O2 Bond Research Articles

Selectivity control in inverse electron demand Diels–Alder reaction of o-Quinone methides catalyzed by chiral N,N′-Dioxide–Sc(III) complex

The reaction mechanism and origin of asymmetric induction in inverse electron demand Diels-Alder (IEDDA) reaction of ortho-quinone methide (o-QM) and fulvene mediated by chiral N,N'-dioxide–Sc(III) catalyst were rationalized using B3LYP-D3(BJ) functional with def2-TZVP basis set. The uncatalyzed IEDDA reaction was concerted but highly asynchronous with activation barriers of 29.8∼31.8 kcal mol−1. Good linear relationship between the Hammett substituent constant (σP) of o-QM and the activation barrier (ΔG≠) of DA reaction was discovered. The secondary orbital interaction (SOI) between the conjugated diene of o-QM and fulvene moiety stabilized the endo-transition state, contributing to high endo-selectivity. The catalytic asymmetric IEDDA reaction occurred via a stepwise mechanism, including the construction of Cβ−C4 bond, followed by the formation of Cα−O1 bond. The bulky substituents (i.e., adamantyl or triphenylmethyl) in amide moiety of ligand furnished sufficient steric shielding for re-face of diene, inducing the attack of fulvene from si-face in endo-pathway. The substituent at exocyclic methylene of the unsymmetrical fulvene was crucial for the adjustment of E/Z selectivity. The steric repulsion between cyclohexyl group in fulvene and aromatic ring in o-QM raised the destabilizing strain energy (ΔEstrain) at the transition state in Z-configuration, contributing to the predominant E-product.

Dehydration behavior of a natural hydrated Ca-chabazite studied by in-situ high-temperature single-crystal X-ray diffraction

Abstract Dehydration behavior of a natural hydrated Ca-chabazite with composition (Ca1.57Na0.49)(Al3.39Si8.55)O24·12.47H2O has been studied by the structure analyses based on the in-situ high-temperature single-crystal X-ray diffraction data at 100, 200 and 250 °C. Among the five water sites (OW1–OW5) and four exchangeable-cation sites (Ca1–Ca4) found previously in the room-temperature (RT) structure, the present structure analyses revealed the presence of OW1, OW4, Ca3 sites at 100 °C and of OW3, Ca3, Ca4 sites at 200 and 250 °C. The presence of the new extraframework sites was also revealed: OW2’ site at 100 °C and Ca5 site at 200 and 250 °C. From the temperature dependence of the occupation for these extraframework sites, the dehydration and cation migration processes are summarized and rationalized as follows. With heating up to 100 °C, OW3 water molecules, having no bonds to exchangeable cations, are completely desorbed. Simultaneously, Ca1 and Ca2 cations lose the large amounts of water molecules as ligands (OW1 and OW5 for Ca1, OW4 for Ca2); this brings about the simultaneous migration of both cations to adjacent Ca3 site, surrounded by more ligands. With further heating up to 250 °C, OW2’ water molecules, having no bonds to exchangeable cations, are completely desorbed. Simultaneously, most of the Ca3 cations migrate to adjacent Ca4 and Ca5 sites by losing the large amounts of water molecules as ligands (OW1, OW4). A part of the water molecules desorbed in the temperature range of 100–250 °C migrate to OW3 site to be bonded to the remaining Ca3 cations. The peculiar temperature dependence of Si/Al−O1, −O2 and −O3 bond lengths in the framework structure can be explained in terms of their bond-valence variations due to such dehydration and cation migration processes. On the other hand, the variation of Si/Al−O4 bond length is dominated by thermal expansion effect rather than the bond valence effect.

Fluorine Effects for Tunable C–C and C–S Bond Cleavage in Fluoro-Julia–Kocienski Intermediates

Classical Julia–Kocienski fluoroolefination represents an indispensable platform for the construction of monofluoroalkenes. Nevertheless, its complex multistep mechanistic manifold along with the u...

Structural characteristics and elasticities of coesite and coesite-II at high pressure

Silica, SiO2, has attracted significant interest as one of the most important compounds in the fields of condensed-matter physics, materials science, and geoscience etc. Here, we theoretically investigate crystal structural characteristics and elastic properties of coesite and its high-pressure polymorph coesite-II at high pressures, which is critical to improve the understanding of densification mechanism of silica at high pressure. The driving mechanism for the pressure-induced transition pathways of coesite comes from both structural features and elastic effect. The phase transition of coesite to cosite-II is triggered by the shortening of Si1–O1 bond distance, and accompanied by elastic instability from a combination of softening elastic constants C44 and C46. The structural modulation of coesite-II along the b-axis direction, as proposed experimentally, results from, on the one hand, the bending of Si2–O15–Si3 angles and shortening of Si5–O4 and Si7–O4 distance with increasing pressure and, on the other hand, elastic softening associated with phonon instability. These results provide key insights into the densification mechanism of silica under high pressure.

Straight Forward and Versatile Differentiation of the l-glycero and d-glycero-d-manno Heptose Scaffold.

Bacterial lipopolysaccharides (LPS) are important bio-medical structures, playing a major role in the interaction with human immune systems. Their core regions, containing multiple units of l-glycero-d-manno heptoses (l,d-heptose), are highly conserved structurally (with O3 and O7 glycosidic bonds), making them an epitope of high interest for the potential development of new antibiotics and vaccines. Research in this field has always been restricted by the limited availability of the parent l,d-heptose as well as its biochemical epimeric precursor d-glycero-d-manno heptose (d,d-heptose). This problem of availability has recently been solved by us, through a rapid and efficient practical synthesis of l,d-manno-heptose peracetate demonstrated at scale. Herein we report an optimized, technically simple and versatile synthetic strategy for the differentiation of both the l-glycero and d-glycero-d-manno heptose scaffolds. Our approach is based on an orthoester methodology for the differentiation of all three positions of the sugar core using a O6, O7-tetraisopropyl disiloxyl (TIPDS) protecting group for the exocyclic positions. Furthermore, the regioselective opening toward 7-OH acceptors (6O-FTIPDS ethers) differentiates the exocyclic diol which has been demonstrated with a broader set of substrates and for both manno-heptoses for the first time.

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations.

SummaryMononuclear metal-peroxo species are invoked as the key intermediates in metalloenzymatic or synthetic catalysis. However, either transience or sluggishness reactivity of synthetic analogs of metal-peroxo species impedes our understanding of oxygen activation mechanism. Herein, we designed and characterized a dioxygen-derived mononuclear osmium-peroxo complex, in which the peroxo ligand is stabilized by internally aromatic metallacycle. We demonstrate that the osmium-peroxo species shows catalytic activity toward promoterless alcohol dehydrogenations. Furthermore, computational studies provide a new mechanism for the osmium-peroxo-mediated alcohol oxidation, starting with the concerted double-hydrogen transfer and followed by the generation of osmium-oxo species. Interestingly, the internally aromatic metallacycle also plays a vital role in catalysis, which mediates the hydrogen transfer from osmium center to the distal oxygen atom of Os–OOH moiety, thus facilitating the Os–OOH→Os=O conversion. We expect that these insights will advance the development of aromatic metallacycle toward aerobic oxidation catalysis.

Mechanism investigations on CO oxidation catalyzed by Fe-doped graphene: A theoretical study

The reaction mechanism in heterogeneous catalysis which is involving O2 or atomic oxygen has been dicussed intensely because the spin change of O2 molecule and atomic O during the reaction. To understand the mechanism of CO oxidation is key for design of advanced materials. Here we demonstrate spin conversion occurs in the path of CO oxiation on Fe-doped graphene (Fe-Gr) with or without electric field according to our DFT investigations. Moreover, the reations proceed via the similar favourable spin state along the pathways both without and with the considered electric field. Additionally, the negative eletric field applied is revealed to promote CO oxidation due to its improving activation of O-O bond in O2, but also induce a competition between the atomic O removal via CO + O* → CO2 for the catalyst regeneration and the carbonate-like species formation via CO2 + O* →CO3* when the negative electric field stronger than −0.25 V/Å. Therefore, a more appropriate for CO oxidation on Fe-Gr is proposed as evidenced under −0.10 V/Å with a lower determined reaction barrier of 0.62 eV at 298.15 K and expelling the carbonate-like species formation. These findings offer a new view of CO oxidation on the doped graphene.

Electronic properties and effective masses of hexagonal Bi9O7.5S6: First-principles calculations

The electronic band structure, density of states, bond character and effective masses of hexagonal Bi9O7.5S6 crystal have been systematically investigated by applying first-principles calculations based on the density functional theory. Approving agreement between experimental and computational results for lattice parameters shows that the GGA-PBE method is suitable. Bi9O7.5S6 is an indirect band gap semiconductor with the value of 1.383eV using the GGA-PBE functional. The detailed analysis of Mulliken populations in Bi9O7.5S6 demonstrates the covalent nature orders for Bi1–S1, Bi2–O1(1), Bi2–O1(3) and Bi2–O2 bond are Bi1–S1>Bi2–O1(3)>Bi2–O2>Bi2–O1(1) on account of bond population values. More interestingly, the electron effective masses are larger in the interlayer direction than in the intralayer one, while the hole effective masses are lower in the interlayer direction than in the intralayer one. The photoinduced carriers for Bi9O7.5S6 have the high mobility that can be inferred from the effective masses of electron and hole in comparation with Bi2O3 and Bi2S3. The Bi9O7.5S6 is a strong photo-oxidation catalyst but the reduction power is limited in aqueous solutions.

The mechanism for CO2 reduction over Fe-modified Cu(100) surfaces with thermodynamics and kinetics: a DFT study.

The adsorption, activation and reduction of CO2 over Fex/Cu(100) (x = 1–9) surfaces were examined by density functional theory. The most stable structure of CO2 adsorption on the Fex/Cu(100) surface was realized. The electronic structure analysis showed that the doped Fe improved the adsorption, activation and reduction of CO2 on the pure Cu(100) surface. From the perspective of thermodynamics and kinetics, the Fe4/Cu(100) surface acted as a potential catalyst to decompose CO2 into CO with a barrier of 32.8 kJ mol−1. Meanwhile, the first principle molecular dynamics (FPMD) analysis indicated that the decomposition of the C–O1 bond of CO2 on the Fe4/Cu(100) surface was only observed from 350 K to 450 K under a CO2 partial pressure from 0 atm to 10 atm. Furthermore, the results of FPMD analysis revealed that CO2 would rather decompose than hydrogenate when CO2 and H co-adsorbed on the Fe4/Cu(100) surface.

Syntheses, crystal structures and spectroscopic properties of two Cu2+-doped single crystals containing 3,14-diethyl-2,6,13,17-tetraazatricyclo[16.4.0.07,12]docosane

Abstract In this study, two Cu2+-doped single crystals, [Cu0.02(H1.96L)](ClO4)2 (1) and [Cu0.91(H0.18L)](ClO4)2 (2) (L = 3,14-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.07,12)docosane), were prepared and characterized by elemental and X-ray crystal structural analyses, along with UV–visible, infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies. The asymmetric units of compounds 1 and 2 contain one half of the molecular dication and one doubly disordered perchlorate anion. The macrocyclic ligand in both the Cu2+-doped compounds adopts the most stable trans-III conformation. The Cu–N bond lengths in the range 2.010 (2)–2.055 (3) A are normal, but the long axial Cu1–O3 bonds of 2.836 (3) and 2.729 (3) A for 1 and 2, respectively, may be due to a combination of the Jahn-Teller effect and strong in-plane ligand field. The crystals are stabilized by hydrogen bonds involving the secondary N–H, the N atoms of the macrocycle, and the O atoms of the perchlorate groups.

A reaction pathway to compound 0 intermediates in oxy-myoglobin through interactions with hydrogen sulfide and His64

Myoglobin (Mb) binds oxygen with high affinity as a low spin singlet complex and thus functions as an oxygen storage protein. Yet, hybrid Density Functional Theory/Molecular Mechanical (DFT/MM) calculations of oxy-Mb models predict that the O2 bond is much less resistant to breaking in the presence of hydrogen sulfide (H2S) compared with water. Specifically, a hydrogen atom from H2S can be transferred to the distal oxygen atom through homolytic cleavage of the S–H bond to form the intermediate Compound (Cpd) 0 structure and a thiyl radical. In the presence of a neutral His64 (Nε protonation, His64-ε) and H2S, only a metastable Cpd 0 would be formed as the active site is devoid of any additional proton donor to fully break the O2 bond. In contrast, the calculations predict that the triplet state is significantly favored over the open shell singlet diradical state throughout the entire reaction coordinate in the presence of H2S and a positively charged His64. Furthermore, a positively charged His64 can readily donate a proton to Cpd 0 to fully break the O2 bond resulting in a configuration analogous to reported reaction models of a hemoglobin mutant bound to H2O2 with H2S present. Typically, exotic techniques are required to generate Cpd 0 but under the conditions just described the intermediate is readily detected in UV–Vis spectra at room temperature. The effect is observed as a 2 nm red shift of the Soret band from 414 nm to 416 nm (pH 5.0, His64-εδ) and from 416 nm to 418 nm (pH 6.6, His64-ε).

Study on the NO2 production pathways and the role of NO2 in fast selective catalytic reduction DeNOx at low-temperature over MnOx/TiO2 catalyst

Selective catalytic reduction of NO by NH3 (SCR) at low temperature usually considered to be the combination of NO oxidation and “Fast SCR” process. However, the NO oxidation mechanism and the role of NO2 over “Fast SCR” process are controversial. Focusing on these problems, the in-deep studies were performed over the typical MnOx/TiO2 catalysts. Combining in-situ DRIFT, TPD-MS and DFT calculation results, an integrated NO oxidation mechanism was proposed based on an “Nitro-assisted oxygen activation” concept where the activation and transformation of O2 is linked with the transformation of nitro species. The OO bond of O2 can be weakened by generating a variation of bidentate nitrates, which can promote the break of OO bond and the formation of NO2. The formation of the variation of bidentate nitrates was the rate-determining step over NO oxidation. Additionally, we further reveal that the crucial role of NO2 over “Fast SCR” process lies in participating the hydrogen abstraction from coordinated NH3 and promoting the production of NH2 active species which can direct react with NO to generate N2 and H2O. The removability of NO2 can coupling the MnO2 (NO oxidation active component) and TiO2 support (“Fast SCR” active component) together. Our findings provide insight into the design and improvement of catalysts or reaction pathways over the low-temperature SCR process.

High Ionic Conductivity in Oxygen-Deficient Ti-Substituted Sodium Niobates and the Key Role of Structural Features

NaNb1–xTixO3–0.5x (0 < x ≤ 0.15) compounds were prepared using a two-step synthesis process involving a hydrothermal route at T = 200 °C in an autoclave followed by heat treatments under air or reductive conditions. Rietveld structural refinements from X-ray diffraction data combined with 23Na and 93Nb nuclear magnetic resonance evidenced the formation of new complex oxides crystallizing in the P21ma space group. Ti substitution for Nb atoms contributes to stabilize the acentric polymorph rather than the well-known thermodynamically stabilized network (Pbma space group) of sodium niobate. Taking into account the competitive bond sequence Na1(2)–O1(2)–Nb–O3(4), the large variation of Na1–O1 and Na2–O2 bond lengths after Ti substitution leads to reduction of the Nb/Ti–O1 and Nb/Ti–O2 apical distortion (elongation in one direction) and consequently exaltation of the distortion in the equatorial plane. Then, transition metal crystal field splitting, as well as the second-order Jahn–Teller effect, increases, a...

Global Potential Energy Surface for HO2+ Using the CHIPR Method.

An analytical potential energy function for the title ion based on the combined hyperbolic inverse power representation (CHIPR) method and its characteristics are discussed at length in the present work. The curves of two diatomic ions, O2+ and OH+, are also obtained within the same approach. The model PES so obtained exhibits extraordinary flexibility in describing with subchemical accuracy even the weak topological features near the higher energy regions. Thus, structural properties predicted by the model may help spectroscopists who want to compare their experimental values with the ones from theory. The relaxed PESs in various coordinates have been calculated by relaxing the O2 bond distance using the present model, thus throwing light on all the possible isomers and their interconversions. The latest estimates of IR frequencies for three vibrational modes have been compared with the computed frequencies using the present model, and the agreement seems encouraging.

The Role of the Fe/Mo Cations Ordering Degree and Oxygen Non‐Stoichiometry on the Formation of the Crystalline and Magnetic Structure of Sr2FeMoO6−δ

Single‐phase Sr2FeMoO6−δ powders with various oxygen indices (δ) and degrees of the superstructural ordering (P) of the Fe/Mo cations are obtained from SrFeO2.52 and SrMoO4 reagents via solid‐state synthesis. It has been established by means of the X‐ray and neutron diffraction that, upon reducing the oxygen content and enhancing the superstructural ordering, the lengths of the Fe–O1 and Mo–O2 bonds in the crystal lattice increase, whereas the Fe–O2 and Mo–O1 bond lengths decrease. At the same time, the volume of the unit cell is reduced, which indicates an enhancement of the covalency degree of the bonds and stimulates a redistribution of the electron density, as well as an increase of the concentration of the spin‐down charge carriers located in the conduction band on the Mo(t2g)↓ orbitals. This circumstance leads to an increase of the density of states at the Fermi level, accompanied by an amplification of the exchange interaction and elevation of the Curie temperature, which points to the leading role of the spin‐polarized charge carriers at the Fermi level in the exchange interaction.

H-bonding in lazulite: a single-crystal neutron diffraction study at 298 and 3 K

The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, Dawson mining district, Yukon, Canada) was investigated by electron microprobe analysis in wavelength-dispersive mode (EMPA) and single-crystal neutron diffraction at 298 and 3 K. Its empirical formula, based on EMPA data, is: (Mg0.871Fe0.127)Σ0.998Al2.030(P1.985Ti0.008Si0.007O4)2(OH)2. The neutron diffraction experiments at room and low T proved that the H-free structural model of lazulite previously reported, on the basis of X-ray structure refinement, is correct: the building unit of the lazulite structure consists of a group of three face-sharing (Al-octahedron) + (Mg,Fe-octahedron) + (Al-octahedron), connected to the adjacent one via a corner-shared OH-group and two corner-shared oxygen sites of the P-tetrahedron, to form a dense 3D-edifice. Only one crystallographically independent H site occurs in the structure of lazulite, forming a hydroxyl group with the O5 oxygen, with O5–H = 0.9997 A at room temperature (corrected for riding motion effect). The H-bonding scheme in the structure of lazulite is now well defined: a bifurcated bonding scheme occurs with the O4 and O2 oxygen sites as acceptors. The two H-bonds are energetically different, as shown by their bonding geometry: the H-bond with the O2 site as acceptor is energetically more favorable, being O5–H···O2 = 152.67(9)°, O5···O2 = 3.014(1) A and H···O2 = 2.114(1) A, whereas that with O4 as acceptor is energetically more costly, being O5–H···O4 = 135.73(8)°, O5···O4 = 3.156(1) A and H···O4 = 2.383(1) A, at room temperature. No T-induced phase transition occurs within the T-range investigated. At low temperature, the O5–H···O2 bond is virtually identical to the room-T one, whereas the effects of T on O5–H···O4 are more pronounced, with significant differences of the Odonor···Oacceptor and H···Oacceptor distances. The experimental findings of this study do not support the occurrence of HPO4 or H2PO4 units into the structure of lazulite, recently reported on the basis of infrared and Raman spectra.

Effects of ZrO2 substitution on crystal structure and microwave dielectric properties of Zn0.15Nb0.3(Ti1-xZrx)0.55O2 ceramics

In this study, the influence of substitution of Zr for Ti on crystal structure and microwave dielectric properties of Zn0.15Nb0.3(Ti1-xZrx)0.55O2 (0 ≤ x ≤ 0.7) ceramics were discussed through Rietveld refinement, normalized bond analysis and complex chemical bond theory. Rietveld refinement analysis indicated that a composite could be formed for x ≤ 0.22. Pure orthorhombic type (O-type) solid solutions are prepared in the region of x = 0.3–0.7. With the increasing amount of ZrO2, cell volume of O-type phase increases, and it results in the enlarging and compressing behaviors of chemical bonds. Under the circumstances, the dielectric polarizability is deteriorated because of the reducing chemical bond covalency value (fc), and this finding is also confirmed by Clausius-Mosotti equation. Lattice energy of Zr–O1 bonds and grain size distribution of sintered specimens are mainly responsible for the variations of quality factor (Q × f) value. Chemical bond energy (E) is closely related with the temperature coefficient of resonant frequency value (τf). Typical microwave dielectric properties of Zn0.15Nb0.3(Ti1-xZrx)0.55O2 ceramics (x = 0.22) were obtained when sintered at 1100 °C: εr = 46.31, Q × f = 30,297 GHz, τf = -8.24 ppm/°C.

Modeling the Self-Assembly of 5-Hydroxy-6-methyluracil within Electrostatic Potential Approach

The sites of reactions in 5-hydroxy-6-methyluracil (HMU) and its dimeric and tetrameric associates are studied theoretically in terms of the electrostatic potential (ESP). The ESP approach allows us to construct the most stable combinations of HMU associates and to study HMU self-assembly without simulating all possible associates. A self-assembly scheme is proposed for HMU. N1–H···O1 bonds initially form between the molecules. If the first recognition center (RC) is occupied, N3–H···O2 bonds form with the participation of the third RC. The resulting ribbons with alternating contacts 1–1 and 3–3 are cross-linked through the electrostatic attraction of hydroxyl groups and the forces of dispersion between methyl groups.

Oxidation of Benzylic Alcohols and Lignin Model Compounds with Layered Double Hydroxide Catalysts

Alcohol oxidation to carbonyl compounds is one of the most commonly used reactions in synthetic chemistry. Herein, we report the use of base metal layered double hydroxide (LDH) catalysts for the oxidation of benzylic alcohols in polar solvents. These catalysts are ideal reagents for alcohol oxidations due to their ease of synthesis, tunability, and ease of separation from the reaction medium. LDHs synthesized in this study were fully characterized by means of X-ray diffraction, NH3-temperature programmed desorption (TPD), pulsed CO2 chemisorption, N2 physisorption, electron microscopy, and elemental analysis. LDHs were found to effectively oxidize benzylic alcohols to their corresponding carbonyl compounds in diphenyl ether, using O2 as the terminal oxidant. LDH catalysts were also applied to the oxidation of lignin β-O-4 model compounds. Typically, for all catalysts, only trace amounts of the ketone formed from benzylic alcohol oxidation were observed, the main products comprising benzoic acids and phenols arising from β-aryl ether cleavage. This observation is consistent with the higher reactivity of the ketones, resulting from weakening of the Cβ–O4 bond that was shown to be aerobically cleaved at 180 °C in the absence of a catalyst.

Photoinduced Breaking of the Fe–O2 Bond in Hemoglobin: Dissociation Quantum Yield, Excited Electronic States, and Nonradiative Relaxation Processes

Bimolecular recombination of molecular oxygen with human hemoglobin upon excitation in different spectral ranges is studied by nanosecond laser kinetic spectroscopy. The results obtained are fundamentally different; i.e., rather efficient photodissociation takes place in the case of excitation into the Q-band of the ππ* nature and is practically absent upon excitation into the near-IR band corresponding to the electron transfer from the porphyrin a2u orbital to a mixed orbital formed of the iron d orbital and the free π orbital of molecular oxygen. Analysis of these data, together with data obtained previously using the techniques of nano-, pico-, and femtosecond spectroscopy, allowed us to describe the mechanism and dynamics of the photodissociation reaction and intraheme relaxation processes.