On the pressure dependence of viscosity, especially for fluids that have a tendency to form glasses.

Understanding fluid viscosity is crucial for applications including lubrication and chemical kinetics. A commonality of molecular models is that they describe fluid flow based on the availability of vacant space. The proposed analysis builds on Goldstein's idea that viscous transport must involve the concerted motion of a molecular ensemble, referred to as cooperatively rearranging regions (CRRs) by Adam and Gibbs in their entropy-based viscosity model for liquids close to their glass transition.

Mechanistic and Kinetic Measurements of Elementary Surface Reactions Using Temperature-Programmed X-ray Photoelectron Spectroscopy.

This paper describes a method by which surface-reaction kinetics can be measured by slowly and precisely ramping up the surface temperature at a constant rate while simultaneously collecting X-ray photoelectron spectra (XPS). This approach results in the collection of a large amount of data over relatively small temperature steps to produce quasi-continuous kinetic data. The method is illustrated for the desorption and reaction of diethyl disulfide (DEDS) on a Au(111) substrate in ultrahigh vacuum, where the results can be compared with previous conventional temperature-programmed desorption (TPD) data from Au(111).

Surface chemistry at the solid-solid interface: mechanically induced reaction pathways of C carboxylic acid monolayers on copper.

Mechano- or tribochemical processes are often induced by the large pressures, of the order of 1 GPa, exerted at contacting asperities at the solid-solid interface. These tribochemical process are not very well understood because of the difficulties of probing surface-chemical reaction pathways occurring at buried interfaces. Here, strategies for following surface reaction pathways in detail are illustrated for the tribochemical decomposition of 7-octenoic and octanoic acid adsorbed on copper.

Adsorption and reaction pathways of 7-octenoic acid on copper.

The surface structure and reaction pathways of 7-octenoic acid are studied on a clean copper substrate in ultrahigh vacuum using a combination of reflection-absorption infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning-tunneling microscopy, supplemented by first-principles density functional theory calculations. 7-Octenoic acid adsorbs molecularly on copper below ∼260 K in a flat-lying configuration at low coverages, becoming more upright as the coverage increases. It deprotonates following adsorption at ∼300 K to form an η2-7-octenoate species.

Insights into the Mechanism of the Mechanochemical Formation of Metastable Phases.

The mechanochemical reaction kinetics of sulfur with copper to form a metastable copper sulfide phase at room temperature is investigated in ultrahigh vacuum by modifying the properties of the copper during cleaning in vacuum. The measured kinetics is in agreement with a theory first proposed by Karthikeyan and Rigney that predicts that the rate depends linearly both on the contact time and on the strain-rate sensitivity of the substrate. The mechanism for this process was investigated using thin samples of copper fabricated using a focused-ion-beam and by measuring the crystal structure and elemental composition of the copper subsurface region by electron microscopy after reaction.

Measuring and modelling mechanochemical reaction kinetics.

Quasi-static density functional theory calculations of the rate of mechanochemical decomposition of methyl thiolate species adsorbed on Cu(100) accurately reproduce the experimental normal-stress dependent rates measured in ultrahigh vacuum by an atomic force microscopy tip. This allows precise analytical models for mechanochemical reaction kinetics to be developed.

Chemical self-assembly strategies for designing molecular electronic circuits.

Design principles are demonstrated for fabricating molecular electronic circuits using the inherently self-limiting growth of molecular wires between gold nanoparticles from the oligomerization of 1,4-phenylene diisocyanide.

Development of a ReaxFF Force Field for Cu/S/C/H and Reactive MD Simulations of Methyl Thiolate Decomposition on Cu (100).

It has been shown that the rate of decomposition of methyl thiolate species on copper is accelerated by sliding on a methyl thiolate covered surface in ultrahigh vacuum at room temperature. The reaction produces small gas-phase hydrocarbons and deposits sulfur on the surface. Here, a new ReaxFF potential was developed to enable investigation of the molecular processes that induce this mechanochemical reaction by using density functional theory calculations to tune force field parameters for the model system.

Modeling Mechanochemical Reaction Mechanisms.

The mechanochemical reaction between copper and dimethyl disulfide is studied under well-controlled conditions in ultrahigh vacuum (UHV). Reaction is initiated by fast S-S bond scission to form adsorbed methyl thiolate species, and the reaction kinetics are reproduced by two subsequent elementary mechanochemical reaction steps, namely a mechanochemical decomposition of methyl thiolate to deposit sulfur on the surface and evolve small, gas-phase hydrocarbons, and sliding-induced oxidation of the copper by sulfur that regenerates vacant reaction sites. The steady-state reaction kinetics are monitored in situ from the variation in the friction force as the reaction proceeds and modeled using the elementary-step reaction rate constants found for monolayer adsorbates.

Enhanced hydrogenation activity and diastereomeric interactions of methyl pyruvate co-adsorbed with R-1-(1-naphthyl)ethylamine on Pd(111).

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity. Here, this effect is explored on a R-1-(1-naphthyl)ethylamine (NEA)-modified Pd(111) model catalyst where temperature-programmed desorption experiments reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. NEA+MP docking complexes are imaged using scanning tunnelling microscopy supplemented by density functional theory calculations to allow the most stable docking complexes to be identified.

Determination of Adsorbate Structures from 1,4-Phenylene Diisocyanide on Gold.

The structure of the 1-D oligomer chains that form on a Au(111) surface following adsorption of 1,4-phenylene diisocyanide (PDI) is explored using reflection-absorption infrared spectroscopy and scanning tunneling microscopy (STM). The experimental work is complemented by first-principles density functional theory calculations, which indicate that the previously proposed gold-PDI oligomer chains in which the PDI molecule bridged gold adatoms are thermodynamically stable. In addition, the calculated vibrational modes for this structure are in excellent agreement with the experimental infrared data.

Chemisorptive enantioselectivity of chiral epoxides on tartaric-acid modified Pd(111): three-point bonding.

The chemisorption of two chiral molecules, propylene oxide and glycidol, is studied on tartaric-acid modified Pd(111) surfaces by using temperature-programmed desorption to measure adsorbate coverage. It is found that R-glycidol shows preferential enantioselective chemisorption on (S,S)-tartaric acid modified Pd(111) surfaces, while propylene oxide does not adsorb enantioselectively. The enantioselectivity of glycidol depends on the tartaric acid coverage, and is exhibited for low tartaric acid coverages indicating that the bitartrate phase is responsible for the chiral recognition.

Linking gold nanoparticles with conductive 1,4-phenylene diisocyanide-gold oligomers.

It is demonstrated that 1,4-phenylene diisocyanide (PDI)-gold oligomers can spontaneously bridge between gold nanoparticles on mica, thereby providing a strategy for electrically interconnecting nanoelectrodes. The barrier height of the bridging oligomer is 0.10 ± 0.

Surface chemistry of isopropoxy tetramethyl dioxaborolane on Cu(111).

The surface chemistry of isopropoxy tetramethyl dioxaborolane (ITDB), tetramethyl dioxaborolane (TDB), and 2-propanol is studied on a clean Cu(111) single crystal using temperature-programmed desorption (TPD). 2-Propanol is found to have two competing reactions on the copper surface. Dehydration results in water and propene formation, and dehydrogenation results in the formation of acetone and hydrogen.

Efficient transport of gold atoms with a scanning tunneling microscopy tip and a linker molecule.

A thiophene-containing molecule attached to a scanning tunneling microscopy (STM) tip is used to transport gold atoms on a Au(111) surface. The molecule contains eight thiophene rings and therefore has sulfur atoms that are known to bind to gold atoms. Using a gold-coated tip, the molecules previously deposited on the surface bind to the lower-coordination gold atoms of the tip.

Low-temperature, shear-induced tribofilm formation from dimethyl disulfide on copper.

The frictional properties of a sliding copper-copper interface exposed to dimethyl disulfide (DMDS) are measured in UHV under conditions at which the interfacial temperature rise is <1 K. A significant reduction in friction is found from the clean-surface values and sulfur is found on the surface and below the surface in the wear scar region by Auger spectroscopy. Because the interfacial temperature rise under the experimental conditions used to measure friction is very small, tribofilm formation is not thermally induced.

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.

The surface chemistry of dimethyl disulfide on copper.

The surface chemistry of dimethyl disulfide (DMDS) is studied on a Cu(111) single crystal and a polished copper foil in ultrahigh vacuum as a basis for understanding its tribological chemistry using a combination of temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Low-energy electron diffraction reveals that the polished foil becomes ordered on heating in vacuo and displays identical surface chemistry to that found on the Cu(111) surface. Dimethyl disulfide reacts with the copper surface at 80 K to form thiolate species.

Coverage effects on the palladium-catalyzed synthesis of vinyl acetate: comparison between theory and experiment.

The high adsorbate coverages that form on the surfaces of many heterogeneous catalysts under steady-state conditions can significantly lower the activation energies for reactions that involve the coupling of two adsorbed intermediates while increasing those which result in adsorbate bond-breaking reactions. The influence of the surface coverage on the kinetics of metal-catalyzed reactions is often ignored in theoretical and even in some ultrahigh vacuum experimental studies. Herein, first principle density functional theoretical calculations are combined with experimental surface titration studies carried out over well-defined Pd(111) surfaces to explicitly examine the influence of coverage on the acetoxylation of ethylene to form vinyl acetate over Pd.

A new method for performing polarization modulation infrared reflection-adsorption spectroscopy of surfaces.

A new and relatively simple polarization modulation technique is presented and tested that enables the whole spectral range to be detected between 400 and 4000 cm(-1). This experiment is conventionally carried out using a photoelastic modulator that modulates incident plane polarized light through 90 degrees . This suffers from the drawback that it enables spectra to be collected only over a relatively narrow spectral range.

Ethylene decomposition at undercoordinated sites on Cu(410).

We demonstrate the selective, low-temperature chemistry of ethylene on the strongly undercoordinated sites of Cu(410) by investigating its adsorption by high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). After dosing ethylene at approximately 110 K, apart from the expected pi-bonded species adsorbed on terraces, di-sigma-bonded ethylene and carbon are formed at the step edges. The latter product results from the complete dehydrogenation of ethylene and blocks sites for further dissociation and/or di-sigma-adsorption.