32 research outputs found
Incorporation of Technetium into Spinel Ferrites
Technetium
(<sup>99</sup>Tc) is a problematic fission product for
the long-term disposal of nuclear waste due to its long half-life,
high fission yield, and to the environmental mobility of pertechnetate,
the stable species in aerobic environments. One approach to preventing <sup>99</sup>Tc contamination is using sufficiently durable waste forms.
We report the incorporation of technetium into a family of synthetic
spinel ferrites that have environmentally durable natural analogs.
A combination of X-ray diffraction, X-ray absorption fine structure
spectroscopy, and chemical analysis reveals that Tc(IV) replaces Fe(III)
in octahedral sites and illustrates how the resulting charge mismatch
is balanced. When a large excess of divalent metal ions is present,
the charge is predominantly balanced by substitution of Fe(III) by
M(II). When a large excess of divalent metal ions is absent, the charge
is largely balanced by creation of vacancies among the Fe(III) sites
(maghemitization). In most samples, Tc is present in Tc-rich regions
rather than being homogeneously distributed
Influence of Pyrazolate vs <i>N</i>‑Heterocyclic Carbene Ligands on the Slow Magnetic Relaxation of Homoleptic Trischelate Lanthanide(III) and Uranium(III) Complexes
Two
isostructural series of trigonal prismatic complexes, M(Bp<sup>Me</sup>)<sub>3</sub> and M(Bc<sup>Me</sup>)<sub>3</sub> (M = Y,
Tb, Dy, Ho, Er, U; [Bp<sup>Me</sup>]<sup>−</sup> = dihydrobis(methypyrazolyl)borate;
[Bc<sup>Me</sup>]<sup>−</sup> = dihydrobis(methylimidazolyl)borate)
are synthesized and fully characterized to examine the influence of
ligand donor strength on slow magnetic relaxation. Investigation of
the dynamic magnetic properties reveals that the oblate electron density
distributions of the Tb<sup>3+</sup>, Dy<sup>3+</sup>, and U<sup>3+</sup> metal ions within the axial ligand field lead to slow relaxation
upon application of a small dc magnetic field. Significantly, the
magnetization relaxation is orders of magnitude slower for the <i>N</i>-heterocyclic carbene complexes, M(Bc<sup>Me</sup>)<sub>3</sub>, than for the isomeric pyrazolate complexes, M(Bp<sup>Me</sup>)<sub>3</sub>. Further, investigation of magnetically dilute samples
containing 11–14 mol % of Tb<sup>3+</sup>, Dy<sup>3+</sup>,
or U<sup>3+</sup> within the corresponding Y<sup>3+</sup> complex
matrix reveals thermally activated relaxation is favored for the M(Bc<sup>Me</sup>)<sub>3</sub> complexes, even when dipolar interactions are
largely absent. Notably, the dilute species U(Bc<sup>Me</sup>)<sub>3</sub> exhibits <i>U</i><sub>eff</sub> ≈ 33 cm<sup>–1</sup>, representing the highest barrier yet observed for
a U<sup>3+</sup> molecule demonstrating slow relaxation. Additional
analysis through lanthanide XANES, X-band EPR, and <sup>1</sup>H NMR
spectroscopies provides evidence that the origin of the slower relaxation
derives from the greater magnetic anisotropy enforced within the strongly
donating <i>N-</i>heterocyclic carbene coordination sphere.
These results show that, like molecular symmetry, ligand-donating
ability is a variable that can be controlled to the advantage of the
synthetic chemist in the design of single-molecule magnets with enhanced
relaxation barriers
Impact of Natural Organic Matter on Uranium Transport through Saturated Geologic Materials: From Molecular to Column Scale
The risk stemming from human exposure to actinides via
the groundwater
track has motivated numerous studies on the transport of radionuclides
within geologic environments; however, the effects of waterborne organic
matter on radionuclide mobility are still poorly understood. In this
study, we compared the abilities of three humic acids (HAs) (obtained
through sequential extraction of a peat soil) to cotransport hexavalent
uranium (U) within water-saturated sand columns. Relative breakthrough
concentrations of U measured upon elution of 18 pore volumes increased
from undetectable levels (<0.001) in an experiment without HAs
to 0.17 to 0.55 in experiments with HAs. The strength of the HA effect
on U mobility was positively correlated with the hydrophobicity of
organic matter and NMR-detected content of alkyl carbon, which indicates
the possible importance of hydrophobic organic matter in facilitating
U transport. Carbon and uranium elemental maps collected with a scanning
transmission X-ray microscope (STXM) revealed uneven microscale distribution
of U. Such molecular- and column-scale data provide evidence for a
critical role of hydrophobic organic matter in the association and
cotransport of U by HAs. Therefore, evaluations of radionuclide transport
within subsurface environments should consider the chemical characteristics
of waterborne organic substances, especially hydrophobic organic matter
Theory and X‑ray Absorption Spectroscopy for Aluminum Coordination Complexes – Al K‑Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR<sub>2</sub>, and (BDI)AlX<sub>2</sub> Complexes
Polarized
aluminum K-edge X-ray absorption near edge structure
(XANES) spectroscopy and first-principles calculations were used to
probe electronic structure in a series of (BDI)Al, (BDI)AlX<sub>2</sub>, and (BDI)AlR<sub>2</sub> coordination compounds (X = F, Cl, I;
R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral
interpretations were guided by examination of the calculated transition
energies and polarization-dependent oscillator strengths, which agreed
well with the XANES spectroscopy measurements. Pre-edge features were
assigned to transitions associated with the Al 3p orbitals involved
in metal–ligand bonding. Qualitative trends in Al 1s core energy
and valence orbital occupation were established through a systematic
comparison of excited states derived from Al 3p orbitals with similar
symmetries in a molecular orbital framework. These trends suggested
that the higher transition energies observed for (BDI)AlX<sub>2</sub> systems with more electronegative X<sup>1–</sup> ligands
could be ascribed to a decrease in electron density around the aluminum
atom, which causes an increase in the attractive potential of the
Al nucleus and concomitant increase in the binding energy of the Al
1s core orbitals. For (BDI)Al and (BDI)AlH<sub>2</sub> the experimental
Al K-edge XANES spectra and spectra calculated using the eXcited electron
and Core–Hole (XCH) approach had nearly identical energies
for transitions to final state orbitals of similar composition and
symmetry. These results implied that the charge distributions about
the aluminum atoms in (BDI)Al and (BDI)AlH<sub>2</sub> are similar
relative to the (BDI)AlX<sub>2</sub> and (BDI)AlMe<sub>2</sub> compounds,
despite having different formal oxidation states of +1 and +3, respectively.
However, (BDI)Al was unique in that it exhibited a low-energy feature
that was attributed to transitions into a low-lying p-orbital of b<sub>1</sub> symmetry that is localized on Al and orthogonal to the (BDI)Al
plane. The presence of this low-energy unoccupied molecular orbital
on electron-rich (BDI)Al distinguishes its valence electronic structure
from that of the formally trivalent compounds (BDI)AlX<sub>2</sub> and (BDI)AlR<sub>2</sub>. The work shows that Al K-edge XANES spectroscopy
can be used to provide valuable insight into electronic structure
and reactivity relationships for main-group coordination compounds
Theory and X‑ray Absorption Spectroscopy for Aluminum Coordination Complexes – Al K‑Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR<sub>2</sub>, and (BDI)AlX<sub>2</sub> Complexes
Polarized
aluminum K-edge X-ray absorption near edge structure
(XANES) spectroscopy and first-principles calculations were used to
probe electronic structure in a series of (BDI)Al, (BDI)AlX<sub>2</sub>, and (BDI)AlR<sub>2</sub> coordination compounds (X = F, Cl, I;
R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral
interpretations were guided by examination of the calculated transition
energies and polarization-dependent oscillator strengths, which agreed
well with the XANES spectroscopy measurements. Pre-edge features were
assigned to transitions associated with the Al 3p orbitals involved
in metal–ligand bonding. Qualitative trends in Al 1s core energy
and valence orbital occupation were established through a systematic
comparison of excited states derived from Al 3p orbitals with similar
symmetries in a molecular orbital framework. These trends suggested
that the higher transition energies observed for (BDI)AlX<sub>2</sub> systems with more electronegative X<sup>1–</sup> ligands
could be ascribed to a decrease in electron density around the aluminum
atom, which causes an increase in the attractive potential of the
Al nucleus and concomitant increase in the binding energy of the Al
1s core orbitals. For (BDI)Al and (BDI)AlH<sub>2</sub> the experimental
Al K-edge XANES spectra and spectra calculated using the eXcited electron
and Core–Hole (XCH) approach had nearly identical energies
for transitions to final state orbitals of similar composition and
symmetry. These results implied that the charge distributions about
the aluminum atoms in (BDI)Al and (BDI)AlH<sub>2</sub> are similar
relative to the (BDI)AlX<sub>2</sub> and (BDI)AlMe<sub>2</sub> compounds,
despite having different formal oxidation states of +1 and +3, respectively.
However, (BDI)Al was unique in that it exhibited a low-energy feature
that was attributed to transitions into a low-lying p-orbital of b<sub>1</sub> symmetry that is localized on Al and orthogonal to the (BDI)Al
plane. The presence of this low-energy unoccupied molecular orbital
on electron-rich (BDI)Al distinguishes its valence electronic structure
from that of the formally trivalent compounds (BDI)AlX<sub>2</sub> and (BDI)AlR<sub>2</sub>. The work shows that Al K-edge XANES spectroscopy
can be used to provide valuable insight into electronic structure
and reactivity relationships for main-group coordination compounds
Probing the Interfacial Interaction in Layered-Carbon-Stabilized Iron Oxide Nanostructures: A Soft X‑ray Spectroscopic Study
We have stabilized the iron oxide
nanoparticles (NPs) of various
sizes on layered carbon materials (Fe-oxide/C) that show excellent
catalytic performance. From the characterization of X-ray absorption
spectroscopy (XAS), X-ray emission spectroscopy (XES), scanning transmission
X-ray microscopy (STXM) and X-ray magnetic circular dichroism spectroscopy
(XMCD), a strong interfacial interaction in the Fe-oxide/C hybrids
has been observed between the small iron oxide NPs and layered carbon
in contrast to the weak interaction in the large iron oxide NPs. The
interfacial interaction between the NPs and layered carbon is found
to link with the improved catalytic performance. In addition, the
Fe <i>L</i>-edge XMCD spectra show that the large iron oxide
NPs are mainly γ-Fe<sub>2</sub>O<sub>3</sub> with a strong ferromagnetic
property, whereas the small iron oxide NPs with strong interfacial
interaction are mainly α-Fe<sub>2</sub>O<sub>3</sub> or amorphous
Fe<sub>2</sub>O<sub>3</sub> with a nonmagnetic property. The results
strongly suggest that the interfacial interaction plays a key role
for the catalytic performance, and the experimental findings may provide
guidance toward rational design of high-performance catalysts
High-Resolution Imaging of Polymer Electrolyte Membrane Fuel Cell Cathode Layers by Soft X‑ray Spectro-Ptychography
Polymer
electrolyte membrane fuel cells (PEMFCs) are a promising and sustainable
alternative to internal combustion engines for automotive applications.
Polymeric perfluorosulfonic acid (PFSA) plays a key role in PEMFCs
as a proton conductor in the anode and cathode catalyst layers and
in the electrolyte membrane. In this study, spectroscopic scanning
coherent diffraction imaging (spectro-ptychography) and spectro-ptychographic
tomography were used to quantitatively image PFSA ionomers in PEMFC
cathodes in both two and three dimensions. We verify that soft X-ray
ptychography gives significant spatial resolution improvement on soft
matter polymeric materials. A two-dimensional spatial resolution of
better than 15 nm was achieved. With better detectors and brighter
and more coherent X-ray beams, radiation-sensitive PFSA ionomers will
be visualized with acceptable levels of chemical and structural modification.
This work is a step toward visualization of ionomers in PEMFC cathodes
at high spatial resolution (presently sub-15 nm, but ultimately below
10 nm), which will be transformative with respect to optimization
of PEMFCs for automotive use
Synthesis and Characterization of Eight Compounds of the MU<sub>8</sub>Q<sub>17</sub> Family: ScU<sub>8</sub>S<sub>17</sub>, CoU<sub>8</sub>S<sub>17</sub>, NiU<sub>8</sub>S<sub>17</sub>, TiU<sub>8</sub>Se<sub>17</sub>, VU<sub>8</sub>Se<sub>17</sub>, CrU<sub>8</sub>Se<sub>17</sub>, CoU<sub>8</sub>Se<sub>17</sub>, and NiU<sub>8</sub>Se<sub>17</sub>
The solid-state MU<sub>8</sub>Q<sub>17</sub> compounds ScU<sub>8</sub>S<sub>17</sub>, CoU<sub>8</sub>S<sub>17</sub>, NiU<sub>8</sub>S<sub>17</sub>, TiU<sub>8</sub>Se<sub>17</sub>, VU<sub>8</sub>Se<sub>17</sub>, CrU<sub>8</sub>Se<sub>17</sub>, CoU<sub>8</sub>Se<sub>17</sub>, and NiU<sub>8</sub>Se<sub>17</sub> were synthesized from the reactions
of the elements at 1173 or 1123 K. These isostructural compounds crystallize
in space group <i>C</i><sub>2<i>h</i></sub><sup>3</sup> - <i>C</i>2<i>/m</i> of the monoclinic system in the CrU<sub>8</sub>S<sub>17</sub> structure
type. X-ray absorption near-edge structure spectroscopic studies of
ScU<sub>8</sub>S<sub>17</sub> indicate that it contains Sc<sup>3+</sup>, and hence charge balance is achieved with a composition that includes
U<sup>3+</sup> as well as U<sup>4+</sup>. The other compounds charge
balance with M<sup>2+</sup> and U<sup>4+</sup>. Magnetic susceptibility
measurements on ScU<sub>8</sub>S<sub>17</sub> indicate antiferromagnetic
couplings and a highly reduced effective magnetic moment. Ab Initio
calculations find the compound to be metallic. Surprisingly, the Sc–S
distances are actually longer than all the other M–S interactions,
even though the ionic radii of Sc<sup>3+</sup>, low-spin Cr<sup>2+</sup>, and Ni<sup>2+</sup> are similar
High-Resolution Imaging of Polymer Electrolyte Membrane Fuel Cell Cathode Layers by Soft X‑ray Spectro-Ptychography
Polymer
electrolyte membrane fuel cells (PEMFCs) are a promising and sustainable
alternative to internal combustion engines for automotive applications.
Polymeric perfluorosulfonic acid (PFSA) plays a key role in PEMFCs
as a proton conductor in the anode and cathode catalyst layers and
in the electrolyte membrane. In this study, spectroscopic scanning
coherent diffraction imaging (spectro-ptychography) and spectro-ptychographic
tomography were used to quantitatively image PFSA ionomers in PEMFC
cathodes in both two and three dimensions. We verify that soft X-ray
ptychography gives significant spatial resolution improvement on soft
matter polymeric materials. A two-dimensional spatial resolution of
better than 15 nm was achieved. With better detectors and brighter
and more coherent X-ray beams, radiation-sensitive PFSA ionomers will
be visualized with acceptable levels of chemical and structural modification.
This work is a step toward visualization of ionomers in PEMFC cathodes
at high spatial resolution (presently sub-15 nm, but ultimately below
10 nm), which will be transformative with respect to optimization
of PEMFCs for automotive use
Vanadium Bisimide Bonding Investigated by X‑ray Crystallography, <sup>51</sup>V and <sup>13</sup>C Nuclear Magnetic Resonance Spectroscopy, and V L<sub>3,2</sub>-Edge X‑ray Absorption Near-Edge Structure Spectroscopy
Syntheses
of neutral halide and aryl vanadium bisimides are described.
Treatment of VCl<sub>2</sub>(N<i>t</i>Bu)[NTMS(N<sup><i>t</i></sup>Bu)], <b>2</b>, with PMe<sub>3</sub>, PEt<sub>3</sub>, PMe<sub>2</sub>Ph, or pyridine gave vanadium bisimides via
TMSCl elimination in good yield: VCl(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>3</b>, VCl(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>4</b>, VCl(PMe<sub>2</sub>Ph)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>5</b>, and VCl(Py)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>6</b>. The
halide series (Cl–I) was synthesized by use of TMSBr and TMSI
to give VBr(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>7</b> and VI(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>8</b>. The
phenyl derivative was obtained by reaction of <b>3</b> with
MgPh<sub>2</sub> to give VPh(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>9</b>. These neutral complexes
are compared to the previously reported cationic bisimides [V(PMe<sub>3</sub>)<sub>3</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>][Al(PFTB)<sub>4</sub>] <b>10</b>, [V(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>][Al(PFTB)<sub>4</sub>] <b>11</b>, and [V(DMAP)(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>][Al(PFTB)<sub>4</sub>] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray
diffraction, <sup>13</sup>C NMR, <sup>51</sup>V NMR, and V L<sub>3,2</sub>-edge X-ray absorption near-edge structure (XANES) spectroscopy provides
a description of the electronic structure in comparison to group 6
bisimides and the bent metallocene analogues. The electronic structure
is dominated by π bonding to the imides, and localization of
electron density at the nitrogen atoms of the imides is dictated by
the cone angle and donating ability of the axial neutral supporting
ligands. This phenomenon is clearly seen in the sensitivity of <sup>51</sup>V NMR shift, <sup>13</sup>C NMR Δδ<sub>αβ</sub>, and L<sub>3</sub>-edge energy to the nature of the supporting phosphine
ligand, which defines the parameters for designing cationic group
5 bisimides that would be capable of breaking stronger σ bonds.
Conversely, all three methods show little dependence on the variable
equatorial halide ligand. Furthermore, this analysis allows for quantification
of the electronic differences between vanadium bisimides and the structurally
analogous mixed Cp/imide system CpV(N<sup><i>t</i></sup>Bu)X<sub>2</sub> (Cp = C<sub>5</sub>H<sub>5</sub><sup>1–</sup>)