A single atom change turns insulating saturated wires into molecular conductors

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements show tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. Here, we demonstrate experimentally that $$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$ β ∝ 1 / ε r , suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

A Single Atom Change Turns Insulating Saturated Wires into Molecular Conductors

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10−18)X, current densities (J) increases > 4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements shows tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drop at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. We demonstrate experimentally that β ∝1/√εr, suggesting highly-polarizable terminal-atom sites act as charge traps as proposed by Berlin and Ratner. Our work shows the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Terrestrial Trapping of the Interstellar Gas, Phosphorus Nitride

The N2 analogue phosphorus nitride (PN) was the first phosphorus containing compound to be detected in the interstellar medium, however this thermodynamically unstable compound has a fleeting existence on Earth. Here, we show that reductive coupling of iron(IV) nitride and molybdenum(VI) phosphide complexes assembles PN as a bridging ligand in a structurally-characterized bimetallic complex. Reaction with C≡N^tBu releases the mononuclear complex [(N₃N)Mo-PN]⁻, N₃N = [(Me₃SiNCH₂CH₂)₃N]³⁻), which undergoes light-induced linkage isomerization to provide [(N₃N)Mo-NP]⁻, as revealed by photocrystallography. While structural and spectroscopic characterization, supported by electronic structure calculations reveal PN multiple bond character, coordination to molybdenum creates nucleophilic character at the terminal atom of the PN/NP ligands. Indeed, the linkage isomers can be trapped in solution by reaction with a Rh(I) electrophile.

Terrestrial Trapping of the Interstellar Gas, Phosphorus Nitride

The N2 analogue phosphorus nitride (PN) was the first phosphorus containing compound to be detected in the interstellar medium, however this thermodynamically unstable compound has a fleeting existence on Earth. Here, we show that reductive coupling of iron(IV) nitride and molybdenum(VI) phosphide complexes assembles PN as a bridging ligand in a structurally-characterized bimetallic complex. Reaction with C≡N^tBu releases the mononuclear complex [(N₃N)Mo-PN]⁻, N₃N = [(Me₃SiNCH₂CH₂)₃N]³⁻), which undergoes light-induced linkage isomerization to provide [(N₃N)Mo-NP]⁻, as revealed by photocrystallography. While structural and spectroscopic characterization, supported by electronic structure calculations reveal PN multiple bond character, coordination to molybdenum creates nucleophilic character at the terminal atom of the PN/NP ligands. Indeed, the linkage isomers can be trapped in solution by reaction with a Rh(I) electrophile.

NO_x cycle and tropospheric ozone isotope anomaly: an experimental investigation

The oxygen isotope composition of nitrogen oxides (NOx) in the atmosphere may be a useful tool for understanding the oxidation of NOx into nitric acid/nitrate in the atmosphere. A set of experiments were conducted to examine changes in isotopic composition of NOx due to O3-NOx photochemical cycling. At low NO2/O2 mixing ratios, NO2 becomes progressively and nearly equally enriched in 17O and 18O over time until it reaches a steady state with Δ17O values of 40.6 ± 1.9‰ and δ18O values of 84.2 ± 4‰, relative to the isotopic composition of the O2 gas. As the mixing ratio increases, isotopic exchange between O atoms and O2 and NOx suppresses the isotopic enrichments. A kinetic model simulating the observed data shows that the isotope effects during ozone formation play a more dominant role compared to kinetic isotope effects during NO oxidation or exchange of NO2. The model results are consistent with the data when the NO + O3 reaction occurs mainly via the transfer of the terminal atom of O3. The model predicts that under tropospheric concentrations of the three reactants, the timescale of NOx isotopic equilibrium ranges from hours (ppbv mixing ratios) to days/weeks (pptv) and yields steady state Δ17O and δ18O values of 46‰ and 115‰ respectively with respect to Vienna Standard Mean Ocean Water. Interpretation of tropospheric nitrate isotope data can now be done with the derived rate coefficients of the major isotopologue reactions at various pressures.

INFLUENCE OF ELECTRODE–MOLECULE INTERFACE ON ELECTRON TRANSPORT

We have studied the metal-molecule interface linkage effects of metal-molecule-metal systems by first principles method, which is based on density functional theory (DFT) and nonequilibrium Green's functions (NEGF) to calculate the electron transport of open metal-molecule-metal systems. Metal electrodes are described through 3-D atomic model instead of a non-atomic (like Jellium model) description. Several open systems are constructed, optimized and simulated. Sulphur atom (S) and cyano-group (CN) are employed to connect Au electrodes and molecule borazine/benzene. The density of states (DOS) and the transmission functions (TF) of constructed systems are investigated. Results show that DOS of the systems are affected little but transmission properties a lot by terminal groups CN and S. The peaks in the TF mismatch with those in DOS and the mismatch resulting from terminal group CN is larger than it from terminal atom S. Calculated transmission functions show that the systems with terminal group CN present better conductance at lower bias and this effect is much more significant in the borazine systems than that in the benzene systems.

Molecular Geometry Approximations for Chlorinated Dibenzodioxins by Fourier Transform Infrared Spectroscopy

Reference infrared vapor-phase spectra of 15 polychlorinated dibenzodioxin- p-dioxin (PCDD) isomers were recorded at low microgram concentrations. Ether linkage (COC) bond angles for these isomers and for the 22 tetrachlorodibenzodioxin (TCDD) isomers were calculated from infrared data, with the use of mass approximations for the terminal atom in a nonlinear XY2 model and by neglecting the valence force equation symmetric stretch bending term. Calculated bond angles show a good correlation with x-ray diffraction (XRD) and carbon-13 nuclear magnetic resonance (13C NMR) relaxation results. Molecular geometries in PCDD isomers, as defined by the COC bond angle and the COC stretching frequencies, were found to range from near planar, in laterally substituted isomers with high electron withdrawing capacity aromatic rings, to tetrahedral, for isomers with low electron withdrawing capacity rings. Non-bonded interactions were also found to influence molecular geometry. Molecular geometry was used to assign structures for the 1,2,3,6,7,8- and 1,2,3,7,8,9-hexachlorodibenzodioxin isomer mixture and was found to be an important factor in estimations of COC bond strength from empirical data. Correlations between infrared data and PCDD LD50 values suggest that molecular geometry, polarizability, and steric PCDD/receptor interactions are associated with isomer toxicity.