C-O Bond Activation in Mononuclear Lanthanide Oxocarbonyl Complexes OLn(η2-CO) (Ln = La, Ce, Pr, and Nd).

Fundamental investigation of metal-CO interactions is of great importance for the development of high-performance catalysts to CO activation. Herein, a series of side-on bonded mononuclear lanthanide (Ln) oxocarbonyl complexes OLn(η2-CO) (Ln = La, Ce, Pr, and Nd) have been prepared and identified in solid argon matrices. The complexes exhibit uncommonly low C-O stretching bands near 1630 cm-1, indicating remarkable C-O bond activation in these Ln analogues. The η2-CO ligand in OLn(η2-CO) can be claimed as an anion on the basis of the experimental observations and quantum chemistry investigations, although the CO anion is commonly considered to be unstable with electron auto-detachment. The CO activation in OLn(η2-CO) is attributed to the photoinduced intramolecular charge transfer from LnO to CO rather than the generally accepted metal → CO π back-donation, which conforms to the traditional Dewar-Chatt-Duncanson motif. Energy decomposition analysis combined with natural orbitals for chemical valence calculations demonstrates that the bonding between LnO and η2-CO arises from the combination of dominant ionic forces (>76%) and normal Lewis "acid-base" interactions. The fundamental findings provide guidelines for the catalyst design of CO activation.

CO Oxidation to CO2 on Uranium Dioxide Molecules through Intermediate O2U(η1-CO).

Investigations on the reactions of uranium oxide molecules with CO offer new inspiration for the design of promising high-efficiency catalysts for CO activation using actinide materials. Herein, we contribute a combined matrix-isolation infrared spectroscopic and theoretical study of CO oxidation to CO2 on uranium dioxide (UO2) molecules in solid argon. The reaction intermediate O2U(η1-CO) is generated spontaneously at the bands of 1893.0, 870.6, and 801.3 cm-1 during codeposition and annealing. Upon the following irradiation, CO2 is substantially produced by the consumption of O2U(η1-CO), indicating the catalytic conversion of CO to CO2 through the intermediate O2U(η1-CO). In C18O isotopic substitution experiments, the yields of 16OC18O convincingly confirm that one of the oxygen atoms in CO2 derives from UO2. The reaction pathways are discussed based on the theoretical and experimental results.

Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO3(η1-NN)1-4.

Investigations of the interactions of uranium trioxide (UO3) with other species are expected to provide a new perspective on its reaction and bonding behaviors. Herein, we present a combined matrix-isolation infrared spectroscopy and theoretical study of the geometries, vibrational frequencies, electronic structures, and bonding patterns for a series of dinitrogen (N2) complexes with UO3 moieties UO3(η1-NN)1-4. The complexes are prepared by reactions of laser-ablated uranium atoms with O2/N2 mixtures or laser-ablated UO3 molecules with N2 in solid argon. UO3(η1-NN)1-4 are classified as "nonclassical" metal-N2 complexes with increased Δν(N2) values according to the experimental observations and the computed blue-shifts of N-N stretching frequencies and N-N bond length contractions. Electronic structure analysis suggests that UO3(η1-NN)1-4 are σ-only complexes with a total lack of π-back-donation. The energy decomposition analysis combined with natural orbitals for chemical valence calculations reveal that the bonding between the UO3 moiety and N2 ligands in UO3(η1-NN)1-4 arises from the roughly equal electrostatic attractions and orbital mixings. The inspection of orbital interactions from pairwise contributions indicates that the strongest orbital stabilization comes from the σ-donations of the 4σ*- and 5σ-based ligand molecular orbitals (MOs) into the hybrid 7s/6dx2-y2 MO of the U center. The electron polarization induced by electrostatic effects in the Ninner ← Nouter direction provides complementary contributions to the orbital stabilization in UO3(η1-NN)1-4. In addition, the reactions of UO3 with N2 ligands and the origination of the nonclassical behavior in UO3(η1-NN)1-4 are discussed.

Insights into the Metal-CO Bond in O2M(η1-CO) (M = Cr, Mo, W, Nd, and U) Complexes.

Investigations on the structures and bonding properties of metal carbonyl compounds provide fundamental understandings on the origin of small-molecule activations. Herein, the geometry and bonding trends of a series of isovalent metal oxocarbonyl complexes O2M(η1-CO) (M = Cr, Mo, W, Nd, and U) were studied by combined matrix-isolation infrared spectroscopy and advanced quantum chemical calculations. The title complexes present red shift of C-O stretching bands in the range from 122 to 244 cm-1, indicating the difference of CO activation ability for the series of isovalent metal dioxides. Density functional theory calculations predict T-shaped structures with a C2v symmetry for all the title molecules. O2Nd(η1-CO) bears little resemblance to the other complexes in bonding characters because of the weak interactions between the NdO2 and CO moiety. For the other complexes, natural localized molecular orbital analysis reveals a gradual increase of covalent character in M-CO bonds along the metal series Cr → Mo → W→ U. Energy decomposition analysis with natural orbitals for chemical valence calculations demonstrates that the M-CO bonding patterns conform to the conventional Dewar-Chatt-Duncanson motif. The contributions from orbital interactions in total attractions increase from Cr (41.7%) to U (52.7%). The breakdown of the orbital term into pairwise interactions shows that contributions of the M ← CO σ donation decrease from Cr (59.2%) to U (28.4%), while the M → CO π* backdonation increases significantly from Cr (23.8%) to U (67.3%). The more effective overlap and the better energy matching of U 5f and U 6d valence orbitals with CO π* orbitals result in much stronger U → CO π backdonation than the other metal elements.

Intermediates of Carbon Monoxide Oxidation on Praseodymium Monoxide Molecules: Insights from Matrix-Isolation IR Spectroscopy and Quantum-Chemical Calculations.

Identifying reaction intermediates in gas-phase investigations will provide understanding for the related catalysts in fundamental aspects including bonding interactions of the reaction species, oxidation states (OSs) of the anchored atoms, and reaction mechanisms. Herein, carbon monoxide (CO) oxidation by praseodymium monoxide (PrO) molecules has been investigated as a model reaction in solid argon using matrix-isolation IR spectroscopy and quantum-chemical calculations. Two reaction intermediates, OPr(η1-CO) and OPr(η2-CO), have been trapped and characterized in argon matrixes. The intermediate OPr(η2-CO) shows an extremely low C-O stretching band at 1624.5 cm-1. Quantum-chemistry studies indicate that the bonding in OPr(η1-CO) is described as "donor-acceptor" interactions conforming to the Dewar-Chatt-Duncanson motif. However, the bonding in OPr(η2-CO) results evidently from a combination of dominant ionic forces and normal Lewis "acid-base" interactions. The electron density of the singly occupied bonding orbital is strongly polarized to the CO fragment in OPr(η2-CO). Electronic structure analysis suggests that the two captured species exhibit Pr(III) OSs. Besides, the pathways of CO oxidation have been discussed.

Theoretical Insight into Coordination Chemistry of Am(VI) and Am(V) with Phenanthroline Ligand: Implications for High Oxidation State Based Minor Actinide Separation.

Despite continuing and burgeoning interest in americium (Am) coordination chemistry in recent years, investigations of the electronic structures and bonding chemistry of high oxidation state americium complexes and their implications for minor actinide separation remain relatively less explored to date. Here, we used density functional theory (DFT) to create high oxidation states of americium but experimentally feasible models of Am(V) and Am(VI) complexes of phenanthroline ligand (DAPhen) as [AmO2(L)]1+/2+ and [AmO3(L)]1+ (L = 2,9-bis[(N,N-dimethyl)-carbonyl]-1,10-phenanthroline (oxo-DAPhen, LO) and 2,9-bis[(N,N-dimethyl)-thio-carbonyl]-1,10-phenanthroline (thio-DAPhen, LS)), meanwhile comparing these with [UO2(L)]2+. On the basis of the calculations, the Am(V) and Am(VI) oxidation state are thermodynamically feasible and can be stabilized by DAPhen ligands. From a comparative study, the strength of thio-DAPhen in the separation of high oxidation state Am emerges better than does oxo-DAPhen, which relates to the nature, energy level, and spatial arrangement of their frontier orbitals. This study provides fundamental knowledge toward understanding the transuranic separations processes, which has implications in designing new, more selective extraction processes for the separation of Am from curium (Cm) as well as lanthanide.

Probing the Electronic Structure and Chemical Bonding of Uranium Nitride Complexes of NU-XO (X = C, N, O).

Uranium(III) compounds are very reactive and exhibit a broad range of chemical-bonding tendencies owing to the spatially diffused valence orbitals of uranium. A systematic study on the geometries, electronic structures, and chemical bonding of NU-XO (X = C, N, O) is performed using relativistic quantum chemistry approaches. The NU-CO and NU-NO complexes have an end-on structure, that is, (NU) (η1-CO) and (NU) (η1-NO), whereas NU-OO adopts a side-on ((NU) (η2-O2)) structure. The electronic structure analysis shows that UN exhibits efficient activation reactivity to molecules, especially to NO and O2, because of the significant U 7s/5f → XO 2π* electron transfer. Thus, the oxidation state of U is +V with the dianion ligand NO2- and O22- in NU-NO and NU-OO, respectively. Instead, U retains its usual +III oxidation state in NU-CO with a neutral CO ligand. The significant stability of NU-XO (X = C, N, O) is determined by the covalent U-X bonding which contains both X → U σ-, π-donation from the X lone pair and U 5f → XO 2π* back-donation contributions. The significant back-donation to the antibonding X-O 2π* orbital results in the obvious weakening of the X-O bonding.

Correction: Insights into the enhanced Ce[triple bond, length as m-dash]N triple bond in the HCe[triple bond, length as m-dash]N molecule.

Correction for 'Insights into the enhanced Ce[triple bond, length as m-dash]N triple bond in the HCe[triple bond, length as m-dash]N molecule' by Zhen Pu et al., Phys. Chem. Chem. Phys., 2017, 19, 8216-8222.

Ammonia Activation by Ce Atom: Matrix-Isolation FTIR and Theoretical Studies.

The activation of ammonia by cerium atom has been investigated in solid argon using infrared spectroscopy and density functional theoretical calculations. The results reveal that the spontaneous formation of CeNH3 complex on annealing is the initial step in the reactions of cerium atoms with ammonia. The CeNH3 complexes rearrange to generate the inserted HCeNH2 molecules on irradiation. A "triplet-singlet" spin conversion occurs along the reaction path in which HCeNH2 (3A″) isomerizes into H2CeNH (1A'). The H2CeNH molecules finally decompose to yield HCeN + H2 upon photolysis. The periodic trend and differences for the M + NH3 (M = Ti, Zr, Hf, Ce, Th) systems are discussed on the basis of the present and previous works. DFT calculations predict that the most stable ground state for HHfNH2 and HThNH2 is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH2, HZrNH2, and HCeNH2 is triplet. Besides, the H2-elimination process is different for Ce and M (Ti, Zr, Hf, Th) cases.

Insights into the enhanced Ce[triple bond, length as m-dash]N triple bond in the HCe[triple bond, length as m-dash]N molecule.

Herein, an experimental study of the vibrational spectra of HCeN was carried out in solid argon, followed by theoretical investigations of molecular structures and the nature of Ce[triple bond, length as m-dash]N bond. The absorption band at 937.7 cm-1 with the 1.0311 14N/15N isotopic shift ratio is characteristic of Ce[triple bond, length as m-dash]N stretching band for HCeN, showing a 94 cm-1 higher shift relative to that of the diatomic CeN molecule. This large frequency shift indicates a much stronger Ce[triple bond, length as m-dash]N bond in HCeN, which is confirmed by DFT calculations. Qualitative orbital interaction and orbital composition analyses suggest that the addition of the H ligand to the Ce center will activate the 4f valence shell and strengthen the covalent bond between Ce and N, which may contribute to enhance the Ce[triple bond, length as m-dash]N triple bond in the HCeN molecule.

Enhancing the sensitivity for weak radioactive source detection.

Considering the difficulties of the low signal-to-noise ratio in weak radioactive source detections, this study proposes an abandon Gaussian tails method based on the analysis of the characteristic information denoted by the full-energy peak of the gamma spectrum of a gamma-emitting radioactive source. Based on the study of the signal-to-background ratio and the statistical fluctuations in the signal of the weak radioactive source, a factor ζ, incorporating the statistical fluctuations of signal and background and the signal-to-background ratio, is suggested to characterize the sensitivity of a radioactive source detection. When ζ reaches its maximum value, the optimal counting window around the centroid of the full-energy peak can be obtained. To evaluate the effectiveness of the proposed approach, comparisons between the proposed abandon Gaussian tails, the conventional full-energy counting, and other experiential methods were performed. The results show that the sensitivity can be significantly improved. Further, experiments with different intensity of radiation sources and duplicated experiments were conducted to examine the stability of the proposed method.

Pd-Ru nanocatalysts derived from a Pd-induced aerogel for dramatic boosting of hydrogen release.

Alloyed bimetallic Pd-Ru nanocatalysts prepared by in situ reduction of a mixture of a Ru(iii) source and a Pd(ii)@alkyne-PVA aerogel and characterized by TEM and XPS exhibit very highly catalytic activity towards hydrogen release from ammonia borane hydrolysis with a TOF value of 578.2 molH2 molcat-1 min-1.

Infrared induced repeatable self-healing and removability of mechanically enhanced graphene-epoxy flexible materials.

A repeatable self-healing epoxy composite mechanically enhanced by graphene nanosheets (GNS) was prepared from an epoxy monomer with Diels-Alder (DA) bonds, octanediol glycidyl ether (OGE) and polyether amine (D230). The GNS/epoxy composites, with a maximum tensile modulus of 14.52 ± 0.45 MPa and elongation at break more than 100%, could be healed several times under Infrared (IR) light with the healing efficiency as high as 90% through the molecule chain mobility and the rebonding of reversible DA bonds between furan and maleimide. Also, they displayed excellent recyclable ability by transforming into a soluble polymer, which offers a wide range of possibilities to produce epoxy flexible materials with healing and removable abilities.

Atomistic and Experimental Investigation of the Effect of Depth of Cut on Diamond Cutting of Cerium.

The ultra-precision diamond cutting process exhibits strong size effects due to the ultra-small depth of cut that is comparable with the cutting edge radius. In the present work, we elucidate the underlying machining mechanisms of single crystal cerium under diamond cutting by means of molecular dynamics simulations, with an emphasis on the evaluation of the effect of depth of cut on the cutting process by using different depths of cut. Diamond cutting experiments of cerium with different depths of cut are also conducted. In particular for the smallest depth of cut of 0.2 nm, shallow cutting simulations varying the sharpness of the cutting edge demonstrate that an atomically sharp cutting edge leads to a smaller machining force and better machined surface quality than a blunt one. Simulation results indicate that dislocation slip is the dominant deformation mechanism of cerium under diamond cutting with each depth of cut. Furthermore, the analysis of the defect zone based on atomic radial distribution functions demonstrates that there are trivial phase transformations from γ-Ce to δ-Ce occurred in both the machined surface and the formed chip. It is found that there is a transition of material removal mode from plowing to cutting with the increase of the depth of cut, which is also consistent with the diamond cutting experiments of cerium with different depths of cut.

Molecular Dynamics Investigation of Residual Stress and Surface Roughness of Cerium under Diamond Cutting.

Machined surface quality in terms of residual stress and surface roughness has an important influence on the performance of devices and components. In the present work, we elucidate the formation mechanisms of residual stress and surface roughness of single crystalline cerium under ultraprecision diamond cutting by means of molecular dynamics simulations. Influences of machining parameters, such as the rake angle of a cutting tool, depth of cut, and crystal orientation of the workpiece on the machined surface quality were also investigated. Simulation results revealed that dislocation activity and lattice distortion are the two parallel factors that govern the formation of both residual stress and surface roughness. It was found that both distributions of residual stress and surface roughness of machined surface are significantly affected by machining parameters. The optimum machining parameters for achieving high machined surface quality of cerium by diamond cutting are revealed.

Patternable Poly(chloro-p-xylylene) Film with Tunable Surface Wettability Prepared by Temperature and Humidity Treatment on a Polydimethylsiloxane/Silica Coating.

Poly(chloro-p-xylylene) (PPXC) film has a water contact angle (WCA) of only about 84°. It is necessary to improve its hydrophobicity to prevent liquid water droplets from corroding or electrically shorting metallic circuits of semiconductor devices, sensors, microelectronics, and so on. Herein, we reported a facile approach to improve its surface hydrophobicity by varying surface pattern structures under different temperature and relative humidity (RH) conditions on a thermal curable polydimethylsiloxane (PDMS) and hydrophobic silica (SiO2) nanoparticle coating. Three distinct large-scale surface patterns were obtained mainly depending on the contents of SiO2 nanoparticles. The regularity of patterns was mainly controlled by the temperature and RH conditions. By changing the pattern structures, the surface wettability of PPXC film could be improved and its WCA was increased from 84° to 168°, displaying a superhydrophobic state. Meanwhile, it could be observed that water droplets on PPXC film with superhydrophobicity were transited from a "Wenzel" state to a "Cassie" state. The PPXC film with different surface patterns of 200 µm × 200 µm and the improved surface hydrophobicity showed wide application potentials in self-cleaning, electronic engineering, micro-contact printing, cell biology, and tissue engineering.

Molecular Dynamics Modeling and Simulation of Diamond Cutting of Cerium.

The coupling between structural phase transformations and dislocations induces challenges in understanding the deformation behavior of metallic cerium at the nanoscale. In the present work, we elucidate the underlying mechanism of cerium under ultra-precision diamond cutting by means of molecular dynamics modeling and simulations. The molecular dynamics model of diamond cutting of cerium is established by assigning empirical potentials to describe atomic interactions and evaluating properties of two face-centered cubic cerium phases. Subsequent molecular dynamics simulations reveal that dislocation slip dominates the plastic deformation of cerium under the cutting process. In addition, the analysis based on atomic radial distribution functions demonstrates that there are trivial phase transformations from the γ-Ce to the δ-Ce occurred in both machined surface and formed chip. Following investigations on machining parameter dependence reveal the optimal machining conditions for achieving high quality of machined surface of cerium.