Reactivity of Sodium Pentaphospholide Na[cyclo-P5 ] towards C≡E (E=C, N, P) Triple Bonds.

A diglyme solution of Na[cyclo-P5 ] (1) reacts with alkynes and isolobal nitriles and phosphaalkynes to afford the otherwise elusive (aza)phospholide anions 2 a-c, 4 a,b, and 6. The reaction of Na[cyclo-P5 ] with alkynes and nitriles was studied by means of DFT methods, which suggested a concerted mechanism for the formation of 2 a and 4 b. The anions 2 a-c, 4 a,b, and 6 coordinate in an η5 -fashion towards FeII to give the sandwich (aza)phosphametallocenes 3 a-c, 5 a,b and 7 in moderate to good yields. The new compounds were characterized by means of multinuclear NMR spectroscopy, single-crystal X-ray diffraction and cyclic voltammetry.

Searching for Monomeric Nickel Tetrafluoride: Unravelling Infrared Matrix Isolation Spectra of Higher Nickel Fluorides.

Binary transition metal fluorides are textbook examples combining complex electronic features with most fundamental molecular structures. High-valent nickel fluorides are among the strongest known fluorinating and oxidizing agents, but there is a lack of experimental structural and spectroscopic investigations on molecular NiF3 or NiF4 . Apart from their demanding synthesis, also their quantum-chemical description is difficult due to their open shell nature and low-lying excited electronic states. Distorted tetrahedral NiF4 (D2d ) and trigonal planar NiF3 (D3h ) molecules were produced by thermal evaporation and laser ablation of nickel atoms in a fluorine/noble gas mixture and spectroscopically identified by a joint matrix-isolation and quantum-chemical study. Their vibrational band positions provide detailed insights into their molecular structures.

Universal Algorithm for Simulating and Evaluating Cyclic Voltammetry at Macroporous Electrodes by Considering Random Arrays of Microelectrodes.

An algorithm for the simulation and evaluation of cyclic voltammetry (CV) at macroporous electrodes such as felts, foams, and layered structures is presented. By considering 1D, 2D, and 3D arrays of electrode sheets, cylindrical microelectrodes, hollow-cylindrical microelectrodes, and hollow-spherical microelectrodes the internal diffusion domains of the macroporous structures are approximated. A universal algorithm providing the time-dependent surface concentrations of the electrochemically active species, required for simulating cyclic voltammetry responses of the individual planar, cylindrical, and spherical microelectrodes, is presented as well. An essential ingredient of the algorithm, which is based on Laplace integral transformation techniques, is the use of a modified Talbot contour for the inverse Laplace transformation. It is demonstrated that first-order homogeneous chemical kinetics preceding and/or following the electrochemical reaction and electrochemically active species with non-equal diffusion coefficients can be included in all diffusion models as well. The proposed theory is supported by experimental data acquired for a reference reaction, the oxidation of [Fe(CN)6 ]4- at platinum electrodes as well as for a technically relevant reaction, the oxidation of VO2+ at carbon felt electrodes. Based on our calculation strategy, we provide a powerful open source tool for simulating and evaluating CV data implemented into a Python graphical user interface (GUI).

Theory of cyclic voltammetry in random arrays of cylindrical microelectrodes applied to carbon felt electrodes for vanadium redox flow batteries.

In order to quantitatively investigate the kinetic performance and the pore size distribution of carbon felt electrodes for the application in vanadium redox flow batteries, the theory of cyclic voltammetry (CV) is derived for a random network of cylindrical microelectrodes on the base of convolutive modeling. In this context we present an algorithm based on the use of a modified Talbot contour for inverse Laplace transformation, providing the mass transfer functions required for the calculation of the CV responses in external cylindrical finite diffusion space. First-order homogenous chemical kinetics preceding and/or following the electrochemical reactions are implemented in this algorithm as well. The VO2+ oxidation is investigated as model reaction at pristine and electrochemically aged commercial carbon felt electrodes. A fit of simulated data to experimental data clearly shows that an electrochemical aging predominantly affects the kinetics of the electron transfer reaction and that internal electrode surfaces and pore size distributions remain constant. The estimated pore size distributions are in excellent agreement with porosimetry measurements, validating our theory and providing a new strategy to determine electrode porosities and electrode kinetics simultaneously via CV.

Oxygen radical character in group 11 oxygen fluorides.

Transition metal complexes bearing terminal oxido ligands are quite common, yet group 11 terminal oxo complexes remain elusive. Here we show that excited coinage metal atoms M (M = Au, Ag, Cu) react with OF2 to form hypofluorites FOMF and group 11 oxygen metal fluorides OMF2, OAuF and OAgF. These compounds have been characterized by IR matrix-isolation spectroscopy in conjunction with state-of-the-art quantum-chemical calculations. The oxygen fluorides are formed by photolysis of the initially prepared hypofluorites. The linear molecules OAgF and OAuF have a 3Σ - ground state with a biradical character. Two unpaired electrons are located mainly at the oxygen ligand in antibonding O-M π* orbitals. For the 2B2 ground state of the OMIIIF2 compounds only an O-M single bond arises and a significant spin-density contribution was found at the oxygen atom as well.

A computational study of samarium diiodide-induced cyclizations of N-oxoalkyl-substituted methyl indole-3-carboxylates-A rationale of the diastereoselectivity.

A detailed model for the reaction mechanism of the samarium diiodide (SmI2 ) mediated reductive coupling of N-oxoalkyl-substituted methyl indole-3-carboxylates is developed in this study by determining the Gibbs energies for the intermediates of possible reaction pathways. The Gibbs energies at ambient temperature are calculated with dispersion corrected density functional theory in combination with implicit (D-COSMO-RS) and explicit solvent description. Temperature dependent ro-vibrational contributions are considered with the help of statistical thermodynamics. In contrast to previous proposals for the reaction mechanism, the high diastereoselectivity in the cyclization is found to be due to the formation of an energetically highly favorable chelate complex in which the final relative configuration is already preformed. After cyclization and a second electron transfer, alkylation of the resulting anion takes place under kinetic control from the more "open" face whereas protonation is under thermodynamic control. The calculations are in good agreement with these experimental findings. © 2017 Wiley Periodicals, Inc.

Increasing Radical Character of Large [n]cyclacenes Unveiled by Wave Function Theory.

We have investigated the radicality and the vertical singlet-triplet energy gap of [n]cyclacenes (cyclic polyacenes) as a function of the system size for n even, from 6 to 22. The calculations are performed using the complete active space self-consistent field method and second-order n-electron valence perturbation theory. We present a systematic way for the selection of the active space in order to have a balanced description of the wave function as the size of the system increases. Moreover, we provide didactic insight into the failure of an approach based on a minimal active space. We find that the ground state is an open-shell singlet and its multireference character increases progressively with n. The singlet-triplet gap decreases as a function of the system size and approaches a finite positive value for the limit n → ∞. Finally, an analysis based on the one-particle reduced density matrix suggests a polyradical character for the largest cyclacenes.

Low-Lying Electronic States of CuAu.

Coinage metal diatomic molecules are building blocks for nanostructured materials, electronic devices, and catalytically or photochemically active systems that are currently receiving lively interest in both fundamental and applied research. The theoretical study presented here elucidates the electronic structure in the ground and several low-lying excited states of the diatomic molecule CuAu that result from the combination of the atoms in their ground states nd(10)(n + 1)s(1 2)S and lowest excited d-hole states nd(9)(n + 1)s(2 2)D (n = 3 for Cu, n = 5 for Au). Full and smooth potential energy curves, obtained at the multireference configuration interaction (MRCI) level of theory, are presented for the complete set of the thus resulting 44 Λ-S terms and 86 Ω terms. Our approach is based on a scalar relativistic description using the Douglas-Kroll-Hess (DKH) Hamiltonian, with subsequent perturbative inclusion of spin-orbit (SO) coupling via the spin-orbit terms of the Breit-Pauli (BP) Hamiltonian. The Ω terms span an energy interval of about 7 eV at the ground state's equilibrium distance. Spectroscopic constants, calculated for all terms, are shown to accurately reproduce the observation for those nine terms that are experimentally known.

Low-temperature formation of cubic ß-PbF2: precursor-based synthesis and first-principles phase stability study.

A precursor-based approach to the cubic ß-phase of PbF(2) was developed and allowed the preparation of this high-temperature phase well below the temperature for transition from the orthorhombic α- to the cubic ß-phase. The formation of ß-PbF(2) from the molecular precursors Pb[Se(C(6)H(2)(CF(3))(3))](2) and Pb(C(6)H(2)(CF(3))(3))(2) is facilitated by the presence of several short PbF contacts in these molecules. The cubic form of PbF(2) was obtained as macroscopic crystals as well as nanoparticulate powder. Its formation at relatively low temperature suggested a theoretical re-investigation of the phase stabilities of the two polymorphs. The theoretical results from the Kohn-Sham density functional theory indicate that the energy content for the ß-phase is slightly lower than the one for the α-phase, by 0.5-1.7 kJ mol(-1) depending on the density functional used (zero-point vibrational energy correction included).

Electronic quantum fluxes during pericyclic reactions exemplified for the Cope rearrangement of semibullvalene.

The outcome of a pericyclic reaction is typically represented by arrows in the Lewis structure of the reactant, symbolizing the net electron transfer. Quantum simulations can be used to interpret these arrows in terms of electronic fluxes between neighboring bonds. The fluxes are decomposed into contributions from electrons in so-called pericyclic orbitals, which account for the mutation of the Lewis structure for the reactant into that for the product, in other valence and in core orbitals. Series of time-integrated fluxes of pericyclic electrons can be assigned to the arrows, for example 0.09-0.23 electrons for Cope rearrangement of semibullvalene, with hysteresis-type time evolutions for 27.3 fs. This means asynchronous electronic fluxes during synchronous rearrangement of all the nuclei. These predictions should become observable by emerging techniques of femto- to attosecond time-resolved spectroscopy.

Understanding the hcp anisotropy in Cd and Zn: the role of electron correlation in determining the potential energy surface.

Cadmium crystallises in the hcp structure, but with an anomalously large c/a ratio, indicating a strong distortion away from ideal packing. Coupled cluster calculations within the framework of the method of increments with an embedding scheme for metals were performed to explore the potential energy surface of cadmium with respect to the hexagonal lattice parameters. This potential energy surface is compared to density functional theory based surfaces, as calculated with various functionals. The overall flatness of the potential energy surface over a wide range of values of the lattice parameter c is analogous for both treatments, however only within the method of increments do we quantitatively describe the cohesion. The overall behaviour of the method of increments for cadmium is consistent with previous results for zinc, emphasising the dominant role of electronic correlation in achieving a sufficiently accurate description of bonding properties for the two elements; however, a detailed analysis shows differences. We discuss this in detail in terms of the correlation contributions of the s- and d-electrons.

Molecular dynamics simulations of dendrimer-encapsulated alpha-Keggin ions in trichloromethane solution.

We report on molecular dynamics simulations of dendrimer-encapsulated alpha-Keggin ions in trichloromethane solution. The simulations were done within the NVE ensemble at temperatures around T = 300 K. The eight examined systems are model compounds for dendrizymes, a hybrid material where a polyoxometalate ion (the core) is surrounded by amphiphilic cationic dendrimers (the shell) such that the complete system may exhibit enzyme-like regioselectivity and substrate selectivity, e.g., in olefin oxidation. The influence of dendrimer type, dendrimer generation, and number of dendritic cations bound by electrostatic interaction to the polyoxometalate core on the structure and dynamics of the shell has been studied. It is shown that the resulting distribution of trichloromethane molecules within the shell may serve as an indicator for the shell's permeability for small molecules. The dendritic shell causes a size exclusion effect that influences the access of small molecules to the central polyoxometalate ion, i.e., to that part where the enzyme-like reaction of a dendrizyme is supposed to take place.