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Carbon-sulfur bond strength in methanesulfinate and benzenesulfinate ligands directs decomposition of Np(v) and Pu(v) coordination complexes.
Gas-phase coordination complexes of actinyl(v) cations, AnO2+, provide a basis to assess fundamental aspects of actinide chemistry. Electrospray ionization of solutions containing an actinyl cation and sulfonate anion CH3SO2- or C6H5SO2- generated complexes [(AnVO2)(CH3SO2)2]- or [(AnVO2)(C6H5SO2)2]- where An = Np or Pu. Collision induced dissociation resulted in C-S bond cleavage for methanesulfinate to yield [(AnVO2)(CH3SO2)(SO2)]-, whereas hydrolytic ligand elimination occurred for benzenesulfinate to yield [(AnVO2)(C6H5SO2)(OH)]-. These different fragmentation pathways are attributed to a stronger C6H5-SO2-versus CH3-SO2- bond, which was confirmed for both the bare and coordinating sulfinate anions by energies computed using a relativistic multireference perturbative approach (XMS-CASPT2 with spin-orbit coupling). The results demonstrate shutting off a ligand fragmentation channel by increasing the strength of a particular bond, here a sulfinate C-S bond. The [(AnVO2)(CH3SO2)(SO2)]- complexes produced by CID spontaneously react with O2 to eliminate SO2, yielding [(AnO2)(CH3SO2)(O2)]-, a process previously reported for An = U and found here for An = Np and Pu. Computations confirm that the O2/SO2 displacement reactions should be exothermic or thermoneutral for all three An, as was experimentally established. The computations furthermore reveal that the products are superoxides [(AnVO2)(CH3SO2)(O2)]- for An = Np and Pu, but peroxide [(UVIO2)(CH3SO2)(O2)]-. Distinctive reduction of O2- to O22- concomitant with oxidation of U(v) to U(vi) reflects the relatively higher stability of hexavalent uranium versus neptunium and plutonium
Facing the challenge of predicting the standard formation enthalpies of n-butyl-phosphate species with ab initio methods
Tributyl-phosphate (TBP), a ligand used in the PUREX liquid-liquid separation
process of spent nuclear fuel, can form explosive mixture in contact with
nitric acid, that might lead to violent explosive thermal runaway. In the
context of safety of a nuclear reprocessing plant facility, it is crucial to
predict the stability of TBP at elevated temperatures. So far, only the
enthalpies of formation of TBP is available in the literature with a rather
large uncertainties, while those of its degradation products, di-(HDBP) and
mono-(HMBP}) are unknown. In this goal, we have used state-of-the art
quantum chemical methods to compute the formation enthalpies and entropies of
TBP and its degradation products di-(HDBP), mono-(HMBP) in gas and liquid
phases. Comparisons of levels of quantum chemical theory revealed that there
are significant effects of correlation on their electronic structures, pushing
for the need of not only high level of electronic correlation treatment, namely
local coupled cluster with single and double excitation operators and
perturbative treatment of triple excitations [LCCSD(T)], but also
extrapolations to the complete basis to produce reliable and accurate
thermodynamics data. Solvation enthalpies were computed with the conductor like
screening model for real solvents [COSMO-RS], for which we observe errors not
exceeding 22 kJ mol. We thus propose with final uncertainty of about 20
kJ mol standard enthalpies of formation of TBP, HDBP, and HMBP which
amounts to -1281.724.4, -1229.419.6 and -1176.714.8 kJ
mol, respectively, in the gas phase. In the liquid phase, the predicted
values are -1367.324.4, -1348.719.6 and -1323.814.8 kJ
mol, to which we may add about -22 kJ mol error from the COSMO-RS
solvent model. From these data, we predict the complete hydrolysis of TBP to be
nearly thermoneutral
Coordination and thermodynamic properties of aqueous protactinium(V) by first-principle calculations
Protactinium (Z = 91) is a very rare actinide with peculiar physico-chemical
properties. Indeed, although one may naively think that it behaves similarly to
either thorium or uranium by its position in the periodic table, it may in fact
follow its own rules. Because of the quite small energy gap between its valence
shells (in particular the 5f and 6d ones) and also the strong influence of
relativistic effects on its properties, it is actually a challenging element
for theoretical chemists. In this article, we combine experimental information,
chemical arguments and standard first-principle calculations, complemented by
implicit and explicit solvation, to revisit the stepwise complexation of
aqueous protactinium(V) with sulfate and oxalate dianionic ligands (SO4^2- and
C2O4^2-, respectively). From a methodological viewpoint, we notably conclude
that it is necessary to at least saturate the coordination sphere of
protactinium(V) to reach converged equilibrium constant values. Furthermore, in
the case of single complexations (i.e. with one sulfate or oxalate ligand bound
in the bidentate fashion), we show that it is necessary to maintain the
coordination of one hydroxyl group, thought of in the [PaO(OH)]^3+ precursor,
to obtain coherent complexation constants. Therefore, we predict that this
hydroxyl group is maintained in the formation of 1:1 complexes while we confirm
that it is withdrawn when coordinating three sulfate or oxalate ligands.
Finally, we stress that this work is a first step toward the future use of
theoretical predictions to elucidate the enigmatic chemistry of protactinium in
solution
Excited states of polonium(IV): Electron correlation and spin-orbit coupling in the Po^{4+} free ion and in the bare and solvated [PoCl5]^- and [PoCl6]^{2-} complexes
Polonium (Po, Z = 84) is a main-block element with poorly known
physico-chemical properties. Not much information has been firmly acquired
since its discovery by Marie and Pierre Curie in 1898, especially regarding its
speciation in aqueous solution and spectroscopy. In this work, we revisit the
absorption properties of two complexes, [PoCl5]^- and [PoCl6]^{2-}, using
quantum mechanical calculations. These complexes have the potential to exhibit
a maximum absorption at 418 nm in HCl medium (for 0.5 mol/L concentrations and
above). Initially, we examine the electronic spectra of the Po^{4+} free ion
and of its isoelectronic analogue, Bi^{3+}. In the spin-orbit configuration
interaction (SOCI) framework. Our findings demonstrate that the SOCI matrix
should be dressed with correlated electronic energies and that the quality of
the spectra is largely improved by decontracting the reference states at the
complete active space plus singles (CAS+S) level. Subsequently, we investigate
the absorption properties of the [PoCl5]^- and [PoCl6]^{2-} complexes in two
stages. Firstly, we perform methodological tests at the MP2/def2-TZVP gas phase
geometries, indicating that the decontraction of the reference states can there
be skipped without compromising the accuracy significantly. Secondly, we study
the solution absorption properties by means of single-point calculations
performed at the solvated geometries, obtained by an implicit solvation
treatment or a combination of implicit and explicit solvation. Our results
highlight the importance of saturating the first coordination sphere of the
Po^{IV} ion to obtain a qualitatively correct picture. Finally, we conclude
that the known-for-decades 418 nm peak could be attributed to a mixture of both
the [PoCl5(H2O)]^- and [PoCl6]^{2-} complexes. This finding not only aligns
with the behaviour of the analogous Bi^{III} ion under similar conditions
but..
Conformational Landscape of Oxygen-Containing Naphthalene Derivatives
Polycyclic aromatic compounds (PACs) constitute an important class of
molecules found in various environments and are considered important pollutants
of the Earth's atmosphere. In particular, functionalization of PACs modify the
ring aromaticity, which greatly influences the chemical reactivity of these
species. In this work we studied several oxygen-containing PACs, relevant to
atmospheric chemistry. We investigated the conformational landscape of four
naphthalene-derivative molecules -- namely ,1- and 2-hydroxynaphthalene and 1-
and 2-naphthaldehyde -- by means of rotational and vibrational spectroscopy
supported by quantum chemical calculations. For 1-hydroxynaphthalene and
1-naphthaldehyde, intramolecular hydrogen bonding and steric effects drive the
conformational preferences while for 2-hydroxynaphthalene and 2-naphthaldehyde,
the charge distributions allow us to understand the conformational landscape.
This work not only demonstrates how the localization of the substitution group
in the ring influences the conformational relative energies and but also
constitutes a step toward a better understanding of the different chemical
reactivity of such functionalized PACs
Solvation effects on halides core spectra with Multilevel Real-Time quantum embedding
In this work we introduce a novel subsystem-based electronic structure
embedding method that combines the projection-based block-orthogonalized
Manby-Miller embedding (BOMME) with the density-based Frozen Density Embedding
(FDE) methods. Our approach is effective for systems in which the building
blocks interact at varying strengths while still maintaining a lower
computational cost compared to a quantum simulation of the entire system. To
evaluate the performance of our method, we assess its ability to reproduce the
X-ray absorption spectra (XAS) of chloride and fluoride anions in aqueous
solutions (based on a 50-water droplet model) via real-time time-dependent
density functional theory (rt-TDDFT) calculations. We employ an ensemble
approach to compute XAS for the K- and L-edges, utilizing multiple snapshots of
configuration space obtained from classical molecular dynamics simulations with
a polarizable force field. Configurational averaging influences both the
broadening of spectral features and their intensities, with contributions to
the final intensities originating from different geometry configurations. We
found that embedding models that are too approximate for halide-water specific
interactions, as in the case of FDE, fail to reproduce the experimental
spectrum for chloride. Meanwhile, BOMME tends to overestimate intensities,
particularly for higher energy features because of finite-size effects.
Combining FDE for the second solvation shell and retaining BOMME for the first
solvation shell mitigates this effect, resulting in an overall improved
agreement within the energy range of the experimental spectrum. Additionally,
we compute the transition densities of the relevant transitions, confirming
that these transitions occur within the halide systems. Thus, our real-time
QM/QM/QM embedding method proves to be a promising approach for modeling XAS of
solvated systems
Post-stroke inhibition of induced NADPH oxidase type 4 prevents oxidative stress and neurodegeneration
Ischemic stroke is the second leading cause of death worldwide. Only one moderately effective therapy exists, albeit with contraindications that exclude 90% of the patients. This medical need contrasts with a high failure rate of more than 1,000 pre-clinical drug candidates for stroke therapies. Thus, there is a need for translatable mechanisms of neuroprotection and more rigid thresholds of relevance in pre-clinical stroke models. One such candidate mechanism is oxidative stress. However, antioxidant approaches have failed in clinical trials, and the significant sources of oxidative stress in stroke are unknown. We here identify NADPH oxidase type 4 (NOX4) as a major source of oxidative stress and an effective therapeutic target in acute stroke. Upon ischemia, NOX4 was induced in human and mouse brain. Mice deficient in NOX4 (Nox4(-/-)) of either sex, but not those deficient for NOX1 or NOX2, were largely protected from oxidative stress, blood-brain-barrier leakage, and neuronal apoptosis, after both transient and permanent cerebral ischemia. This effect was independent of age, as elderly mice were equally protected. Restoration of oxidative stress reversed the stroke-protective phenotype in Nox4(-/-) mice. Application of the only validated low-molecular-weight pharmacological NADPH oxidase inhibitor, VAS2870, several hours after ischemia was as protective as deleting NOX4. The extent of neuroprotection was exceptional, resulting in significantly improved long-term neurological functions and reduced mortality. NOX4 therefore represents a major source of oxidative stress and novel class of drug target for stroke therapy
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