Activation Barrier Research Articles

Growth Kinetics of Graphene on Cu(111) Foils from Methane, Ethyne, Ethylene, and Ethane

AbstractChemical vapor deposition of carbon precursors on Cu‐based substrates at temperatures exceeding 1000 °C is currently a typical route for the scalable synthesis of large‐area high‐quality single‐layer graphene (SLG) films. Using molecules with higher activities than CH4 may afford lower growth temperatures that might yield fold‐ and wrinkle‐free graphene. The kinetics of growth of graphene using hydrocarbons other than CH4 are of interest to the scientific and industrial communities. We measured the growth rates of graphene islands on Cu(111) foils by using C2H2, C2H4, C2H6 and CH4, respectively (each mixed with H2). From such kinetics data we obtain the activation enthalpy (ΔH≠) of graphene growth as shown in parentheses (C2H2 (0.93±0.09 eV); C2H4 (2.05±0.19 eV); C2H6 (2.50±0.11 eV); CH4 (4.59±0.26 eV)); C2Hy (y=2, 4, 6) show similar growth behavior but CH4 is different. Computational fluid dynamics and density functional theory simulations suggest that C2Hy differs from CH4 due to different values of adsorption energy and the lifetime of relevant carbon precursors on the Cu(111) surface. Combining experimental and simulation results, we find that the rate determining step (RDS) is the dissociation of the first C−H bond of CH4 molecules in the gas phase, while the RDS using C2Hy is the first dehydrogenation of adsorbed C2Hy that happens with assistance of H atoms adsorbed on the Cu(111) surface. By using C2H2 as the carbon precursor, high‐quality single‐crystal adlayer‐free SLG films are achieved on Cu(111) foils at 900 °C.

Growth Kinetics of Graphene on Cu(111) Foils from Methane, Ethyne, Ethylene, and Ethane.

Chemical vapor deposition of carbon precursors on Cu-based substrates at temperatures exceeding 1000 °C is currently a typical route for the scalable synthesis of large-area high-quality single-layer graphene (SLG) films. Using molecules with higher activities than CH4 may afford lower growth temperatures that might yield fold- and wrinkle-free graphene. The kinetics of growth of graphene using hydrocarbons other than CH4 are of interest to the scientific and industrial communities. We measured the growth rates of graphene islands on Cu(111) foils by using C2H2, C2H4, C2H6 and CH4, respectively (each mixed with H2). From such kinetics data we obtain the activation enthalpy (ΔH≠) of graphene growth as shown in parentheses (C2H2 (0.93±0.09 eV); C2H4 (2.05±0.19 eV); C2H6 (2.50±0.11 eV); CH4 (4.59±0.26 eV)); C2Hy (y=2, 4, 6) show similar growth behavior but CH4 is different. Computational fluid dynamics and density functional theory simulations suggest that C2Hy differs from CH4 due to different values of adsorption energy and the lifetime of relevant carbon precursors on the Cu(111) surface. Combining experimental and simulation results, we find that the rate determining step (RDS) is the dissociation of the first C-H bond of CH4 molecules in the gas phase, while the RDS using C2Hy is the first dehydrogenation of adsorbed C2Hy that happens with assistance of H atoms adsorbed on the Cu(111) surface. By using C2H2 as the carbon precursor, high-quality single-crystal adlayer-free SLG films are achieved on Cu(111) foils at 900 °C.

Dielectric relaxation study of pyridine-n-propyl alcohol mixture using time domain reflectometry technique

ABSTRACT The Time Domain Reflectometry (TDR) technique was used to analyse the complex permittivity spectra of the Pyridine-n-propyl alcohol mixture across the concentration and temperature range of 10–25°C. The Cole-Davidson model was used to match the Pyridine- n-propyl alcohol complex permittivity spectra. The nonlinear least squares fit approach has been used to obtain the Static dielectric constant (ε0), Relaxation time (τ), Effective Kirkwood correlation factor (geff) Excess permittivity ε 0 E , Thermodynamic parameters (activation enthalpy and activation entropy), and Bruggeman factor (fB). Furthermore, the analysis of excess characteristics and the Bruggeman factor points towards the presence of hydrogen bonding between the pyridine and n-propyl alcohol molecules.

Towards a complete description of the reaction mechanisms between nitrenium ions and water.

Nitrenium ions are intermediates in the metabolic routes producing the highly carcinogenic nitrosamines and binding to DNA molecules. The reaction mechanism of nitrenium molecules with explicit water molecules is sensibly dependent on the number of waters: when a second molecule is involved, it acts as a catalyst for the reaction, lowering intrinsic activation barriers regardless of the substituent. For all cases, the reaction force constants and reaction electron flux indicate highly synchronous reactions for . Conversely, for highly non-synchronous reactions are obtained, involving two separate proton transfers happening early and late in the reaction path. As a test case, for the simplest reactions, orbital interactions within the NBO paradigm, bond orders, and their derivatives indicate that each individual proton transfer is highly synchronous. Molecular geometries were optimized and characterized at the B3LYP/6-311++G(d, p) level. Intrinsic reaction coordinates were calculated. CCSD(T) single point energies with the same basis were computed on all stationary points. The reaction force, reaction force constant, and reaction electron flux are used to study the evolution of the reacting systems. Natural bond orbitals are used to understand the primitive changes driving the reaction.

Experimental and computational studies of steric factors in the isomerization of internal alkynes within the coordination environment of half-sandwich metal complexes.

The isomerization of internal alkynes Ar1C≡CAr2 within the coordination environment of low-valent half-sandwich [Ru(dppe)Cp]+ complexes via a 1,2-migration process affords vinylidene species [Ru{=C=C(Ar1)Ar2}(dppe)Cp]+. The rearrangement reactions of symmetrically and asymmetrically substituted substrates featuring different electron-donating and -withdrawing groups and of varying steric bulk were modelled using density functional theory (DFT), and the conclusions supported by experimental observations. Examination of the reaction pathway and associated activation barriers reveal a high solvent dependency for the generation of the key intermediate species [Ru(dppe)Cp]+ from [RuCl(dppe)Cp] by Na+ induced halide abstraction , with the lattice enthalpy of the NaCl by-product playing a critical role in the overall thermochemical balance of the reaction. The activation barriers associated with the reaction of [Ru(dppe)Cp]+ with Ar1C≡CAr2, and the relative energies of the alkyne complexes [Ru(h2-Ar1C≡CAr2)(dppe)Cp]+, are sensitive to the electron density of the alkyne and conformational changes associated with the 'bend-back' of the substrate. The latter differs by up to 66.1 kJ/mol, which in turn impacts the barrier height of the subsequent 1,2-migration step . The relative importance of these factors is evinced by the successful rearrangement of the very sterically congested 1(9-anthryl)-2(9-phenanthryl) acetylene into the fully characterized diaryl vinylidene complex, which was isolated in 89% yield.

Converting second-order saddle points to transition states: New principles for the design of 4π photoswitches.

Molecular photoswitches have demonstrated potential for storing solar energy at the molecular level, with power densities comparable to commercial batteries and hydroelectric energy storage. However, development of efficient photoswitches is hindered by limitations in cyclability and optical properties of existing materials. We here demonstrate that certain limitations in photoswitches based on electrocyclizations stem from the issue of controlling competition between Woodward-Hoffmann allowed and forbidden pathways. Our approach moves beyond the traditional view of activation barriers and reveals that second-order saddle points are crucial in dictating the competition between disrotatory and conrotatory pathways. These insights suggest new opportunities to manipulate the competition between these pathways through geometric constraints, fundamentally altering the connectivity of the potential energy surface. Our study also emphasizes the necessity of multi-reference methods and the need to conduct higher-dimensional explorations for competing pathways beyond photoswitch design.

Mesoporous, high surface area and microrod-shaped cobalt (II) coordination polymer for assessment of molecular structure, thermolysis and non-isothermal kinetics parameters

This study is significant as it presents the assessment of molecular identification, thermal degradation, and non-isothermal kinetics of a cobalt (II) coordination polymer [Co (II) CPs] synthesized from suberoyl bis (2-ethoxybenzamide) (sbebz) and cobalt acetate through the condensation process. The condensation product sbebz was obtained from suberoyl chloride and 2-ethoxybenzamide. The XRD, FT-IR, Raman, and XPS investigated as aspects to identify its structure, purity, and chemical state. The morphological facet was confirmed by SEM and TEM. The morphology data revealed a microrod-shaped morphology of Co (II) CPs with an average particle size 0.7 µm to 1 µm. The pore size, and surface properties were examined by Brunauer-Emmett-Teller (BET) and atomic force microscopy (AFM), revealing a highly large surface area (1076 m2/g), mesoporous and monodisperse nature. The molecular interaction of ligand was estimated using protein molecule. The thermal-decomposition behavior and non-isothermal kinetics of Co (II) CPs were estimated at different heating rates (5 °C/min, 10 °C/min, 15 °C/min, and 20 °C/min) under nitrogen atmosphere by thermogravimetry/Derivative thermogravimetry (TG/DTG). As-synthesized Co (II) CPs show enhanced thermal stability compared to the ligand (sbebz). The multiple heating TG/DTG curve exhibits a more or less identical degradation profile/pattern. The kinetic parameters like order of reaction (n), pre-exponential Arrhenius factor (A), activation entropy (∆s), activation energy (Ea), activation free-energy (∆G), and activation enthalpy (∆H) were calculated using Coats-Redfern (CR) method.

Exploring the potential of two-dimensional MB4 (M=Cr, Mo, and W) monolayer as a high-capacity negative electrode material for Al-ion batteries

Rechargeable metal-ion batteries can employ two-dimensional transition metal tetraboride monolayers because of their numerous accommodating sites, high specific surface area, and layered structure. Li-ion batteries (LIBs) can be efficiently replaced by aluminum ion batteries (AIBs) because aluminum is a cheap and abundant resource. In this work, the density functional theory approach has been used for Al-ion batteries to predict MB4(M= Cr, Mo, W) monolayers performance as anode material. We analyze these monolayers' structures, which are mechanically, thermally, and dynamically stable. Utilizing the energy-efficient adsorption of six Al-atom layers and their lightweight properties, with large storage capacities of 5066 mA h g−1, 3466 mA h g−1, and 2124 mA h g−1, with relatively low average open circuit voltages of 0.18 V, 0.15 V, and 0.11 V for CrB4, MoB4, and WB4, respectively, these monolayers show significant superiority over other 2D anode materials. Likewise, MB4 monolayer showed faster Al ion mobility as seen by the low activation barriers of 0.95 eV, 0.83 eV, and 0.49 eV for CrB4, MoB4, and WB4, respectively. Additionally, even after the full content of Al ions was adsorbed, the metallic characteristics of the materials were mostly retained. These traits suggest that monolayer CrB4, MoB4, and WB4 are promising anode materials for AIBs, with impressive rate capacities.

Trimetallic Alloys as an Electrocatalyst for Fuel Cells: The Case of Methyl Formate on Pt3Pd3Sn2.

The shift toward renewable energy sources plays a central role in the quest for a circular economy. In this context, methyl formate (MF) has garnered attention as a compelling hydrogen carrier and alternative fuel, because of its remarkable characteristics (energy density, ease of storage and transport, and low boiling point). In this study, DFT calculations supported by online electrochemical mass spectroscopy (OE-MS) were performed to investigate the MF electro-oxidation (MFEO) on Pt3Pd3Sn2 (111). The DFT calculations provide insight into the role of Pt, Pd, and Sn atoms in MFEO. Pt and Pd together provide a preferred active site for initiating MFEO through the O-H bond scission, and Sn plays an essential role in the mitigation of CO through oxygenation or water activation. By comparing the reaction energies and activation barriers for all possible reactions in MFEO, the suggested path necessitates a minimum energy of 0.14 eV to initiate the MFEO. This value was supported by the experimental results, showing that the oxidation wave of MF starts at 0.15 V (70 °C). Density functional theory (DFT) results, supported by OE-MS, indicate that the hydrolysis of MF prior to MFEO is not preferred on Pt3Pd3Sn2 (111) surfaces, although the formation of methanol is plausible via a CH3O intermediate. Among the three small organic molecules (SOMs) studied─MF, methanol, and formic acid─MF has the lowest activation energy for the initial bond breaking that starts the whole oxidation process (0.13 eV), compared to formic acid (0.45 eV) and methanol (0.61 eV); thus, MF is the preferred fuel on Pt3Pd3Sn2 (111).

The Electrical Properties of Dacite Mixed with Various Pyrite Contents and Its Geophysical Applications for the High-Conductivity Duobaoshan Island Arc

In this work, a series of electrical conductivities of pyrite-bearing dacite were measured under 10−1–106 Hz, 573–973 K, 1.0–3.0 GPa, and different pyrite contents ranging from 0 vol.% to 20 vol.%) using a Solartron–1260 A impedance analyzer. For the dacite sample with 5 vol.% pyrite, the electrical conductivity of the dacite increased with temperature but slightly decreased when the pressure was increased from 1.0 GPa to 3.0 GPa. In the temperature range of 573–973 K, the bulk electrical conductivity of the pyrite-bearing dacite gradually increased with increasing pyrite percentage from 0 vol.% to 20 vol.% at 1.0 GPa. Thus, a positive correlation between the electrical conductivity of the sample and the pyrite content was typically observed. In light of the significant enhancement in the electrical conductivity of the interconnected pyrite in the dacite, the value of the percolation threshold was determined as 7 vol.%. Furthermore, the dominant conduction mechanism of the small polaron for pyrite-bearing dacite was proposed from our obtained results on the chemical compositions and activation enthalpies under high-temperature and high-pressure conditions. A comprehensive consideration of our constructed electrical conductivity–depth profile based on the electrical conductivity of the pyrite-bearing dacite, can provide a good constraint on the volume of pyrite in dacite for high-conductivity Duobaoshan island arc. In conclusion, the presence of pyrite in dacite can provide a reasonable explanation for the high-conductivity anomaly observed in the region of Duobaoshan island arc.

Triggering Asymmetric Layer Displacement Polarization and Redox Dual-Sites Activation by Inside-Out Anion Substitution for Efficient CO2 Photoreduction.

Sluggish bulk charge transfer and barren catalytic sites severely hinder the CO2 photoreduction process. Seeking strategies for accelerating charge dynamics and activating reduction and oxidation sites synchronously presents a huge challenge. Herein, an inside-out chlorine (Cl) ions substitution strategy on the layered polar Bi4O5Br2 is proposed for achieving layer structure-dependent polarization effect and redox dual-sites activation. Cl ions in the bulk phase shrink the halogen layer interspace by 8‰, triggering asymmetric [Bi4O5]2+ layer displacement polarization, prolonging the average photocharge lifetime to 201.8ps. Meanwhile, surface substituted Cl ions enhance the electron-donating capability of neighboring Bi atoms, activating the intrinsic Bi reduction sites, and increasing H2O molecule adsorption on nearby intrinsic O oxidation site (cal. by 0.105eV), also self-donating as an alien oxidation site. Besides, Cl upshifts the p-band center closer to the Fermi level, facilitating the reactant adsorption. Therefore, the energy barrier for CO2 activation and rate-limiting *COOH intermediate formation steps are significantly decreased. Without cocatalysts and sacrificial reagents, inside-out Cl-substituted Bi4O5Br2 delivers a remarkable CO2-to-CO photoreduction rate of 50.18µmol g-1 h-1, being one of the state-of-the-art catalysts. This finding offers insights into exploiting polarization at the molecular-level and enhances understanding of catalytic site activation.

Enhanced functionalities of isovalently substituted 0.67(Bi1-xLaxFe0.97Ga0.03O3)-0.33(BaTiO3) relaxor piezoceramics synthesized via air quenching

The isovalently substituted lead-free binary solid solution, 0.67(Bi1-xLaxFe0.97Ga0.03O3)-0.33(BaTiO3) (BF-BT) (x ≤ 0.07), has been synthesized by solid state ceramic method via air quenching sintering. FTIR and XRD along with Rietveld structure refinement confirmed a pure perovskite phase with a pseudocubic structure (space group Pm3‾m) for developed compositions. SEM revealed a dense microstructure with fine grain size (<1.5 μm) and minimal porosity. XPS analysis reveals predominantly Bi+3 and Fe+3 states, along with oxygen vacancies (VO··). Ferroelectric studies demonstrated a maximum remanent polarization (Pr) of 12.3 μC/cm2 and coercive field EC of 30.95 kV/cm for La3+ concentration of x = 0.03. Overall decrement in Pr can be attributed to dipolar defect-induced random fields reducing the activation barrier for domain nucleation. Leakage current measurement reveals improved resistivity (∼108 Ω cm) up to an applied field of 30 kV/cm. Dielectric measurements unveiled two frequency-dependent anomalies in temperature dependence above room temperature, indicative of diffuse phase transitions. The Vogel-Fulcher model depicted an increasing freezing temperature (from 465 K for x = 0.01–551 K for x = 0.07) and decreasing activation energy (from ∼0.61 eV to 0.07 eV) with increasing La3+ concentration. The estimated diffuseness parameter around ∼2 suggested relaxor behavior. Doping with La resulted in increased dielectric permittivity at 10 kHz (from 4100 to 24066), showcasing the efficacy of site engineering in augmenting the functional properties of BF-BT for high-temperature dielectric and transducer applications.

Low-Temperature Activation and Coupling of Methane on MgO Nanostructures Embedded in Cu2O/Cu(111).

The efficient conversion of methane into valuable hydrocarbons, such as ethane and ethylene, at relatively low temperatures without deactivation issues is crucial for advancing sustainable energy solutions. Herein, AP-XPS and STM studies show that MgO nanostructures (0.2-0.5 nm wide, 0.4-0.6 Å high) embedded in a Cu2O/Cu(111) substrate activate methane at room temperature, mainly dissociating it into CHx (x = 2 or 3) and H adatoms, with minimal conversion to C adatoms. These MgO nanostructures in contact with Cu2O/Cu(111) enable C-C coupling into ethane and ethylene at 500 K, a significantly lower temperature than that required for bulk MgO catalysts (>700 K), with negligible carbon deposition and no deactivation. DFT calculations corroborate these experimental findings. The CH4,gas → *CH3 + *H reaction is a downhill process on MgO/Cu2O/Cu(111) surfaces. The activation of methane is facilitated by electron transfer from copper to MgO and the existence of Mg and O atoms with a low coordination number in the oxide nanostructures. The formation of O-CH3 and O-H bonds overcomes the energy necessary for the cleavage of a C-H bond in methane. DFT studies reveal that smaller Mg2O2 model clusters provide stronger binding and lower activation barriers for C-H dissociation in CH4, while larger Mg3O3 clusters promote C-C coupling due to weaker *CH3 binding. All of these results emphasize the importance of size when optimizing the catalytic performance of MgO nanostructures in the selective conversion of methane.

A Computational DFT Study of the Stereoinversion of Succinimide Residues Formed in Proteins and Peptides Catalyzed by a Hydrogen Phosphate Ion: An Unsymmetrical SE1 Mechanism

Succinimide residues formed spontaneously from aspartic acid (Asp) and asparagine (Asn) residues in proteins and peptides are stereochemically unstable, undergoing partial l-to-d stereoinversion, and this is responsible for the d-Asp and d-β-Asp residues found in long-lived proteins. These stereoinverted abnormal amino acid residues are believed to be related to aging and some age-related diseases such as cataracts. Although the succinimide stereoinversion is nonenzymatic, a catalyst is required for it to occur at physiological temperature. In this study, it was found by density functional theory (DFT) calculations that a hydrogen phosphate ion (HPO42−) can effectively catalyze the stereoinversion of the succinimide intermediate. The HPO42− ion abstracts a proton from the asymmetric carbon atom of the succinimide residue to form an enolate intermediate. Then, while the resultant dihydrogen phosphate ion (H2PO4−) remains bound to the enolate ion, a water molecule donates a proton to the enolate intermediate on the opposite side from the phosphate (which is the rate-determining step) to produce the inverted carbon atom. The calculated activation barrier (ca. 90 kJ mol−1) is consistent with a slow in vivo reaction. The present found mechanism can be termed the “unsymmetrical SE1” or “pseudo-SE2” mechanism.

Self-forming Na3P/Na2O interphase on a novel biphasic Na3Zr2Si2PO12/Na3PO4 solid electrolyte for long-cycling solid-state Na-metal batteries

A novel biphasic Na3Zr2Si2PO12/Na3PO4 solid electrolyte is proposed to effectively address critical anode interface challenges for solid-state Na-metal batteries (SSSMBs). The Na3PO4 phase, at an optimal composition of ∼20 mol%, transforms the interface chemistry throughout the NZSP electrolyte, which results in dense electrolytes with high Young's modulus, rapid ion transport (6.2 × 10−4 S cm-1) at low activation barrier (0.19 eV), negligible electronic conductivity, and excellent sodiophilicity. The AIMD/DFT calculations and XPS analysis reveal a self-formed, (electro)chemically stable mixed Na+/electron-conducting interphase, comprising Na3P and Na2O, at the Na anode interface. The interphase not only homogenizes the Na+ flux distribution and accelerates the interfacial charge transport, but prevents continuous interfacial reactions, thereby stabilizing the anode interface against dendrite formation. Benefiting from the low-impedance, dendrite-free anode interface, Na symmetric cells demonstrate a low interface resistance of 12.7 Ω cm2 and exceptional cyclability of 3000 h. Additionally, full cells with Na3V2(PO4)3 cathodes achieve 93 % capacity retention after 550 cycles at 0.5 C. This research comprehensively elucidates and leverages the critical advantages of Na3PO4 in enhancing the bulk and interface properties of Na3Zr2Si2PO12 solid electrolytes. The design strategy of biphasic solid electrolytes presented here offers new insights into the developing high-performance solid electrolytes for advanced SSSMBs.

Mechanistic Insights into the Reactivity and Activation Barrier Origins for CO2 Capture by Heavy Group-14 Imine Analogues.

Using M06-2X-D3/def2-TZVP, the [2 + 2] cycloaddition reactions of carbon dioxide with the heavy imine analogues G14=N-Rea (G14 = Group 14 element) were investigated. The theoretical evidence reveals that the nature of the doubly bonded G14=N moiety in heavy imine analogues, G14=N-Rea (L1L2G14=N-L3), is characterized by the electron-sharing interaction between triplet L1L2G14 and triplet N-L3 fragments. Based on our theoretical studies, except for the carbon-based imine, all four heavy imine analogues with Si=N, Ge=N, Sn=N, and Pb=N groups can easily engage in [2 + 2] cycloaddition reactions with CO2. Energy decomposition analysis-natural orbitals for chemical valence analyses and the FMO theory strongly suggest that in the CO2 capture reaction by heavy imine analogues G14=N-Rea, the primary bonding interaction is the occupied p-π orbital (G14=N-Rea) → vacant p-π* orbital (CO2) interaction, instead of the empty p-π* orbital (G14=N-Rea) ← filled p-π orbital (CO2) interaction. The activation barrier of the CO2 capture reactions by G14=N-Rea molecules is primarily determined by the deformation energy of CO2. Shaik's valence bond state correlation diagram model, used to rationalize the computed results, indicates that the singlet-triplet energy splitting of G14=N-Rea is a key factor in determining the reaction barrier for the current CO2 capture reactions.

Physicochemical Characterization and Thermodynamic Analysis of Avocado Oil Enhanced with Haematococcus pluvialis Extract

The consumption of fatty acids offers significant health benefits; however, they are prone to degradation by environmental factors. One method to preserve these fatty acids is the addition of synthetic antioxidants. This study focuses on the determination of peroxide and MDA formation rates at temperatures of 25 °C, 45 °C, and 65 °C. The oxidative stability of cold-pressed avocado oil was evaluated using pure astaxanthin, TBHQ, and H. pluvialis extract at concentrations of 100, 500, and 1000 ppm. Kinetic models and thermodynamic analysis were applied to determine the oxidation rate and compare the antioxidant effects of H. pluvialis extract with astaxanthin and TBHQ. The Arrhenius model was used to estimate activation energy (Ea), enthalpy, entropy, and free energy. Avocado oil with 500 ppm of H. pluvialis extract showed antioxidant effects comparable to TBHQ and pure astaxanthin. The activation energy of plain avocado oil was 40.47 kJ mol−1, while with H. pluvialis extract, it was 54.35 kJ mol−1. These findings suggest that H. pluvialis extract offers effective antioxidant properties and could serve as a natural alternative to synthetic antioxidants in food applications, despite the limitations of unprotected astaxanthin.

Superconductor to metal quantum phase transition with magnetic field in Josephson coupled lead islands on Graphene

Abstract Superconductor-to-metal transition with magnetic field and gate-voltage is studied in a Josephson junction array comprising of randomly distributed lead islands on exfoliated single-layer graphene. The superconductivity onset temperature at low magnetic-fields is fitted to the Werthamer-Helfand-Hohenberg theory to model the temperature dependence of the upper critical field. The magnetoresistance in the intermediate temperature and field regime is described using thermally activated flux flow dictated by field dependent activation barrier. The barrier also depends on the gate voltage which dictates the inter-island Josephson coupling and disorder. The magnetoresistance near the upper critical field at low temperatures shows signatures of a gate dependent continuous quantum phase transition between superconductor and metal. The finite size scaling analysis shows that this transition belongs to the $(2+1)$D-XY universality class without disorder.&amp;#xD;

Small Molecule Activation at the acriPNP Pincer-Supported Nickel Sites.

ConspectusNickel pincer systems have recently attracted much attention for applications in various organometallic reactions and catalysis involving small molecule activation. Their exploration is in part motivated by the presence of nickel in natural systems for efficient catalysis. Among such systems, the nickel-containing metalloenzyme carbon monoxide dehydrogenase (CODH) efficiently and reversibly converts CO2 to CO at its active site. The generated CO moves through a channel from the CODH active site and is transported to a dinuclear nickel site of acetyl-coenzyme A synthase (ACS), which catalyzes organometallic C-S and C-C bond forming reactions. An analogous C-S bond activation process is also mediated by the nickel containing enzyme methyl-coenzyme M reductase (MCR). The nickel centers in these systems feature sulfur- and nitrogen-rich environments, and in the particular case of lactate racemase, an organometallic nickel pincer motif revealing a Ni-C bond is observed. These bioinorganic systems inspired the development of several nickel pincer scaffolds not only to mimic enzyme active sites and their reactivity but also to further extend low-valent organonickel chemistry. In this Account, we detail our continuing efforts in the chemistry of nickel complexes supported by acridane-based PNP pincer ligands focusing on our long-standing interest in biomimetic small molecule activation. We have employed a series of diphosphinoamide pincer ligands to prepare various nickel(II/I/0) complexes and to study the conversion of C1 chemicals such as CO and CO2 to value-added products. In the transformation of C1 chemicals, the key C-O bond cleavage and C-E bond (E = C, N, O, or S, etc.) formation steps typically require overcoming high activation barriers. Interestingly, enzymatic systems overcome such difficulties for C1 conversion and operate efficiently under ambient conditions with the use of nickel organometallic chemistry. Furthermore, we have extended our efforts to the conversion of NOx anions to NO via the sequential deoxygenation by nickel mediated carbonylation, which was applied to catalytic C-N coupling to produce industrially important organonitrogen compound oximes as a strategy for NOx conversion and utilization (NCU). Notably, the rigidified acriPNP pincer backbone that enforces a planar geometry at nickel was found to be an important factor for diversifying organometallic transformations including (a) homolysis of various σ-bonds mediated by T-shaped nickel(I) metalloradical species, (b) C-H bond activation mediated by a nickel(0) dinitrogen species, (c) selective CO2 reactivity of nickel(0)-CO species, (d) C-C bond formation at low-valent nickel(I or 0)-CO sites with iodoalkanes, and (e) catalytic deoxygenation of NOx anions and subsequent C-N coupling of a nickel-NO species with alkyl halides for oxime production. Broadly, our results highlight the importance of molecular design and the rich chemistry of organonickel species for diverse small molecule transformations.

Co+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy and Theory.

Co+(C2H2)n (n = 1-6) complexes produced with laser vaporization in a supersonic molecular beam are studied with infrared photodissociation spectroscopy and computational chemistry. Infrared spectra are measured in the C-H stretching region using the method of tagging with argon atoms to enhance the photodissociation yields. C-H stretch vibrations for all clusters studied are shifted to lower frequencies than those of the well-known acetylene vibrations from ligand → metal charge transfer interactions. The magnitude of the red shifts decreases in the larger clusters as the interaction is distributed over more ligands. Computational studies identify various unreacted complexes with individual acetylene ligands in cation-π bonding configurations as well as reacted isomers in which ligand coupling reactions have taken place. Infrared spectra are consistent only with unreacted structures, even though the formation of reacted structures such as the metal ion-benzene complex is highly exothermic. Large activation barriers are predicted by theory along the reaction coordinates for the n = 2 and 3 complexes, which inhibit reactions in these smaller clusters, and this situation is presumed to persist in larger clusters.