Extension of the Polarizable Charge Equilibration Model to Higher Oxidation States with Applications to Ge, As, Se, Br, Sn, Sb, Te, I, Pb, Bi, Po, and At Elements

We recently developed the Polarizable Charge Equilibration (PQEq) model to predict accurate electrostatic interactions for molecules and solids and optimized parameters for H, C, N, O, F, Si, P, S, and Cl elements to fit polarization energies computed by quantum mechanics (QM). Here, we validate and optimize the PQEq parameters for other p-block elements including Ge, As, Se, Br, Sn, Sb, Te, I, Pb, Bi, Po, and At using 28 molecular structures containing these elements. For elements in the Se column of the periodic table, we now include molecules with higher oxidation states: III and V for the As column, IV and VI for the Se column, -I, III, and V for the Br column. We find that PQEq predicts polarization energies in excellent agreement with QM

Electronic Structure of Tetracyanonickelate(II)

Tetracyanonickelate(II) has been a poster child of ligand field theory for several decades. We have revisited the literature assignments of the absorption spectrum of [Ni(CN) ₄]²⁻ and the calculated ordering of orbitals with metal d character. Using low-temperature single-crystal absorption spectroscopy and accurate ab initio and density functional quantum mechanical methods (NEVPT2-CASSCF, EOM-CCSD, TD-DFT), we find an ordering of the frontier d- and p-orbitals of xy < xz, yz < z² < z < x²–y² < x, y and assign the d-d bands in the absorption spectrum to ¹A_(1g) → ³B_(1g) < ³E_g < ³A_(2g) < ¹B_(1g) < ¹E_g < ¹A_(2g). While differing from all previous interpretations, our assignments accord with an MO model in which strong π-backbonding in the plane of the molecule stabilizes d_(xy) more than out-of-plane bonding stabilizes d_(xz) and d_(yz)

Shock Synthesis of Five-component Icosahedral Quasicrystals

Five-component icosahedral quasicrystals with compositions in the range Al_(68–73)Fe_(11–16)Cu_(10–12)Cr_(1–4)Ni_(1–2) were recently recovered after shocking metallic CuAl_5 and (Mg_(0.75)Fe_(0.25))_2SiO_4 olivine in a stainless steel 304 chamber, intended to replicate a natural shock that affected the Khatyrka meteorite. The iron in those quasicrystals might have originated either from reduction of Fe^(2+) in olivine or from the stainless steel chamber. In this study, we clarify the shock synthesis mechanism of icosahedral quasicrystals through two new shock recovery experiments. When CuAl_5 and Fe^(2+)-bearing olivine were isolated in a Ta capsule, no quasicrystals were found. However, with only metallic starting materials, numerous micron-sized five-component icosahedral quasicrystals, average composition Al_(72)Cu_(12)Fe_(12)Cr_3Ni_1, were found at the interface between CuAl_5 and stainless steel, demonstrating nucleation of quasicrystals under shock without any redox reaction. We present detailed characterization of recovered quasicrystals and discuss possible mechanisms for generating sufficiently high temperatures to reach melting with relatively weak shocks. We discuss the implications of our five-component quasicrystal for the stability of quasicrystals, which have previously only been considered in alloy systems with four or fewer components. Even small amounts of additional metals expand the stability range of the icosahedral phase and facilitate routine syntheses without extraordinary precision in preparation of starting mixtures

CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone

The mechanism originally proposed by Fischer and Tropsch for carbon monoxide (CO) hydrogenative catenation involves C–C coupling from a carbide-derived surface methylidene. A single molecular system capable of capturing these complex chemical steps is hitherto unknown. Herein, we demonstrate the sequential addition of proton and hydride to a terminal Mo carbide derived from CO. The resulting anionic methylidene couples with CO (1 atm) at low temperature (−78 °C) to release ethenone. Importantly, the synchronized delivery of two reducing equivalents and an electrophile, in the form of a hydride (H– = 2e– + H+), promotes alkylidene formation from the carbyne precursor and enables coupling chemistry, under conditions milder than those previously described with strong one-electron reductants and electrophiles. Thermodynamic measurements bracket the hydricity and acidity requirements for promoting methylidene formation from carbide as energetically viable relative to the heterolytic cleavage of H2. Methylidene formation prior to C–C coupling proves critical for organic product release, as evidenced by direct carbide carbonylation experiments. Spectroscopic studies, a monosilylated model system, and Quantum Mechanics computations provide insight into the mechanistic details of this reaction sequence, which serves as a rare model of the initial stages of the Fischer–Tropsch synthesis

Extension of the Polarizable Charge Equilibration Model to Higher Oxidation States with Applications to Ge, As, Se, Br, Sn, Sb, Te, I, Pb, Bi, Po, and At Elements

We recently developed the Polarizable Charge Equilibration (PQEq) model to predict accurate electrostatic interactions for molecules and solids and optimized parameters for H, C, N, O, F, Si, P, S, and Cl elements to fit polarization energies computed by quantum mechanics (QM). Here, we validate and optimize the PQEq parameters for other p-block elements including Ge, As, Se, Br, Sn, Sb, Te, I, Pb, Bi, Po, and At using 28 molecular structures containing these elements. For elements in the Se column of the periodic table, we now include molecules with higher oxidation states: III and V for the As column, IV and VI for the Se column, -I, III, and V for the Br column. We find that PQEq predicts polarization energies in excellent agreement with QM

Accurate non-bonded potentials based on periodic quantum mechanics calculations for use in molecular simulations of materials and systems

Molecular dynamics simulations require accurate force fields (FFs) to describe the physical and chemical properties of complex materials and systems. FF parameters for valence interactions can be determined from high-quality Quantum Mechanical (QM) calculations. However, it has been challenging to extract long-range nonbonded interaction potentials from QM calculations since there is no unambiguous method to separate the total QM energy into electrostatics (polarization), van der Waals (vdW), and other components. Here, we propose to use density functional theory with dispersion corrections to obtain the equation of state for single element solid systems (of H, C, N, O, F, Cl, Br, I, P, He, Ne, Ar, Kr, Xe, and Rn) from which we obtain the pure 2-body vdW nonbonded potentials. Recently, we developed the polarizable charge equilibration (PQEq) model based on QM polarization energy of electric probe dipoles with no contributions from vdW. Together, the vdW and PQEq interactions form the nonbonded potential of our new transferrable reactive FF (RexPoN). They may also be useful to replace the nonbonded parts of standard FFs, such as OPLS, Amber, UFF, and CHARMM. We find that the individual 2-body vdW potential curves can be scaled to a universal vdW potential using just three specific atomic parameters. This simplifies extension to the rest of the periodic table for atoms that do not exhibit molecular packing. We validate the accuracy of these nonbonded interactions for liquid water, energetic, and biological systems. In all cases, we find that our new nonbonded potentials provide good agreement with QM and experimental data

Shock Synthesis of Five-component Icosahedral Quasicrystals

Five-component icosahedral quasicrystals with compositions in the range Al_(68–73)Fe_(11–16)Cu_(10–12)Cr_(1–4)Ni_(1–2) were recently recovered after shocking metallic CuAl_5 and (Mg_(0.75)Fe_(0.25))_2SiO_4 olivine in a stainless steel 304 chamber, intended to replicate a natural shock that affected the Khatyrka meteorite. The iron in those quasicrystals might have originated either from reduction of Fe^(2+) in olivine or from the stainless steel chamber. In this study, we clarify the shock synthesis mechanism of icosahedral quasicrystals through two new shock recovery experiments. When CuAl_5 and Fe^(2+)-bearing olivine were isolated in a Ta capsule, no quasicrystals were found. However, with only metallic starting materials, numerous micron-sized five-component icosahedral quasicrystals, average composition Al_(72)Cu_(12)Fe_(12)Cr_3Ni_1, were found at the interface between CuAl_5 and stainless steel, demonstrating nucleation of quasicrystals under shock without any redox reaction. We present detailed characterization of recovered quasicrystals and discuss possible mechanisms for generating sufficiently high temperatures to reach melting with relatively weak shocks. We discuss the implications of our five-component quasicrystal for the stability of quasicrystals, which have previously only been considered in alloy systems with four or fewer components. Even small amounts of additional metals expand the stability range of the icosahedral phase and facilitate routine syntheses without extraordinary precision in preparation of starting mixtures

Interface Structure in Li-Metal/[Pyr_(14)][TFSI]-Ionic Liquid System from Ab Initio Molecular Dynamics Simulations

Ionic liquids (ILs) are promising materials for application in a new generation of Li batteries. They can be used as electrolyte or interlayer or incorporated into other materials. ILs have the ability to form a stable solid electrochemical interface (SEI), which plays an important role in protecting the Li-based electrode from oxidation and the electrolyte from extensive decomposition. Experimentally, it is hardly possible to elicit fine details of the SEI structure. To remedy this situation, we have performed a comprehensive computational study (density functional theory-based molecular dynamics) to determine the composition and structure of the SEI compact layer formed between the Li anode and [Pyr_(14)][TFSI] IL. We found that the [TFSI] anions quickly reacted with Li and decomposed, unlike the [Pyr_(14)] cations which remained stable. The obtained SEI compact layer structure is nonhomogeneous and consists of the atomized S, N, O, F, and C anions oxidized by Li atoms