12 research outputs found
Exceptional uranium(VI)-nitride triple bond covalency from <sup>15</sup>N nuclear magnetic resonance spectroscopy and quantum chemical analysis.
From Europe PMC via Jisc Publications RouterHistory: ppub 2021-09-01, epub 2021-09-24Publication status: PublishedFunder: RCUK | Engineering and Physical Sciences Research Council (EPSRC); Grant(s): EP/M027015/1, EP/K024000/1, EP/S033181/1Funder: European Research Council; Grant(s): 612724Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies
Covalent bond shortening and distortion induced by pressurization of thorium, uranium, and neptunium tetrakis aryloxides
Covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of the actinides, but the ionic versus covalent nature of metal-ligand bonding in actinide complexes remains controversial. The tetrakis 2,6-di-tert-butylphenoxide complexes of Th, U and Np form an isostructural series of crystal structures containing approximately tetrahedral MO(4) cores. We show that up to 3âGPa the Th and U crystal structures show negative linear compressibility as the OMO angles distort. At 3âGPa the angles snap back to their original values, reverting to a tetrahedral geometry with an abrupt shortening of the M-O distances by up to 0.1âĂ
. The Np complex shows similar but smaller effects, transforming above 2.4âGPa. Electronic structure calculations associate the M-O bond shortening with a change in covalency resulting from increased contributions to the M-O bonding by the metal 6d and 5f orbitals, the combination promoting MO(4) flexibility at little cost in energy
Exceptional uranium(VI)-nitride triple bond covalency from 15 N nuclear magnetic resonance spectroscopy and quantum chemical analysis
From Springer Nature via Jisc Publications RouterHistory: received 2021-06-23, accepted 2021-09-06, registration 2021-09-07, pub-electronic 2021-09-24, online 2021-09-24, collection 2021-12Publication status: PublishedFunder: RCUK | Engineering and Physical Sciences Research Council (EPSRC); doi: https://doi.org/10.13039/501100000266; Grant(s): EP/K024000/1, EP/M027015/1, EP/S033181/1Funder: EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council); doi: https://doi.org/10.13039/100010663; Grant(s): 612724Abstract: Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies
Effect of angiotensin-converting enzyme inhibitor and angiotensin receptor blocker initiation on organ support-free days in patients hospitalized with COVID-19
IMPORTANCE Overactivation of the renin-angiotensin system (RAS) may contribute to poor clinical outcomes in patients with COVID-19.
Objective To determine whether angiotensin-converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB) initiation improves outcomes in patients hospitalized for COVID-19.
DESIGN, SETTING, AND PARTICIPANTS In an ongoing, adaptive platform randomized clinical trial, 721 critically ill and 58 nonâcritically ill hospitalized adults were randomized to receive an RAS inhibitor or control between March 16, 2021, and February 25, 2022, at 69 sites in 7 countries (final follow-up on June 1, 2022).
INTERVENTIONS Patients were randomized to receive open-label initiation of an ACE inhibitor (nâ=â257), ARB (nâ=â248), ARB in combination with DMX-200 (a chemokine receptor-2 inhibitor; nâ=â10), or no RAS inhibitor (control; nâ=â264) for up to 10 days.
MAIN OUTCOMES AND MEASURES The primary outcome was organ supportâfree days, a composite of hospital survival and days alive without cardiovascular or respiratory organ support through 21 days. The primary analysis was a bayesian cumulative logistic model. Odds ratios (ORs) greater than 1 represent improved outcomes.
RESULTS On February 25, 2022, enrollment was discontinued due to safety concerns. Among 679 critically ill patients with available primary outcome data, the median age was 56 years and 239 participants (35.2%) were women. Median (IQR) organ supportâfree days among critically ill patients was 10 (â1 to 16) in the ACE inhibitor group (nâ=â231), 8 (â1 to 17) in the ARB group (nâ=â217), and 12 (0 to 17) in the control group (nâ=â231) (median adjusted odds ratios of 0.77 [95% bayesian credible interval, 0.58-1.06] for improvement for ACE inhibitor and 0.76 [95% credible interval, 0.56-1.05] for ARB compared with control). The posterior probabilities that ACE inhibitors and ARBs worsened organ supportâfree days compared with control were 94.9% and 95.4%, respectively. Hospital survival occurred in 166 of 231 critically ill participants (71.9%) in the ACE inhibitor group, 152 of 217 (70.0%) in the ARB group, and 182 of 231 (78.8%) in the control group (posterior probabilities that ACE inhibitor and ARB worsened hospital survival compared with control were 95.3% and 98.1%, respectively).
CONCLUSIONS AND RELEVANCE In this trial, among critically ill adults with COVID-19, initiation of an ACE inhibitor or ARB did not improve, and likely worsened, clinical outcomes.
TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT0273570
Effect of Amino Acid Ligands on the Structure of Iron Porphyrins and Their Ability to Bind Oxygen
Density functional theory is used
to study a series of model iron
porphyrins in the gas phase. In the first part of this study, three
range-separated hybrid density functionals developed by Chai and Head-Gordon
were assessed; ÏB97, ÏB97X, and ÏB97XD. The effects
of including full HartreeâFock exchange at long-range and dispersion
corrections are reported with respect to the geometries and binding
energies of oxygen to the iron porphyrin systems. The functionals
all correctly predict the quintet ground state for the deoxy-iron
porphyrins, where typically hybrid functionals fail and predict a
triplet ground state. Including dispersion in ÏB97XD is shown
to give the best results for the O<sub>2</sub> binding energy and
geometrical parameters. The second part of the study employs ÏB97XD
to study iron porphine systems with different amino acids in the axial
position. Geometrical parameters are reported and compared to experimental
data, where available. Binding energies of the systems with oxygen
are also reported and discussed
Balancing Exchange Mixing in Density-Functional Approximations for Iron Porphyrin
Predicting
the correct ground-state multiplicity for ironÂ(II) porphyrin,
a high-spin quintet, remains a significant challenge for electronic-structure
methods, including commonly employed density functionals. An even
greater challenge for these methods is correctly predicting favorable
binding of O<sub>2</sub> to ironÂ(II) porphyrin, due to the open-shell
singlet character of the adduct. In this work, the performance of
a modest set of contemporary density-functional approximations is
assessed and the results interpreted using Bader delocalization indices.
It is found that inclusion of greater proportions of HartreeâFock
exchange, in hybrid or range-separated hybrid functionals, has opposing
effects; it improves the ability of the functional to identify the
ground state but is detrimental to predicting favorable dioxygen binding.
Because of the uncomplementary nature of these properties, accurate
prediction of both the relative spin-state energies and the O<sub>2</sub> binding enthalpy eludes conventional density-functional approximations
Polarised Covalent Thorium(IV)- and Uranium(IV)-Silicon Bonds
We report the synthesis and characterisation of isostructural thorium(IV)- and uranium(IV)-
silanide complexes, providing the first structurally authenticated Th-Si bond and a rare example of a
molecular U-Si bond. These complexes therefore present the first opportunity to directly compare the
chemical bonding of Th-Si and U-Si bonds. Quantum chemical calculations show significant and
surprisingly similar 7s, 6d, and 5f orbital contributions from both actinide (An) elements in polarised
covalent An-Si bond