Requirements for the formation of a chiral template

The chemisorptive enantioselectivity of propylene oxide is examined on Pd(111) surfaces templated by chiral 2-methylbutanoate and 2-aminobutanoate species. It has been found previously that chiral propylene oxide is chemisorbed enantiospecifically onto Pd(111) surfaces modified by either (R)- or (S)-2-butoxide. The enantiomeric excess (ee) varied with template coverage, reaching a maximum of approximately 31%. Templating the surface using 2-methylbutanoate, where the chiral center is identical to that in the 2-butoxide species, but is now anchored to the surface by a carboxylate rather than an alkoxide linkage, shows no enantiospecificity. The enantioselectivity is restored when the methyl group is replaced by an amine group, where a maximum ee value of approximately 27% is found. DFT calculations and infrared measurements suggest that the structures of the butyl group on the surface are similar for both 2-butoxide and 2-methylbutanoate species, implying that gross conformational changes are not responsible for differences in chemisorptive enantioselectivity. There is no clear correlation between the location of the chiral center and enantioselectivity, suggesting that differences in the template adsorption site are also not responsible for the lack of enantioselectivity. It is proposed that the 2-butyl group in 2-methylbutanoate species is less rigidly bonded to the surface than that in 2-butoxides, allowing the chiral center to rotate azimuthally. It is postulated that the role of the amino group in 2-aminobutanoate species is to anchor the chiral group to the surface to inhibit azimuthal rotation.     

Vinyl acetate formation by the reaction of ethylene with acetate species on oxygen-covered Pd(111)

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.     

The adsorption site and orientation of CH3S on Ni(111)

The angle-resolved X-ray photoelectron spectra for 0.15 monolayers (ML) of sulfur, and 0.25 ML methyl thiolate formed at 100 K and annealed to 150 and 250 K, on Ni(111) are analyzed to determine the structures of these species. It is found that sulfur adsorbs on the face-centered cubic hollow site on Ni(111) with a S---Ni bond length of 2.20±0.02 Å. The thiolate species formed at 150 K has the C---S bond tilted at 35° to the surface normal with a C---S bond length of 1.85±0.02 Å and a S---Ni bond length similar to that for adsorbed sulfur (2.2 Å). The methyl group is tilted toward the bridge site and the thiolate appears to be adsorbed on the face-centered cubic site although there may also be adsorption in the hexagonal close packed site. The species formed at 250 K adsorbs on a reconstructed surface where the chemical shift of the S 2p core level indicates that it adsorbs at a four-fold site and the angle-resolved XPS data indicate that the C---S bond is oriented normal to the surface. The calculated angular variations in intensity are consistent with this interpretation but cannot distinguish between the various models proposed for the reconstructed surface.     

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. ITDB directly adsorbed on the surface reacts completely and does not molecularly desorb. TDB and 2-propanol decompose desorbing mainly 2,3-dimethyl 2-butene and acetone, respectively. Both of those products desorb above room temperature and are present in TPDs of ITDB. An additional acetone desorption peak was observed for ITDB at higher temperatures than acetone desorption from 2-propanol. This higher temperature peak at ～391 K was attributed to two acetone molecules forming from the tetramethyl end group resulting from a stronger bound surface species in ITDB compared to TDB despite their identical end groups. The copper surface seems to be reactive enough toward ITDB at room temperature that a potential boron-containing tribofilm could be produced for copper-copper sliding contacts. Despite their similarities, ITDB and TDB have different surface species present at room temperature, so their tribological properties will be investigated in the future.     

The adsorption and reaction of 2-butanol on clean and oxygen-covered Pd(100)

The surface chemistry of 2-butanol is explored on clean Pd(100), c(2 × 2)-O/Pd(100) and p(2 × 2)-O/Pd(100) surfaces by means of temperature-programmed desorption, reflection–absorption infrared and X-ray photoelectron spectroscopies. 2-Butanol adsorbs molecularly on clean and oxygen-covered Pd(100) below ～ 190 K, but then appears to react to form 2-butoxide species at ～ 200 K. Both 2-butanone and 2-butanol desorb from the clean surface at ～ 226 K, by β-hydride elimination from the 2-butoxide species and rehydrogenation of the 2-butoxide, respectively. In contrast, almost exclusively 2-butanone is formed on oxygen-covered surfaces. Butanone desorbs at ～ 195 K and ～ 260 K from c(2 × 2)-O/Pd(100) with the 195 K peak being the most intense. However, on p(2 × 2)-O/Pd(100), 2-butanone desorbs at ～ 195 K and ～ 295 K, and the latter peak is the most intense. The ～ 195 K, 2-butanone state is proposed to occur due to abstraction by adsorbed atomic oxygen and the change in relative intensity of these features is ascribed to the lower ability of surface hydroxyl groups to facilitate β-hydride elimination on oxygen-covered surfaces. Further heating results in the formation of hydrogen and carbon monoxide and leaves a small amount of carbon deposited on the surface.     

The surface chemistry of propylene, 1-iodopropane, and 1,3-diiodopropane on MoAl alloy thin films formed on dehydroxylated alumina

The adsorption of C3 hydrocarbons propylene, 1-iodopropane, and 1,3-diiodopropane is studied in ultrahigh vacuum on a molybdenum-aluminum alloy formed by molybdenum hexacarbonyl reaction with a planar alumina film grown on a Mo(100) substrate. Carbon-iodine bond scission occurs below approximately 200 K to deposit iodine, and form propyl species from 1-iodopropane and a C3 metallacycle from 1,3-diiodopropane. Propyl species either undergo beta-hydride elimination to yield propylene or hydrogenate to form propane. Propylene adsorbs as both pi- and di-sigma-bonded species, and the di-sigma form hydrogenates to yield propane, where the addition of the first hydrogen to form propyl species is slower than the second hydrogenation step to yield propane. Propylene also thermally decomposes on the surface to desorb hydrogen and deposit carbon where the methylyne group is the most, and the methyl group the least reactive. The metallacyclic intermediate reacts to give an allylic intermediate, which forms propylene, but also decomposes by C-C bond cleavage to evolve ethylene and deposit methylene species on the surface. This is a key step in the mechanism proposed for heterogeneously catalyzed olefin metathesis and this is the first time that this chemistry has been directly identified in ultrahigh vacuum.     

An infrared spectroscopic investigation of thin alumina films: measurement of acid sites and surface reactivity

The surface chemical activity of an alumina films grown on Mo(100) by oxidation of aluminum evaporated onto the surface and oxidized using water is examined using Auger, X-ray photoelectron and reflection/absorption infrared spectroscopies. The formation of alumina is confirmed using Auger and X-ray photoelectron spectroscopy from the positions and intensities of the aluminum features and using reflection-absorption infrared spectroscopy from the longitudinal optical modes of the Al–O bonds measured at 935 cm⁻¹. The presence of surface hydroxyls is monitored by forming films using D₂O which are evidenced by a feature at 2700 cm⁻¹. Ammonia adsorption on a dehydroxylated surface yields a single peak at 1260 cm⁻¹ due to ammonia adsorbed at a surface Lewis site where the principle symmetry axis of ammonia is oriented perpendicularly to the surface plane. Ammonia also appears to adsorb at Lewis sites on a hydroxylated surface with a slightly different adsorption geometry from that on a dehydroxylated surface. Finally, the chemistry of trimethyl aluminum adsorbed on the planar hydroxylated alumina surface is compared with that found on high-surface-area γ-alumina where the spectra and the chemistry found in both régimes is exactly identical except that the low-frequency methyl bending modes (at 769 and 718 cm⁻¹) are not obscured on the thin film by the intense substrate whereas they are on the high-surface-area support.     

The chemistry of CH2I2 on MoAl alloy thin films formed on dehydroxylated alumina: insight into methylene insertion reactions

The chemistry of diiodomethane is explored in ultrahigh vacuum on a MoAl alloy film grown on planar, dehydroxylated alumina by reaction with molybdenum hexacarbonyl. The majority of the diiodomethane forms methylene species below approximately 250 K, although a small proportion forms CH(2)I((ads)), which hydrogenates to form iodomethane. The majority ( approximately 90%) of the adsorbed methylene species thermally decomposes to carbon and hydrogen. The remainder undergoes several reactions, including partial hydrogenation to form adsorbed methyl species or total hydrogenation to form methane. The methyl species can couple forming ethane or undergo methylene insertion reactions to form alkyl species up to C(4). These form alkenes via a beta-hydride elimination reaction. This chemistry is relatively unique, only having been found previously for Ni(110) surfaces. No such chemistry is found on Ni(100) and Ni(111).