Colloque Davy-Weyl: Electron solvation and electron transfer - satellite meeting

Chair

Dr Matthias Schadel, GSI, Germany

Welcome and opening remarks

Professor Peter Edwards FRS, University of Oxford, UK

Electron locales and interactions in electrides

Professor James Dye, Michigan State University, USA

Abstract

Solvated electrons are among the most studied species in chemistry but their detailed nature is still open to question. In the most studied systems, alkali metals in liquid ammonia, dilute solutions are ionic with the solvated electron moving about 5 times as fast as the cation. But the properties change from ionic to metallic as the concentration increases. The study of solvated electrons in many other solvents was hampered for 60 years (1918-1968) by the presence of other un-identified species, now known to be alkali metal anions. By using cation complexants such as crown ethers and cryptands, alkali metal solubilities in solvents such as ethers, THF, amines, etc. became high enough to do “real chemistry”. In addition, by appropriate choice of alkali metals and complexants, it became possible to control whether alkali metal anions or solvated electrons were the dominant species. This also permitted the synthesis of a number of solvent-free crystalline ionic solids that contain complexed cations and either alkali metal anions (alkalides) or trapped electrons (electrides).

This presentation will focus on studies of electron distributions and properties in electrides and the comparison with solvated electrons. The crystal structures, spectra, conductivities, magnetic behavior, and comparison with iso-structural alkalides and halides will be reviewed. Particular emphasis will be on studies made since the last Colloque Weyl (VII) in 1991. The ability to study single crystals has permitted us to make detailed pictures of trapped electrons whose major electron density is in anionic sites, but with various amounts of “leaking” to adjacent sites. The amount of inter-electron coupling depends strongly on the nature of the channels that connect adjacent sites. 

Chair

The structure and dynamics of metal-ammonia systems by neutron scattering

Professor Neal Skipper, University College London, UK

Abstract

Neutron scattering, in conjunction with isotopic labelling and classical three-dimensional computer modelling, has been used to investigate the structure and dynamics of metal-ammonia solutions over the full composition range. These studies have revealed in detail the nature of the hydrogen bonding and cation and electron solvation in these systems. For example, we have been able to elucidate the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium−ammonia solutions. We have also demonstrated that rapid quenching from above the liquid-liquid phase separated regime leads to the formation of homogenous glassy solids. Recently we have applied these techniques to understand the ability of metal-ammonia solutions to act as solvents for a variety of carbon nanostructures: fullerenes, graphenes, and single-walled carbon nanotubes. Electron transfer reduces these nanoparticles, and thereby makes them highly soluble in a variety of polar aprotic solutes

Spectroscopy of metal-solvent clusters: a gas phase journey to solvated electrons

Professor Andrew Ellis, University of Leicester, UK

Abstract

We have applied mass-selective optical spectroscopy to metal-solvent complexes, with particular emphasis on species formed by combining alkali metal atoms with ammonia molecules. Action spectroscopic methods, based on tuneable laser excitation in combination with UV laser photoionization, allow mass-selective spectra of neutral complexes in the gas phase to be obtained. The scientific goal is to link the information drawn on the complexes with the known behaviour of alkali metals when dissolved in bulk liquid ammonia. For example, IR spectra can provide information on the solvent structure around the metal atom while electronic spectra explore the ‘solvated’ electron directly. The emphasis in this presentation will be on the interaction of Li with ammonia, and in particular the complex Li(NH3)4, where both vibrational and electronic spectra will be presented. Some brief discussion of complexes for other metals will also be given.

A molecular orbital analysis of electron solvation

Dr Eva Zurek, University of Buffalo, USA

Abstract

Density Functional Theory calculations have been undertaken to explore the electronic structure of various species likely to be important constituents of lithium-ammonia solutions, solid Li(NH3)4 at pressures ranging from atmospheric to 200 GPa, and alkali metals interacting with ethylenediamine. In metal-ammonia solutions we find that the valence electrons of the metal atoms enter a diffuse orbital built up largely from the lowest unoccupied MOs of the ammonia molecules. The electronic structure of a great variety of species, likely to be important components of metal-ammonia solutions, can be understood by considering the overlap of these diffuse MOs. TD-DFT calculations on a number of species are able to account for the characteristic adsorption spectrum of dilute metal-ammonia solutions. We further explore the electronic structure of various phases of solid Li(NH3)4, and show that a Jortner-type model can be used to explain the peculiar behavior of their electronic structure under pressure. Finally, calculations indicate that superalkali dimers, [M(en)3+ • M-], may be important constituents of solutions containing Na, K, Rb, or Cs and ethylenediamine (en).

TBC

Professor Bernd Abel, University of Leipzig, Germany

Controlling substrate access to the diiron sites in soluble methane monooxygenase

Professor Stephen Lippard, Massachusetts Institute of Technology, USA

TBC

Professor Harry Gray ForMemRS, Caltech, USA

The liquid-vapour transition of mercury: Landau-Zeldovich revisited

Professor Friedrich Hensel, Philipps-Universitat Marburg, Germany

Abstract

It is now more than 70 years that Landau and Zeldovich first called attention to the interrelation between the liquid-vapour and metal-nonmetal transition in fluid metals .Renewed interest in this problem is now motivated by recent measurements of very accurate sound velocity,small angle X-ray,and eletrical conductivity data for fluid mercury.We will make an attempt to connect these experimental results with the long sought- for effect of the interplay between the liquid-vapour and metal -dielectric transition in fluid mercury.

Chair

Professor Nicholas Long, Imperial College London, UK

Photoinduced electron transfer applied in sensing and logic devices

Professor Prasana A.P. de Silva, Queens University Belfast, UK

Abstract

The combination of a fluorescent unit dealing with photons and a receptor engaging chemical species leads to chemical sensors with a fluorescence output. The fluorescent unit and the receptor are separated by a spacer/bridge so that photoinduced electron transfer (PET) becomes possible. The PET rate is modulated by the occupants of the receptor site, so that the fluorescence signal is switched ‘on’ only when the receptor captures a target species. Sensors can be understood as single-input Boolean logic devices. Double-input logic devices are constructed by building PET-based systems containing two receptors. Being molecular in nature, these logic devices can operate in small and biorelevant spaces where semiconductor-based logic devices cannot go. Fluorescent PET sensors are already being used worldwide for point-of-care blood diagnostics in hospitals and ambulances, where electrolytes are being measured in 30 seconds. Multiple-input logic devices allow the development of ‘lab on a molecule’ systems where several medically relevant parameters are evaluated simultaneously in order to produce a preliminary diagnosis of a disease without a doctor’s intervention.

Electron tunnelling through organic molecules

Professor Oliver Wenger, University of Basel, Switzerland

Abstract

The dependence of electron transfer rates (kET) on donor-acceptor distance has been investigated in various settings. An exponential decrease of kET with increasing distance is frequently observed, in line with a tunneling phenomenon. The steepness of the exponential decrease of kET depends on the donor-acceptor couple and on the intervening medium separating the two reactants. From work on photoinduced electron transfer between randomly dispersed donors and acceptors in frozen glasses insight regarding the inefficiency of electron tunneling across van-der-Waals gaps can be gained. Related work on covalent donor-bridge-acceptor molecules underscores the importance of “hard-wired” systems for efficient electron tunneling. Seemingly subtle substituent changes on oligo-p-phenylene bridges can have an important impact on the rates for long-range electron transfer, highlighting the importance of the so-called tunneling-energy gap. When deprotonatable electron donors such as phenols are used, it becomes possible to investigate proton-coupled electron transfer (PCET) reactions which may involve long-range electron tunneling occurring in concert with short-range proton transfer events. The dependence of such concerted proton-electron transfers on donor-acceptor distance is no steeper than that of “simple” (i. e., not proton-coupled) electron tunneling processes

TBC

Professor Lee Cronin, University of Glasgow, UK

Chair

Professor Susumu Kitagawa, Kyoto University, Japan

Are mixed-valence systems completely classy, or devoid of class (and does it matter)?

Professor Paul Low, University of Western Australia, Australia

Abstract

Mixed-valence complexes are import in helping us to understand electron-transfer processes, and have attracted attention of experimentalists and theoreticians since the ground-breaking work on the Creutz-Taube ion in the 1960s. In the important case of two redox centers linked by a bridging ligand, the description of electronic structure is usually made within the Robin-Day scheme. The three primary Robin-Day classes are simply denoted Class I, II and III and correspond to situations in which there is no coupling between the redox centers, there are weakly interacting centers with evidence for a degree of coupling between them, or the redox centres are strongly coupled with charge and spin delocalized over both redox centers. However, the thermal population of a conformational phase space encompassing both localized and delocalized charge distributions limits the usefulness of a description n terms of a single, static molecular geometry and assignment to a single Robin-Day class. Here we describe several examples of mixed-valence complexes in which accurate explanation of the spectroscopic properties and electronic characteristics requires consideration of the internal rotational dynamics of the molecule and description in terms of a continuum of class II and class III states rather than a specific single class.

In search of structure-activity relationships in molecular wires

Professor John McGrady, University of Oxford, UK

Abstract

Much of the recent momentum in the field of molecular electronics has been centred on organic components, where the conjugated  systems typically provide the dominant transport pathways. Transition metal based systems, in contrast, have been somewhat overlooked despite the fact that their innately flexible electronic structure offers enormous potential. Ligand field effects, changes in redox and spin state and metal-metal bonding all play a critical role in determining the nature of the transport channels near the Fermi level in a putative molecular electronic device. Structure-function relationships – principles that can guide synthetic effort towards target molecules – are not as well developed in the context of transition metal electronics as they are in the organic field. The design of rectifiers, compounds that allow current in only one direction, has been a holy grail since Aviram and Ratner first proposed the concept 40 years ago.[1] It is clear that asymmetry, either in the molecule or in its contacts to the electrodes, precludes the possibility that current flow is rigorously symmetric, but to what extent does the compositional asymmetry actually perturb the transport channels of interest? Arrays of redox-active transition metals are a promising avenue for further study: intramolecular electron transfer can support large internal electric fields which localize channels on one side or other of the molecule. Subtle changes in the left-right delocalization of a channel can then be controlled through spin polarization, and this can lead to substantial rectification ratios. In this presentation I will review recent computational work that seeks to provide a set of guidelines for structure-activity relationships in metal atom chains.[2]

Chair

Professor Dieter Fenske, University of Karlsruhe, Germany

TBC

Professor Hideo Hosono, Tokyo Institute of Technology, Japan

Carbon Nanostructures - Direct Synthesis and Counterintuitive Transport Properties

Professor Martin Jansen, Max Planck Institute for Solid State Research, Germany

Abstract

Carbon based nanostructures such as fullerenes, nanotubes and graphenes attract considerable attention because of their unique properties and potential applications. Despite great success in the total synthesis of many complex organic molecules, the rational construction of fullerenes, and nanotubes still remains challenging. The main task of direct synthesis is the controlled generation of the desired nanostructures by rational chemical methods. This approach is of practical interest for the production of individual higher fullerenes and nanotubes with defined chiralities, and thus specific physical properties. Our methodology is based on the synthesis of polycyclic aromatic hydrocarbons which already contain all necessary carbon atoms in appropriate positions. Subsequent intramolecular condensation leads to the desired nanostructures, the carbon connectivity of which is fully predefined by the precursor molecule. We demonstrate on the examples of C60 and the 6/6 SWCNT that fully selective synthesis of such carbon nanostructures has become feasible.

The ability of fullerenes to accept excess electrons, up to twelve in the case of C60, is among the most conspicuous aspects of their chemistry and physics. The series of compounds as formed with alkali metals, AxC60 (A= Na, K, Rb, Cs; x = 1-6), is particularly interesting. Since except for A6C60, the conduction bands are partly filled for all of them, they are expected to show metallic conductivity. Surprisingly, A2C60 and A4C60 are reported to be insulating, while A3C60 becomes superconducting at quite elevated transition temperatures. In the case of C602- one faces the so-called open-shell problem. The two added electrons partially  occupy the lowest unoccupied molecular orbital (LUMO) of the neutral C60 which has  tiu symmetry and is three-fold degenerate. Yet, no evidence for a magnetic state has been found for C602- compounds. A singulet ground state can only be achieved by a Jahn-Teller (J-T) distortion. However, structural evidence for such a J-T effect has so far escaped detection. By analysing bond length alternation patterns in a series of high precision crystal structure data sets of  ionic fullerides C602-, we became aware of a systematic anisotropy of the bond alternation lowering the point symmetry to D3d. By theoretical calculations it has been shown that these patterns unequivocally determine a particular singulet state as the electronic ground state of the C602- ions. Thus experimental data and calculations clearly confirm the first manifestation of a static J-T deformation in bulk fullerides. Interestingly, at the example of a (C60 )-   radical anion, we have been able to also trace the transition from  static to dynamic J-T effect, by EPR spectroscopy and temperature dependent x-ray  crystallography. Recently, also for C604-, which is electronically inverse to C602-, evidence for a singulet ground state has been obtained from STM investigations on K4C60 mono-layers.

These findings on C602- and C604- are also relevant for superconductivity in the A3C60 compounds. Here the C60-ions are three-fold negatively charged in the average, however, electron transport implies intermediary charges of –2 and –4. Superconductivity may arise from structural relaxations related to charge fluctuations away from –3, accompanied by distortional modes. In the C602- fullerides treated here, these structural relaxations were seen in a quasi-frozen form.
Finally, we have spotted orbital ordering in a C 60     radical monoanion anion salt, resulting in a counterintuitive anisotropy of the electronic conductivity.

Structure determined catalysis by electron states 2e-e2= in condensed metal-ammonia, layer cuprates and Prussian Turnball's Blue systems

Professor Jennie Acrivos, San Jose State University, USA

Abstract

Growth catalysis for layer superconductors and Prussian-Turnbull’s_Blue is described by their structure, HOMO_LUMO LCAO components, determined by resonance enhanced X-Ray diffraction and absorption (J Kmiec and B Shyam, XRD, XAS Synchrotron measurements at DOE_SLAC_SSRL) near the constituents X-Ray edge Ea, based on 19thC Davy and Weyl chemical activity: bonding by 2e-e2=, also Mott and Alexandrov polaronbi-polaron superconductivity: (Bi1.7Pb0.3:Sr2:Can-1:Cun:O2n+4+δ)2≡2s:2:n-1:n=4-19, n’=2,3 intergrowth by concentrated sun radiation melt synthesis catalysis (DD Gulamova, Sun_Institute_Tashkent), and Prussian_Turnbull’s_Blue B≡Fe4[Fe(CN)6]3•15H2O room temperature annealed films (SC Weaver, SJSU), is determined by a photon absorption: |Cu/Fe:1s2_HOMO|Q0>+hE0,Qo|Cu/Fe:1s_LUMOl=1,/Rydbergl=1,|Q>E<,>Ea, ,at a Bragg orientation Q0, followed by relaxation in t<10-16s, and excited e*(E,p*), excess energy E and momentum p* that Compton scatter a 2nd photon with conservation of energy and momentum, and from neighbors at RM, before t>10-15s vibration thermodynamic equilibrium is reached. 2s:2:n-1:n=4-19 mutually enhanced reflections {Q0,Q*=Q0[1+E/E0cos(p*^Q0)]Eo,Qo,Eo-Ea<100eV,Ea=9keV}={(0016),(200)}n=2 and others identify the reactive sites orientation, and HOMO/LUMO/LCAO Cu:(3d2sp3hybrid):M bonds by the relaxation characteristic core energy, M=SrM2,3:E=270-280eV, CaL2,3:E=349-346eV, and OK:E=525-540eV. B films obtain Ea=7126eV, Fe=Fe coordination valence intermediate between K3Fe(NC)6 and K4Fe(CN)6), resonance absorption Ea-E0=86-10eV mutually enhanced {(10,0,0),(8,6,0),(10,0,2)} and others for excited B*:FCC a*=20.34Å, with charge/spin transfer HOMO/LUMO/LCAO involved in core excitation to anti-bonding LUMO Fe(3d2sp3hybrid):N states, identified by characteristic core relaxation energy NK:E=411-425eV.

Chair

Professor Peter Edwards FRS, University of Oxford, UK