Tunneling currents that increase with molecular elongation

We present a model molecular system with an unintuitive transport-extension behavior in which the tunneling current increases with forced molecular elongation. The molecule consists of two complementary aromatic units (1,4-anthracenedione and 1,4-anthracenediol) hinged via two ether chains and attached to gold electrodes through thiol-terminated alkenes. The transport properties of the molecule as it is mechanically elongated in a single-molecule pulling setting are computationally investigated using a combination of equilibrium molecular dynamics simulations of the pulling with gDFTB computations of the transport properties in the Landauer limit. Contrary to the usual exponential decay of tunneling currents with increasing molecular length, the simulations indicate that upon elongation electronic transport along the molecule increases 10-fold. The structural origin of this inverted trend in the transport is elucidated via a local current analysis that reveals the dual role played by H-bonds in both stabilizing π-stacking for selected extensions and introducing additional electronic couplings between the complementary aromatic rings that also enhance tunneling currents across the molecule. The simulations illustrate an inverted electromechanical single-molecule switch that is based on a novel class of transport-extension behavior that can be achieved via mechanical manipulation and highlight the remarkable sensitivity of conductance measurements to the molecular conformation.     

Interference-induced electron- and hole-conduction asymmetry

Principles established by Shephard and Paddon-Row for optimizing and controlling intramolecular electron transport through the modulation of interfering pathways are employed to design new molecules for steady-state conduction experiments aimed at manifesting electron?Chole conduction asymmetry in a unique way. First, a review of the basic principles is presented through application to a pertinent model system in which a molecule containing donor and acceptor terminal linking groups with an internal multiple-pathway bridge is used to span two metal electrodes. Different interference patterns are produced depending on whether the through-molecule coupling pathways are symmetric or antisymmetric with respect to a topological bisecting plane, giving rise to asymmetric electron and hole conductances at the tight-binding (Hückel) level; this process is also described from a complementary molecular-orbital viewpoint. Subsequently, a new molecular system based on organic polyradicals is designed to allow such asymmetry to be realized in single-molecule conduction experiments. These polyradicals are analyzed using analogous simple models, density-functional theory (DFT) calculations of steady-state transmission, and intermediate neglect of differential overlap (INDO) calculations of intramolecular connectivity, verifying that polyradicals at low temperatures should show experimentally measureable electron?Chole conduction asymmetry. A key feature of this system is that the polyradicals form a narrow partially occupied band of orbitals that lie within and well separated from the HOMO and LUMO orbitals of the surrounding molecular scaffold, allowing for holes and electrons to be transported through the same molecular band.     

The relationship between redox enzyme activity and electrochemical potential-cellular and mechanistic implications from protein film electrochemistry

In protein film electrochemistry a redox protein of interest is studied as an electroactive film adsorbed on an electrode surface. For redox enzymes this configuration allows quantification of the relationship between catalytic activity and electrochemical potential. Considered as a function of enzyme environment, i.e., pH, substrate concentration etc., the activity-potential relationship provides a fingerprint of activity unique to a given enzyme. Here we consider the nature of the activity-potential relationship in terms of both its cellular impact and its origin in the structure and catalytic mechanism of the enzyme. We propose that the activity-potential relationship of a redox enzyme is tuned to facilitate cellular function and highlight opportunities to test this hypothesis through computational, structural, biochemical and cellular studies.     

The synthesis of functionalised chiral bicyclic lactam and lactone N-oxides using a tandem Cope elimination/reverse Cope elimination protocol

Functionalised hydroxylamine derivatives of (S)-proline and (R)-pipecolic acid have been prepared using a Cope elimination. These undergo reverse Cope elimination onto a pendant double bond to give bicyclic lactam and lactone N-oxides containing three contiguous chiral centres, this extends the scope and applicability of the reverse Cope elimination in the synthesis of heterocyclic systems by incorporation of the lactone and lactam structural motifs.     

Scaling law for the elastic response of spherical explosion-containment vessels

The substantial progress made in the development of portable, spherical explosion-containment vessels has led to its adoption for explosives-related transporting and manufacturing. A simple scaling law for engineering-design use has been obtained for the maximum first-cycle elastic-strain response to explosive loading by a centered, spherical charge of a representative high explosive known as Pentolite. Results from a computer program based on the work of Baker have been accurately fitted to a six-product power law that is a function of the density, elastic modulus, diameter, and thickness of the vessel and the weight of detonated explosive. Nonlinear and log-linear fits are compared and found to agree if the role of weighting factors is understood. An impulse approximation is used to indicate that the scaling-law parameters are reasonable.     

VUV photon induced fluorescence study of SF5CF3

The interaction of SF(5)CF(3) with vacuum-UV radiation has been investigated by photon induced fluorescence spectroscopy. Total fluorescence yield and dispersed fluorescence spectra of SF(5)CF(3) were recorded in the 200-1000 nm fluorescence window. In all cases, the fluorescence spectra resemble those of CF(3)X (X = H, F, Cl, and Br) molecules. At photon energies below 20 eV, the emission is attributed to the excited CF(3) and CF(2) fragments. The threshold for the CF(3) emission is 10.2 +/- 0.2 eV, giving an upper limit estimate for the SF(5)-CF(3) bond dissociation energy of 3.9 +/- 0.3 eV. The excitation functions of the CF(3) and CF(2) emissions were measured in the photon energy range 13.6-27.0 eV. The resonant structures observed in SF(5)CF(3) are attributed to electronic transitions from valence to Rydberg orbitals, following similar assignments in CF(3)X molecules. The photoabsorption spectrum of SF(5)CF(3) shows features at the same energies, indicating a strong contribution from Rydberg excitations.     

Chemical markup, XML, and the world wide web. 6. CMLReact, an XML vocabulary for chemical reactions

A set of components (CMLReact) for managing chemical and biochemical reactions has been added to CML. These can be combined to support most of the strategies for the formal representation of reactions. The elements, attributes, and types are formally defined as XMLSchema components, and their semantics are developed. New syntax and semantics in CML are reported and illustrated with 10 examples.     

Bromoiodinanes with an I(III)-Br bond: preparation, X-ray crystallography and reactivity as electrophilic brominating agents

Bromoiodinanes--conveniently and directly prepared from iodobenzenecarbinols and N-bromosuccinimide, and characterised for the first time crystallographically--act as electrophilic bromine donors.     

On the Asymptotic Depth of Multigraded Modules

The aim of this article is to establish, among other results, the asymptotic stability of the depth of the graded pieces of a nonstandard multigraded module. As a corollary, we get the asymptotic stability of the depth of the graded pieces of the multigraded Rees algebra defined by a finite set of ideals and their associated multigraded rings.