The adsorption of CO on a saturated overlayer of 1,4-phenylene diisocyanide (PDI) adsorbed on a Au(111) surface at 300 K is studied using scanning tunneling microscopy (STM), density functional theory (DFT) calculations and reflection absorption infrared spectroscopy (RAIRS). The PDI forms closed-packed rows of gold-PDI chains by extracting gold atoms from the Au(111) substrate. They are imaged by STM and the structure calculated by DFT. The adsorption of CO is studied on the low-coordination gold sites formed on the PDI-covered surface where it adsorbs exhibiting a CO stretching frequency of 2004 cm -1 , consistent with adsorption on an atop site. It is found that CO is stable on heating the sample to *150 K and is only removed from the surface by heating to *180 K. Since low-coordination gold atoms are suggested to be the active catalytic sites on supported gold nanoclusters, ''embossing'' the surface to form similar lowcoordination sites using PDI might offer a strategy for tailoring the catalytic activity of gold.
The molecular mechanisms by which mechanical energy accelerates a chemical reaction at sliding solid−solid interfaces are not well understood because of the experimental difficulties in monitoring chemical processes and their rates, and in controlling parameters such as interfacial temperature. These issues are addressed by measuring the shear-induced rate of methane formation from the decomposition of adsorbed methyl thiolate species on copper in ultrahigh vacuum (UHV), where the frictional heating is negligible. The effect of a constant force F on the energy profile for thiolate decomposition from density functional theory calculations is modeled by superimposing a linear potential, V(x) = −Fx. This enables the change in activation barrier to be calculated as a function of force. The mechanically induced reaction rate is measured by sliding a ball over a methyl thiolate-covered copper surface from the methane yield measured by a mass spectrometer placed in the UHV chamber. Molecular dynamics simulations reveal that a wide distribution of forces are exerted on the thiolates and comparing the measured methyl thiolate decomposition rate with the rate calculated by assuming a wide force distribution reproduces the experimental data. This reveals that only a small proportion of the adsorbed thiolates experience sufficiently high forces to reduce the activation barrier to reproduce the experimentally measured rate constant.
The pathways for the spontaneous self-assembly of one-dimensional oligomeric chains from the adsorption of 1,4-phenylene diisocyanide (PDI) on Au(111) surface are explored using density functional theory. It has been shown previously that the chain comprises repeating −(Au−PDI)− structures. The results show that the chains form from mobile Au−PDI adatom complexes and that chains propagate by the adatom complex coupling to a terminal isocyanide group which lies close to parallel to the surface and the activation barrier for this propagation step is ∼28 kJ/mol. It is also found that the Au−PDI adatom complex is attracted to the terminal isocyanide, thereby facilitating the oligomerization process. The insights into the oligomerization pathway are used to explore whether an external electric field applied to diisocyanide functionalized molecules that contain a dipole moment can be used to align them. It is found that molecules with dipole moments of ∼1 D show significant alignment with an electric field of ∼10 8 V/m and almost complete alignment when the electric field reaches ∼10 9 V/m. This suggests that the selfassembly chemistry of dipolar diisocyanides can be used to create oriented systems.
The structure and self-assembly of alanine on Pd(111) is explored using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), reflection–absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM), and supplemented by density functional theory (DFT) calculations to explore the stability of the proposed surface structures formed by adsorbing alanine on Pd(111) and to simulate the STM images. Both zwitterionic and anionic species are detected using RAIRS and XPS, while DFT calculations indicate that isolated anionic alanine is significantly more stable than the zwitterion. This observation is rationalized by observing dimeric species when alanine is dosed at ∼270 K and then cooled to trap metastable surface structures. The dimers form due to an interaction between the carboxylate group of anionic alanine with the NH3 + group of the zwitterion. Adsorbing alanine at 290 K results in the formation of dimer rows and tetramers resulting in only short-range order, consistent with the lack of additional diffraction spots in LEED. The stability of various structures is explored using DFT, and the simulated STM images are compared with experiment. This enables the dimer rows to be assigned to the assembly of anionic-zwitterionic dimers and the tetramer to the assembly of two dimers in which three of the alanine molecules undergo a concerted rotation by 30°.
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. The results show that diastereomeric interactions between NEA and MP occur predominantly by binding of the C=C of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H2N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds thus providing a rationale for the catalytic observations.
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