<i>x</i></sub> and Ba<sub>12</sub>InC<sub>18</sub>H<sub>4</sub> both crystallize in cubic space group <i>Im</i>3Ì
[<i>a</i> = 9.6055(8) and 11.1447(7) Ă
, respectively].
Their related structures are both built on a body-centered-cubic array
of icosahedral clusters comprised of an indium atom and 12 surrounding
alkaline-earth cations; these clusters are connected by bridging monatomic
anions (either H<sup>â</sup> or C<sup>4â</sup>) and
allenylide anions, C<sub>3</sub><sup>4â</sup>. The allenylide
anions were characterized by Raman spectroscopy and hydrolysis studies.
Density of states and crystal orbital Hamilton population calculations
confirm that both compounds are metallic
Luminescent zero-dimensional organic metal halide hybrids with near-unity quantum efficiency
Single crystalline zero-dimensional (0D) organic-inorganic hybrid materials with perfect host-guest structures have been developed as a new generation of highly efficient light emitters. Here we report a series of lead-free organic metal halide hybrids with a 0D structure, (C4N2H14X)4SnX6 (X = Br, I) and (C9NH20)2SbX5 (X = Cl), in which the individual metal halide octahedra (SnX64â) and quadrangular pyramids (SbX52â) are completely isolated from each other and surrounded by the organic ligands C4N2H14X+ and C9NH20+, respectively. The isolation of the photoactive metal halide species by the wide band gap organic ligands leads to no interaction or electronic band formation between the metal halide species, allowing the bulk materials to exhibit the intrinsic properties of the individual metal halide species. These 0D organic metal halide hybrids can also be considered as perfect host-guest systems, with the metal halide species periodically doped in the wide band gap matrix. Highly luminescent, strongly Stokes shifted broadband emissions with photoluminescence quantum efficiencies (PLQEs) of close to unity were realized, as a result of excited state structural reorganization of the individual metal halide species. Our discovery of highly luminescent single crystalline 0D organic-inorganic hybrid materials as perfect host-guest systems opens up a new paradigm in functional materials design
Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site
The detailed mechanism by which ethylene
polymerization is initiated
by the inorganic Phillips catalyst (Cr/SiO<sub>2</sub>) without recourse
to an alkylating cocatalyst remains one of the great unsolved mysteries
of heterogeneous catalysis. Generation of the active catalyst starts
with reduction of Cr<sup>VI</sup> ions dispersed on silica. A lower
oxidation state, generally accepted to be Cr<sup>II</sup>, is required
to activate ethylene to form an organoCr active site. In this work,
a mesoporous, optically transparent monolith of Cr<sup>VI</sup>/SiO<sub>2</sub> was prepared using solâgel chemistry in order to monitor
the reduction process spectroscopically. Using in situ UVâvis
spectroscopy, we observed a very clean, stepwise reduction by CO of
Cr<sup>VI</sup> first to Cr<sup>IV</sup>, then to Cr<sup>II</sup>.
Both the intermediate and final states show XANES consistent with
these oxidation state assignments, and aspects of their coordination
environments were deduced from Raman and UVâvis spectroscopies.
The intermediate Cr<sup>IV</sup> sites are inactive toward ethylene
at 80 °C. The Cr<sup>II</sup> sites, which have long been postulated
as the end point of CO reduction, were observed directly by high-frequency/high-field
EPR spectroscopy. They react quantitatively with ethylene to generate
the organoCr<sup>III</sup> active sites, characterized by X-ray absorption
and UVâvis spectroscopy, which initiate polymerization
Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site
The detailed mechanism by which ethylene
polymerization is initiated
by the inorganic Phillips catalyst (Cr/SiO<sub>2</sub>) without recourse
to an alkylating cocatalyst remains one of the great unsolved mysteries
of heterogeneous catalysis. Generation of the active catalyst starts
with reduction of Cr<sup>VI</sup> ions dispersed on silica. A lower
oxidation state, generally accepted to be Cr<sup>II</sup>, is required
to activate ethylene to form an organoCr active site. In this work,
a mesoporous, optically transparent monolith of Cr<sup>VI</sup>/SiO<sub>2</sub> was prepared using solâgel chemistry in order to monitor
the reduction process spectroscopically. Using in situ UVâvis
spectroscopy, we observed a very clean, stepwise reduction by CO of
Cr<sup>VI</sup> first to Cr<sup>IV</sup>, then to Cr<sup>II</sup>.
Both the intermediate and final states show XANES consistent with
these oxidation state assignments, and aspects of their coordination
environments were deduced from Raman and UVâvis spectroscopies.
The intermediate Cr<sup>IV</sup> sites are inactive toward ethylene
at 80 °C. The Cr<sup>II</sup> sites, which have long been postulated
as the end point of CO reduction, were observed directly by high-frequency/high-field
EPR spectroscopy. They react quantitatively with ethylene to generate
the organoCr<sup>III</sup> active sites, characterized by X-ray absorption
and UVâvis spectroscopy, which initiate polymerization
Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state
Electronic Structure and Properties of Berkelium Iodates
The
reaction of <sup>249</sup>BkÂ(OH)<sub>4</sub> with iodate under
hydrothermal conditions results in the formation of BkÂ(IO<sub>3</sub>)<sub>3</sub> as the major product with trace amounts of BkÂ(IO<sub>3</sub>)<sub>4</sub> also crystallizing from the reaction mixture.
The structure of BkÂ(IO<sub>3</sub>)<sub>3</sub> consists of nine-coordinate
Bk<sup>III</sup> cations that are bridged by iodate anions to yield
layers that are isomorphous with those found for Am<sup>III</sup>,
Cf<sup>III</sup>, and with lanthanides that possess similar ionic
radii. BkÂ(IO<sub>3</sub>)<sub>4</sub> was expected to adopt the same
structure as MÂ(IO<sub>3</sub>)<sub>4</sub> (M = Ce, Np, Pu), but instead
parallels the structural chemistry of the smaller Zr<sup>IV</sup> cation.
Bk<sup>III</sup>âO and Bk<sup>IV</sup>âO bond lengths
are shorter than anticipated and provide further support for a postcurium
break in the actinide series. Photoluminescence and absorption spectra
collected from single crystals of BkÂ(IO<sub>3</sub>)<sub>4</sub> show
evidence for doping with Bk<sup>III</sup> in these crystals. In addition
to luminescence from Bk<sup>III</sup> in the BkÂ(IO<sub>3</sub>)<sub>4</sub> crystals, a broad-band absorption feature is initially present
that is similar to features observed in systems with intervalence
charge transfer. However, the high-specific activity of <sup>249</sup>Bk (<i>t</i><sub>1/2</sub> = 320 d) causes oxidation of
Bk<sup>III</sup> and only Bk<sup>IV</sup> is present after a few days
with concomitant loss of both the Bk<sup>III</sup> luminescence and
the broadband feature. The electronic structure of BkÂ(IO<sub>3</sub>)<sub>3</sub> and BkÂ(IO<sub>3</sub>)<sub>4</sub> were examined using
a range of computational methods that include density functional theory
both on clusters and on periodic structures, relativistic <i>ab initio</i> wave function calculations that incorporate spinâorbit
coupling (CASSCF), and by a full-model Hamiltonian with spinâorbit
coupling and SlaterâCondon parameters (CONDON). Some of these
methods provide evidence for an asymmetric ground state present in
Bk<sup>IV</sup> that does not strictly adhere to RusselâSaunders
coupling and Hundâs Rule even though it possesses a half-filled
5<i>f</i> <sup>7</sup> shell. Multiple factors contribute
to the asymmetry that include 5<i>f</i> electrons being
present in microstates that are not solely spin up, spinâorbit
coupling induced mixing of low-lying excited states with the ground
state, and covalency in the Bk<sup>IV</sup>âO bonds that distributes
the 5<i>f</i> electrons onto the ligands. These factors
are absent or diminished in other <i>f</i><sup>7</sup> ions
such as Gd<sup>III</sup> or Cm<sup>III</sup>