Studies of large scale unstable growth formed during GaAs(001) homoepitaxy

Atomic force and scanning tunneling microscopy studies have been performed on GaAs(001) films grown by molecular beam epitaxy. Multilayered mounds are seen to evolve when the growth conditions favor island nucleation. As the epilayer thickness is increased, these features grow in all dimensions but the angle of inclination remains approximately constant at 1°. The mounding does not occur on surfaces grown in step-flow. We propose that the multi-layered features are due to an unstable growth mode which relies on island nucleation and the presence of a step edge barrier.     

Entropic barriers in nanoscale adhesion studied by variable temperature chemical force microscopy

Intermolecular interactions drive the vast majority of condensed phase phenomena from molecular recognition to protein folding to particle adhesion. Complex energy barriers encountered in these interactions include contributions from van der Waals forces, hydrogen bonding, and solvent medium. With the spectacular exception of hydrophobic interactions, contributions from the medium are usually considered secondary. We report a variable temperature force microscopy study of the interactions between several hydrogen bonds in different solvents that challenges this point of view. Surprisingly, we observed an increase in the strength of the interaction between carboxylic acid groups in ethanol as the temperature increased. Moreover, when we switched to a nonpolar solvent we observed the opposite behavior: The binding force decreased as the temperature increased. Kinetic model of bond dissociation provided quantitative interpretation of our measurements. We attributed the observed phenomena to a large entropic contribution from the ordered solvent layers that are forming on the probe and sample surfaces upon detachment. The observed reversal in the force vs temperature trend is a manifestation of a transition between thermodynamic and kinetic regimes of unbinding predicted by the model. Our results indicate that entropic barriers dominated by the interactions of solvent molecules with the surface exist in a much wider variety of systems than previously thought.     

Synthesis of Bioactive Side‐Chain Analogues of TAN‐2483B

The fungal metabolite TAN‐2483B has a 2,6‐trans‐relationship across the pyran ring of its furo[3,4‐b]pyran‐5‐one core, which has thwarted previous attempts at its synthesis. We have now developed a chiral pool approach to this core and prepared side‐chain analogues of TAN‐2483B. The synthesis relies on ring expansion of a reactive furan ring‐fused dibromocyclopropane and alkynylation of the resulting pyran. The furan ring is constructed by palladium‐catalysed carbonylative lactonisation. Various side‐chains are appended through Wittig‐type chemistry. The prepared analogues showed micromolar activity towards cancer cell lines HL‐60, 1A9 and MCF‐7 and certain human disease‐relevant kinases, including Bruton's tyrosine kinase (Btk).     

Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine

A new chemical kinetic reaction mechanism has been developed for the oxidation of methylcyclohexane (MCH), combining a new low temperature mechanism with a recently developed high temperature mechanism. Predictions from this kinetic model are compared with new experimentally measured ignition delay times from a rapid compression machine. Computed results were found to be particularly sensitive to isomerization rates of methylcyclohexylperoxy radicals. Three different methods were used to estimate rate constants for these isomerization reactions. Rate constants based on comparable alkylperoxy radical isomerizations corrected for the differences in the structure of MCH and the respective alkane, predicted ignition delay times in very poor agreement with the experimental results. The most significant drawback was the complete absence of a region of negative temperature coefficient (NTC) in the model results using this method, although a prominent NTC region was observed experimentally. Alternative estimates of the isomerization reaction rate constants, based on the results from previous experimental studies of low temperature cyclohexane oxidation, provided much better agreement with the present experiments, including the pronounced NTC behavior. The most important feature of the resulting methylcyclohexylperoxy radical isomerization reaction analysis was found to be the relative rates of isomerizations that proceed through 5-, 6-, and 7-membered transition state ring structures and their different impacts on the chain branching behavior of the overall mechanism. Theoretical implications of these results are discussed, with particular attention paid to how intramolecular H atom transfer reactions are influenced by the differences between linear alkane and cycloalkane structures.     

Experimental and modeling study of methyl cyclohexane pyrolysis and oxidation

Although the combustion chemistry of aliphatic hydrocarbons has been extensively documented, the oxidation of cyclic hydrocarbons has been studied to a much lesser extent. To provide a deeper understanding of the combustion chemistry of naphthenes, the oxidation of methylcyclohexane was studied in a series of high-temperature shock tube experiments. Ignition delay times for a series of mixtures, of varying methylcyclohexane/oxygen equivalence ratios (phi=0.5, 1.0, 2.0), were measured over reflected shock temperatures of 1200-2100 K and reflected shock pressures of 1.0, 2.0, and 4.0 atm. A detailed chemical kinetic mechanism has been assembled to simulate the shock tube results and flow reactor experiments, with good agreement observed.