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Mechanistic Implications of Reductive Co–C Bond Cleavage in B 12 -Dependent Methylmalonyl CoA Mutase
Vitamin B12-dependent enzymes catalyze several difficult radical reactions. However, there are fundamental open questions that need to be addressed to fully understand the formation of highly reactive radical species, its dynamics and interaction with substrate and enzyme. In this work, the possible role of a Proton Coupled Electron Transfer (PCET) pathway concurrent with the Co-C bond activation was investigated. Car-Parrinello molecular dynamics (CPMD) was performed within a QM/MM framework on a reduced AdoCbl cofactor, which was taken as a post-PCET initial step in the activation of the AdoCbl-dependent methylmalonyl CoA mutase (MCM) enzyme. The calculated free energy profile reveals two possible pathways with similar energies, stepwise (I) and concerted (II) for the reductive Co-C cleavage and subsequent H-abstraction. The concerted pathway may be preferred kinetically, because it avoids the formation of a high-energy radical intermediate with possibly a larger recrossing rate. Our results are consistent with the previous proposal of the conductor hypothesis, indicating the explicit role of cob(II)alamin in stabilizing the radical intermediate involved in H-atom transfer.
Chemical bond activation observed with an x-ray laser
The concept of bonding and anti-bonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Lastly, we apply time-resolved soft X-ray spectroscopy at a free-electron laser to directly observe the decreased bonding–anti-bonding splitting following bond-activation using an ultra short optical laser pulse.
Theory of Surface Forces in Multivalent Electrolytes
Aqueous electrolyte solutions containing multivalent ions exhibit various intriguing properties, including attraction between like-charged colloidal particles, which results from strong ion–ion correlations. In contrast, the classical Derjaguin–Landau–Verwey–Overbeek theory of colloidal stability, based on the Poisson–Boltzmann mean-field theory, always predicts a repulsive electrostatic contribution to the disjoining pressure. Here, we formulate a general theory of surface forces, which predicts that the contribution to the disjoining pressure resulting from ion–ion correlations is always attractive and can readily dominate over entropic-induced repulsions for solutions containing multivalent ions, leading to the phenomenon of like-charge attraction. Ion-specific short-range hydration interactions, as well as surface charge regulation, are shown to play an important role at smaller separation distances but do not fundamentally change these trends. The theory is able to predict the experimentally observed strong cohesive forces reported in cement pastes, which result from strong ion–ion correlations involving the divalent calcium ion.
Anisotropic Coarse-Grained Model for Conjugated Polymers: Investigations into Solution Morphologies
The optoelectronic properties of conjugated polymers are dictated by the interplay of multiscale structural features, including intrachain dihedrals, interchain pi-pi stacking, and the complex mesoscale morphology. While much is known about the structures of polymers with isotropically interacting monomers, little is known about polymers with strongly anisotropic backbone monomers, a class of materials to which conjugated polymers belong. Fundamental understanding is further complicated when the semiflexible and molecularly heterogeneous nature of conjugated polymers is taken into account. We present an anisotropic coarse-grained (CG) model for conjugated polymers that incorporates key molecular features (monomer anisotropy, intermonomer dihedrals, and side chains). Finally, the model is employed to characterize the single-chain conformational properties of conjugated polymers, revealing a rich temperature-dependent conformational landscape. These studies provide a critical link between CG molecular descriptors and conformational ordering in conjugated polymers.
Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization
β-agostic Pd complexes are critical intermediates in catalytic reactions, such as olefin polymerization and Heck reactions. Pd β-agostic complexes, however, have eluded structural characterization, due to the fact that these highly unstable molecules are difficult to isolate. In this paper, we report the single-crystal X-ray and neutron diffraction characterization of β-agostic (α-diimine)Pd–ethyl intermediates in polymerization. Short C α –C β distances and acute Pd–C α –C β bond angles combined serve as unambiguous evidence for the β-agostic interaction. Finally, characterization of the agostic structure and the kinetic barrier for β-H elimination offer important insight into the fundamental understanding of agostic bonds and the mechanism of polymerization.
Imaging moiré flat bands in three-dimensional reconstructed WSe 2 /WS 2 superlattices
Moiré superlattices in transition metal dichalcogenide (TMD) heterostructures can host novel correlated quantum phenomena due to the interplay of narrow moiré flat bands and strong, long-range Coulomb interactions. However, microscopic knowledge of the atomically reconstructed moiré superlattice and resulting flat bands is still lacking, which is critical for fundamental understanding and control of the correlated moiré phenomena. Here we quantitatively study the moiré flat bands in three-dimensional (3D) reconstructed WSe 2 /WS 2 moiré superlattices by comparing scanning tunnelling spectroscopy (STS) of high-quality exfoliated TMD heterostructure devices with ab initio simulations of TMD moiré superlattices. A strong 3D buckling reconstruction accompanied by large in-plane strain redistribution is identified in our WSe 2 /WS 2 moiré heterostructures. STS imaging demonstrates that this results in a remarkably narrow and highly localized K-point moiré flat band at the valence band edge of the heterostructure. A series of moiré flat bands are observed at different energies that exhibit varying degrees of localization. Our observations contradict previous simplified theoretical models but agree quantitatively with ab initio simulations that fully capture the 3D structural reconstruction. Furthermore, our results reveal that the strain redistribution and 3D buckling in TMD heterostructures dominate the effective moiré potential and the corresponding moiré flat bands at the Brillouin zone K points.
Signatures of a liquid-crystal transition in spin-wave excitations of skyrmions
Abstract Understanding the spin-wave excitations of chiral magnetic order, such as the skyrmion crystal (SkX), is of fundamental interest to confirm such exotic magnetic order. The SkX is realized by competing Dzyaloshinskii-Moriya and ferromagnetic-exchange interactions with a magnetic field or anisotropy. Here, we compute the dynamical spin structure factor, using Monte Carlo and spin dynamics simulations, extracting the spin-wave spectrum in the SkX, in the vicinity of the paramagnet to SkX transition. Inside the SkX, we find six spin-wave modes, which are supplemented by another mode originating from the ferromagnetic background. Above the critical temperature T s for the skyrmion crystallization, we find a diffusive regime, reminiscent of the liquid-to-crystal transition, revealing that topological spin texture of skyrmionic character starts to develop above T s as the precursor of the SkX. We discuss the opportunities for the detection of the spin waves of the SkX using inelastic-neutron-scattering experiments in manganite-iridate heterostructures.
Computation of high-frequency magnetoelastic waves in layered materials
Here, the direct calculation of magnetoelastic wave dispersion in layered media is presented using an efficient, accurate computational technique. The governing, coupled equations for elasticity and magnetism, the Navier and Landau-Lifshitz equations, respectively, are linearized to form a quadratic eigenvalue problem that determines a complex web of wave-number–frequency dispersion branches and their corresponding mode profiles. Numerical discretization of the eigenvalue problem via a spectral collocation method (SCM) is employed to determine the complete dispersion maps for both a single, finite-thickness magnetic layer and a finite magnetic-nonmagnetic double-layer. The SCM, previously used to study elastic waves in nonmagnetic media, is fast, accurate, and adaptable to a variety of sample configurations and geometries. Emphasis is placed on the extremely high-frequency regimes being accessed in ultrafast magnetism experiments. The dispersion maps and modes provide insight into how energy propagates through the coupled system, including how energy can be transferred between elastic- and magnetic-dominated waves as well as between different layers. The numerical computations for a single layer are further understood by a simplified analytical calculation in the high-frequency, exchange-dominated regime where the resonance condition required for energy exchange (an anticrossing) between quasi-elastic and quasi-magnetic dispersion branches is determined. Nonresonant interactions are shown to be well approximated by the dispersion of uncoupled elastic and magnetic waves. The methods and results provide fundamental theoretical tools to model and understand current and future magnetic devices powering spintronic innovation.
Theoretical calculation of the melting curve of Cu-Zr binary alloys
Helmholtz free energies of the dominant binary crystalline solids found in the Cu-Zr system at high temperatures close to the melting curve are calculated. This theoretical approach combines fundamental measure density functional theory (applied to the hard-sphere reference system) and a perturbative approach to include the attractive interactions. The studied crystalline solids are Cu(fcc), Cu 51 Zr 14 (β), CuZr(B 2 ), CuZr 2 (C11b), Zr(hcp), and Zr(bcc). The calculated Helmholtz free energies of crystalline solids are in good agreement with results from molecular-dynamics (MD) simulations. Using the same perturbation approach, the liquid phase free energies are calculated as a function of composition and temperature, from which the melting curve of the entire composition range of this system can be obtained. Phase diagrams are determined in this way for two leading embedded atom method potentials, and the results are compared with experimental data. Furthermore, theoretical melting temperatures are compared both with experimental values and with values obtained directly from MD simulations at several compositions.
Higher Harmonics in Multipactor Induced Plasma Ionization Breakdown near a Dielectric Surface
In this Letter, the novel physics of higher harmonic (HH) generation of the normal electric field near a dielectric surface is reported in multipactor induced plasma ionization breakdown, as determined by kinetic particle-in-cell simulations. The observed HH frequency is around ten times the fundamental rf driving frequency, but lower than the electron plasma frequency. A theory is constructed which indicates that stream plasma interaction-induced instability is the mechanism of HH generation in the collisional regime. The HH frequency and its corresponding growth rate of the HH oscillation amplitude from the theory are in good agreement with kinetic particle-in-cell simulations.
Learning Many-Body Hamiltonians with Heisenberg-Limited Scaling
Learning a many-body Hamiltonian from its dynamics is a fundamental problem in physics. Here, in this Letter, we propose the first algorithm to achieve the Heisenberg limit for learning an interacting N-qubit local Hamiltonian. After a total evolution time of $\mathscr{O}$(ε –1 ), the proposed algorithm can efficiently estimate any parameter in the N-qubit Hamiltonian to ε error with high probability. Our algorithm uses ideas from quantum simulation to decouple the unknown N-qubit Hamiltonian H into noninteracting patches and learns H using a quantum-enhanced divide-and-conquer approach. The proposed algorithm is robust against state preparation and measurement error, does not require eigenstates or thermal states, and only uses polylog(ε –1 ) experiments. In contrast, the best existing algorithms require $\mathscr{O}$(ε –2 ) experiments and total evolution time. We prove a matching lower bound to establish the asymptotic optimality of our algorithm.
Spectral Similarity Masks Structural Diversity at Hydrophobic Water Interfaces
The air-water and graphene-water interfaces represent quintessential examples of the liquid-gas and liquid-solid boundaries, respectively. While the sum-frequency generation (SFG) spectra of these interfaces show similarities, a consensus on their signals and interpretations has yet to be reached. Leveraging deep learning, we computed first-principles SFG spectra for both systems, addressing experimental discrepancies. Here, our findings reveal that similarities in SFG signals do not translate into comparable interfacial microscopic properties. Instead, graphene-water and air-water interfaces exhibit fundamental differences in SFG-active thicknesses, hydrogen-bonding networks, and surface dynamics. These distinctions underscore roughness suppression and electronic interactions present at the solid-liquid interface but absent at the gas-liquid interface.
Spatiotemporal Bragg gratings forming inside a nonlinear dispersive medium
We show that a spatiotemporal Bragg grating can be created inside a nonlinear dispersive medium (such as silica fibers) by launching a periodic train of pump pulses that travel as fundamental solitons. We develop a theoretical model and use it to find the band structure of such gratings. We study the interaction of a probe pulse with the Bragg grating, both within and outside of momentum gaps. We also show that a photonic analog of the Anderson localization is possible when a disorder is introduced into a spatiotemporal Bragg grating.
Fuel Cell Tech Team Meeting: Engineered Nano-scale Ceramic Supports for PEM Fuel Cells [Slides]
Catalyst support durability is currently a technical barrier for commercialization of polymer electrolyte membrane (PEM) fuel cells, especially for transportation applications. Degradation and corrosion of the conventional carbon supports leads to losses in active catalyst surface area and, consequently, reduced performance. As a result, the goal of this work is to develop support materials that interact strongly with Pt, yet sustain bulk-like catalytic activities with very highly dispersed particles. Ceramic materials that are prepared using conventional solid-state methods have large grain sizes and low surface areas that can only be minimally ameliorated through grinding and ball milling. Other synthesis routes to produce ceramic materials must be investigated and utilized in order to obtain desired surface areas. In this work, several different synthesis methods are being utilized to prepare electronically conductive ceramic boride, nitride, and oxide materials with high surface areas and have the potential for use as PEMFC catalyst supports. Polymer-assisted deposition (PAD) and aerosol-through plasma (A-T-P) torch are among several methods used to obtain ceramic materials with surface areas that are equal to, or exceed Vulcan XC-72R supports. Cubic Mo-based ceramic phases have been prepared with average XRD-determined crystallite sizes as low as 1.6 nm (from full profile, XRD fitting) and a BET surface area exceeding 200 m 2 /g. Additionally, black, sub-stoichiometric TiO 2-x , have been prepared with an average crystallite size in the 4 nm range and surface areas exceeding 250 m 2 /gr. Pt disposition using an incipient wetness approach produced materials with activity for hydrogen redox reactions and ORR. Cyclic voltammetry data will be shown for a variety of potential Pt/ceramic catalysts. Initial experiments indicate enhanced Pt metal-support interactions as well. Plane wave periodic density functional calculations (VASP) are being used to predict the thermodynamic and activation barriers for fundamental electrode processes occurring at platinum surfaces supported on thin films of the ceramic support materials. The results of this work will be used in order to optimize support properties.
ECP ALCC Quarterly Report (Oct-Dec 2017)
The scientific goal of ExaWind Exascale Computing Project (ECP) is to advance our fundamental understanding of the flow physics governing whole wind plant performance, including wake formation, complex terrain impacts, and turbine-turbine-interaction effects. Current methods for modeling wind plant performance fall short due to insufficient model fidelity and inadequate treatment of key phenomena, combined with a lack of computational power necessary to address the wide range of relevant length scales associated with wind plants. Thus, our ten-year exascale challenge is the predictive simulation of a wind plant composed of O(100) multi-MW wind turbines sited within a 100 km2 area with complex terrain, involving simulations with O(100) billion grid points. The project plan builds progressively from predictive petascale simulations of a single turbine, where the detailed blade geometry is resolved, meshes rotate and deform with blade motions, and atmospheric turbulence is realistically modeled, to a multi turbine array in complex terrain. The ALCC allocation will be used continually throughout the allocation period. In the first half of the allocation period, small (e.g., for testing Kokkos algorithms) and medium (e.g., 10K cores for highly resolved ABL simulations) sized jobs will be typical. In the second half of the allocation period, we will also have a number of large submittals for our resolved-turbine simulations. A challenge in the latter period is that small time step sizes will require long wall-clock times for statistically meaningful solutions. As such, we expect our allocation-hour burn rate to increase as we move through the allocation period.
Nuclear Fundamentals Orientation Module 1: Nuclear Weapons Deterrence Concepts [Slides]
Topics covered include: Defining Deterrence, And Why it Matters; Deterrence Today - Challenges of Great Power, Competition and Multi-Polar Nuclear Interaction; Deterrence and Our Mission -- Science underpins Deterrence; and, Questions and Answers.
Lattice QCD for Neutrino Physics
Neutrinos are the most poorly understood particles in the Standard Model but may be key to understanding fundamental mysteries of the universe, including matter-antimatter asymmetry. Most experiments to improve our knowledge of neutrinos depend on neutrino interactions with hadronic matter and therefore rely on a solid theoretical understanding of these interactions. Lattice QCD offers a first-principles, model-independent method of analyzing hadronic systems and their responses to external currents, including the electroweak currents induced by neutrino interactions. In this talk, I will discuss my work on using lattice QCD to understand the hadronic systems needed for both neutrino oscillation and neutrinoless double-beta decay experiments and the impact of this work on neutrino physics.