Dennis W. Bennett

Electronic transport through metal-1,4-phenylene diisocyanide-metal junctions

Abstract We report measurements on through-bond electronic transport properties of 1,4-phenylene diisocyanide–metal junctions. Nanoscale metal–molecule–metal junctions with self-assembled 1,4-phenylene diisocyanide layers were analyzed with variable temperature conductance measurements to reveal the dominant electronic transport mechanisms. Non-Ohmic thermionic emission is the dominant process, with isocyanide–Pd showing the lowest thermionic barrier of 0.22 eV.

A near-edge X-ray absorption fine structure and photoelectron spectroscopic study of the structure of acetylene on Pd(111) at low temperature

Abstract The structure of acetylene adsorbed on Pd(111) below ∼ 200 K is probed using near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet and X-ray photoelectron spectroscopy. The NEXAFS spectra are calculated using a one-electron cluster formalism corrected by an energy-dependent exchange contribution. Experimental spectra agree well with results calculated for an acetylene molecule adsorbed in a three-fold hollow site with a CCH angle of ∼ 117°, a C-C bond length of ∼ 1.3 A and with its H-C-C-H plane tilted slightly at ∼ 22° from normal to the Pd(111) surface. The thermal removal of carbon from the surface is measured using X-ray photoelectron spectroscopy by monitoring the C 1s signal intensity. These results indicate that ∼ 35% of the surface carbon is removed by heating to 600 K. This is the temperature range over which benzene desorption is detected in thermal desorption spectroscopy implying that ∼ 35% of the acetylene initially adsorbed at ∼ 90 K converts to benzene. In addition, a shift in the C 1s peak position on heating to between 200 and 300 K implies the conversion of the adsorbed acetylenic molecule into another surface species.

Molecular wires, switches, and memories.

Abstract: Design and measurements of molecular wires, switches, and memories offer an increased device capability with reduced elements. We report: Measurements on through‐bond electronic transport properties of nanoscale metal‐1,4‐phenylene diisocyanide‐metal junctions are reported, where nonohmic thermionic emission is the dominant process, with isocyanide‐Pd showing the lowest thermionic barrier of 0.22 eV; robust and large reversible switching behavior in an electronic device that utilizes molecules containing redox centers as the active component, exhibiting negative differential resistance (NDR) and large on‐off peak‐to‐valley ratio (PVR) are realized; erasable storage of higher conductivity states in these redox‐center‐containing molecular devices are observed; and a two‐terminal electronically programmable and erasable molecular memory cell with long bit retention time is demonstrated.

NEXAFS identification of a catalytic reaction intermediate: C4H4 on Pd(111)

Abstract Polarisation-dependent NEXAFS data are presented for the reaction intermediate C 4 H 4 chemisorbed on Pd(111). Comparison of these data with calculated spectra indicates that this species is a metallocycle with slightly expanded ( ~ 0.05 A) C-C bonds. Dependence of the resonance intensities on photon incidence angle shows that the molecular plane is tilted at ~ 35° with respect to the metal surface. The assigned chemical identity and absorption geometry are fully consistent with all the reactive behaviour of C 4 H 4 .

One-dimensional supramolecular surface structures: 1,4-diisocyanobenzene on Au(111) surfaces

One-dimensional supramolecular structures formed by adsorbing low coverages of 1,4-diisocyanobenzene on Au(111) at room temperature are obtained and imaged by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. The structures originate from step edges or surface defects and arrange predominantly in a straight fashion on the substrate terraces along the <110> directions. They are proposed to consist of alternating units of 1,4-diisocyanobenzene molecules and gold atoms with a unit cell in registry with the substrate corresponding to four times the lattice interatomic distance. Their long 1-D chains and high thermal stability offer the potential to use them as conductors in nanoelectronic applications.

Model studies toward the synthesis of macrolactin A: Organoiron methodology for introduction of the C1–C11 and C16–C24 segments

Abstract Preparation of the Fe(CO)3 complexed C1–C11 and the C16–C24 segments of macrolactin A has been accomplished from (sorbaldehyde)Fe(CO)3 in 4 steps and 5 steps respectively.

Redox properties of 2[4Fe4S] ferredoxins

Abstract The redox properties of the 2[4Fe4S] ferredoxin isolated from Clostridium pasteurianum were examined directly at an edge pyrolytic graphite electrode using square-wave voltammetry. The direct electrochemical approach was compared with other techniques that have been used to determine the equilibrium reduction potential of this protein. A microcell is described that permits routine electrochemical experiments on small sample volumes (25–50 μl). The electrostatic potentials of a homologous ferrodoxin with a known x-ray crystal structure, which includes the first treatment of net atomic charges in the iron—sulfur clusters, was determined using a macroscopic electrostatic model.

The synthesis of S-(+)-2,2-dimethylcyclopropane carboxylic acid: a precursor for cilastatin

Abstract S -(+)-2,2-Dimethylcyclopropane carboxylic acid, a precursor for cilastatin, was prepared from 2-methylpropene and chiral iron carbene in three steps. Asymmetric cyclopropanation reaction of 2-methylpropene with iron carbene complex having chirality at the carbene ligand, followed by exhaustive ozonolysis, produced S -(+)-2,2-dimethylcyclopropanecarboxylic acid of up to 92% ee. The absolute configuration of complexed chiral cyclopropane (−)- 8 was determined by X-ray crystallographic analysis.

Temperature programmed desorption of co-adsorbed hydrogen and acetylene on Pd(111)

Co-adsorbed acetylene and hydrogen react on Pd(111) to form ethylene and butene. Benzene is also formed and it is found that the low temperature (∼ 280 K) desorption state observed on the clean surface is virtually unaffected by the presence of hydrogen whereas the higher temperature (∼ 520 K) state is modified. However, adsorbing benzene onto a hydrogen pre-covered surface just affects the sticking coefficient but not its desorption properties. The effect of hydrogen on the high-temperature, acetylene-derived benzene desorption state is therefore ascribed to its influence on the chemistry of acetylene rather than being due to the effect of hydrogen on the benzene that has been formed. The reaction between hydrogen and acetylene to form ethylene is shown to be first order in hydrogen coverage and to proceed with an activation energy of ∼ 23 kJ/mol. The trailing edge of the ethylene desorption profile is proposed to be due to the loss of acetylene because of its conversion to a vinylidene species. A fit to the desorption profile using this model gives an activation energy for this reaction of ∼ 16 kJ/mol with a reaction pre-factor of ∼ 600 s−1.

Preparation, Reactions, and Stereochemistry of 4-tert-Butyl-1-chlorophosphorinane 1-Oxide and Derivatives

Abstract Nucleophilic substitution at tetracoordinated phosphorus centers has been extensively investigated.1 Transesterification of acyclic phosphinates proceeds with inversion of configuration,2 whereas four-membered phosphinates react with retention.3 In previous work4 we suggested that transesterification in six-membered phosphinates occurs via an SN2 mechanism. The title compound was synthesized as a model structure to study the stereochemical and mechanistic aspects of nucleophlic substitution at phosphorus in a six-membered ring. A mixture of diastereomers was produced and separated by column chromatography. Both the mixture and separate isomers were used for an investigation of substitution with methanol, methoxide ions, and phenoxide ions; the reactions proceeded with inversion. Transesterification reactions of the resulting thiophosphinates were studied and found to proceed with inversion. Reactions of these with phenyllithium, benzyllithium, lithium phenylacetylide, and the corresponding organo...