Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2005"
TL;DR: It is shown how genetic algorithms, mimicking biological evolution ('survival of the fittest'), can be used to distil reliable model hamiltonian parameters from a database of first-principles calculations.
Abstract: Modern condensed-matter theory from first principles is highly successful when applied to materials of given structure-type or restricted unit-cell size. But this approach is limited where large cells or searches over millions of structure types become necessary. To treat these with first-principles accuracy, one 'coarse-grains' the many-particle Schrodinger equation into 'model hamiltonians' whose variables are configurational order parameters (atomic positions, spin and so on), connected by a few 'interaction parameters' obtained from a microscopic theory. But to construct a truly quantitative model hamiltonian, one must know just which types of interaction parameters to use, from possibly 10(6)-10(8) alternative selections. Here we show how genetic algorithms, mimicking biological evolution ('survival of the fittest'), can be used to distil reliable model hamiltonian parameters from a database of first-principles calculations. We demonstrate this for a classic dilemma in solid-state physics, structural inorganic chemistry and metallurgy: how to predict the stable crystal structure of a compound given only its composition. The selection of leading parameters based on a genetic algorithm is general and easily applied to construct any other type of complex model hamiltonian from direct quantum-mechanical results.
253 citations
TL;DR: This Letter shows that hitherto unaccounted spin selection rules give rise to a highly nonadiabatic behavior in the O(2)/Al(111) interaction, and discusses problems caused by the insufficient accuracy of present-day exchange-correlation functionals.
Abstract: A most basic and puzzling enigma in surface science is the description of the dissociative adsorption of O(2) at the (111) surface of Al. Already for the sticking curve alone, the disagreement between experiment and results of state-of-the-art first-principles calculations can hardly be more dramatic. In this Letter we show that this is caused by hitherto unaccounted spin selection rules, which give rise to a highly nonadiabatic behavior in the O(2)/Al(111) interaction. We also discuss problems caused by the insufficient accuracy of present-day exchange-correlation functionals.
248 citations
TL;DR: The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy, finding competition of nonradiative decay ofexcitons is facilitated by a thermally activated process.
Abstract: The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode.
205 citations
TL;DR: Time-resolved photoluminescence (PL) spectroscopy is used to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes to provide evidence for PL from small diameter core tubes in DWNTs.
Abstract: Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.
201 citations
TL;DR: In certain cases, evidence suggesting that successive adsorption of CO can distort the metal cluster framework is found, and the structure of the saturated Au(n)(CO)(m)(+) complexes is established.
Abstract: We report on the interaction of carbon monoxide with cationic gold clusters in the gas phase. Successive adsorption of CO molecules on the Aun+ clusters proceeds until a cluster size specific saturation coverage is reached. Structural information for the bare gold clusters is obtained by comparing the saturation stoichiometry with the number of available equivalent sites presented by candidate structures of Aun+. Our findings are in agreement with the planar structures of the Aun+ cluster cations with n ≤ 7 that are suggested by ion mobility experiments [Gilb, S.; Weis, P.; Furche, F.; Ahlrichs, R.; Kappes, M. M. J. Chem. Phys. 2001, 116, 4094]. By inference we also establish the structure of the saturated Aun(CO)m+ complexes. In certain cases we find evidence suggesting that successive adsorption of CO can distort the metal cluster framework. In addition, the vibrational spectra of the Aun(CO)m+ complexes in both the CO stretching region and in the region of the Au−C stretch and the Au−C−O bend are measu...
172 citations
TL;DR: Infrared multiple-photon dissociation spectroscopy is effected on the K(+) tagged aromatic amino acids tyrosine and phenylalanine, as well as the K (+) tagged peptides bradykinin fragment 1-5 and [Leu]-enkephalin to determine whether the complex is in the charge solvation (CS) or salt bridge (SB) configuration.
Abstract: Infrared multiple-photon dissociation spectroscopy is effected on the K+ tagged aromatic amino acids tyrosine and phenylalanine, as well as the K+ tagged peptides bradykinin fragment 1-5 and [Leu]-enkephalin. The fingerprint (800−1800 cm-1) infrared spectra of these species are compared to density-functional theory (DFT) calculated spectra to determine whether the complex is in the charge solvation (CS) or salt bridge (SB) (i.e. zwitterionic) configuration. For the aromatic amino acids the CS structure is favored and the tridentate N/O/ring structure is found to be the preferred binding geometry for K+. The experimental and theoretical evidence for bradykinin fragment 1-5 tagged with K+ suggests that the SB structure is favored; the calculations indicate a head-to-tail looped structure stabilized by a salt bridge between the protonated guanidine group and the deprotonated C-terminus, which allows K+ to sit in a binding pocket with five CO electrostatic interactions. For K+ tagged [Leu]-enkephalin the spec...
141 citations
TL;DR: A test of Lorentz invariance performed by comparing the resonance frequencies of one stationary optical resonator and one continuously rotating on a precision air bearing turntable results in an eightfold improvement with respect to previous nonrotating measurements.
Abstract: We report on a test of Lorentz invariance performed by comparing the resonance frequencies of one stationary optical resonator and one continuously rotating on a precision air bearing turntable. Special attention is paid to the control of rotation induced systematic effects. Within the photon sector of the standard model extension, we obtain improved limits on combinations of 8 parameters at a level of a few parts in ${10}^{\ensuremath{-}16}$. For the previously least well known parameter we find ${\stackrel{\texttildelow{}}{\ensuremath{\kappa}}}_{e\ensuremath{-}}^{ZZ}=(\ensuremath{-}1.9\ifmmode\pm\else\textpm\fi{}5.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$. Within the Robertson-Mansouri-Sexl test theory, our measurement restricts the isotropy violation parameter $\ensuremath{\beta}\ensuremath{-}\ensuremath{\delta}\ensuremath{-}\frac{1}{2}$ to $(\ensuremath{-}2.1\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$, corresponding to an eightfold improvement with respect to previous nonrotating measurements.
118 citations
TL;DR: An electrodynamic trap, capable of trapping neutral atoms and molecules in both low-field and high-field seeking states, is realized, and the stability of the trap is studied as a function of the switching frequency.
Abstract: We here report on the realization of an electrodynamic trap, capable of trapping neutral atoms and molecules in both low-field and high-field seeking states. Confinement in three dimensions is achieved by switching between two electric field configurations that have a saddle point at the center of the trap, i.e., by alternating a focusing and a defocusing force in each direction. The ac trapping of ${^{15}\mathrm{N}\mathrm{D}}_{3}$ molecules is experimentally demonstrated, and the stability of the trap is studied as a function of the switching frequency. A 1 mK sample of ${^{15}\mathrm{N}\mathrm{D}}_{3}$ molecules in the high-field seeking component of the $|J,K⟩=|1,1⟩$ level, the ground state of para-ammonia, is trapped in a volume of about $1\text{ }\text{ }{\mathrm{m}\mathrm{m}}^{3}$.
117 citations
TL;DR: In this article, free energy phase diagrams for transition metal systems in (T, p)-space ranging from ultra-high vacuum to technically relevant pressures, p, and temperatures, T, are discussed.
110 citations
TL;DR: This work reports the vibrational spectra of the carbonyl complexes of anionic gold clusters in the range of the CO stretching frequency as measured in the gas phase using IR multiple photon dissociation spectroscopy, and quantifies how theCO stretching frequency varies with the charge state of the gold cluster.
Abstract: We report the vibrational spectra of the carbonyl complexes of anionic gold clusters in the range of the CO stretching frequency as measured in the gas phase using IR multiple photon dissociation spectroscopy. The investigated complexes contain between 3 and 14 Au atoms and up to 7 CO ligands. Special attention is given to the complexes that exhibit saturation CO coverage as well as to the monocarbonyl species. In conjunction with data from the corresponding cationic complexes we quantify how the CO stretching frequency varies with the charge state of the gold cluster. Our results provide a size- and charge-dependent basis to interpret values of the CO stretching frequency measured for CO on deposited gold clusters in terms of the charge states of the clusters.
105 citations
TL;DR: Density functional theory calculations reveal that the O-H in-plane bending vibration is a sensitive probe to hydrogen bonding and, thus, to the folding of the peptide backbone in these structures, enabling the identification of spectroscopic fingerprints for the various conformational structures.
Abstract: To understand the intrinsic properties of peptides, which are determined by factors such as intramolecular hydrogen bonding, van der Waals bonding and electrostatic interactions, the conformational landscape of isolated protein building blocks in the gas phase was investigated. Here, we present IR-UV double-resonance spectra of jet-cooled, uncapped peptides containing a tryptophan (Trp) UV chromophore in the 1000-2000 cm(-1) spectral range. In the series Trp, Trp-Gly and Trp-Gly-Gly (where Gly stands for glycine), the number of detected conformers was found to decrease from six (Snoek et al., PCCP, 2001, 3, 1819) to four and two, respectively, which indicates a trend to relaxation to a global minimum. Density functional theory calculations reveal that the O-H in-plane bending vibration, together with the N-H in-plane bend ing and the peptide C=O stretching vibrations, is a sensitive probe to hydrogen bonding and, thus, to the folding of the peptide backbone in these structures. This enables the identification of spectroscopic fingerprints for the various conformational structures. By comparing the experimentally observed IR spectra with the calculated spectra, a unique conformational assignment can be made in most cases. The IR-UV spectrum of a Trp-containing nonapeptide (Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu) was recorded as well and, although the IR spectrum is less well-resolved (and it probably results from different isomers), groups of amide I (peptide C=O stretching) and amide II (N-H in-plane bending) bands can still be recognised, in agreement with predictions at the AM1 level.
TL;DR: Clear-cut diagnostic features in the spectra of the acetophen one, anisole, and aniline complexes showed the sites of Cr+ attachment to be the carbonyl oxygen site for acetophenone (bis-complex) and the ring-pi site for anisoles and anILine (both mono- and bis-complexes).
Abstract: Infrared spectra were recorded for a series of gas-phase Cr+ complexes using infrared multiphoton dissociation (IRMPD) in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The functionalized aromatic ligands (acetophenone, anisole, aniline, and dimethyl aniline) offer a choice of either aromatic ring-pi or n-donor-base binding sites. Use of the FELIX free electron laser light source allowed convenient, rapid scanning of the chemically informative wavelength range from approximately 500 to 1800 cm(-1), which in many cases characterized the preferred site of metal binding, as well as the electronic spin state of the complex. Mono-complex ions, Cr+(ligand), for anisole, aniline, and dimethyl aniline and bis-complex ions, Cr+(ligand)(2), for anisole, aniline, and acetophenone were produced by ligand attachment to laser-desorbed Cr+ ions in the FT-ICR cell. The photodissociation yields plotted as a function of wavelength were interpreted as approximations to the infrared absorption spectra and were compared with computed spectra of different possible geometries and spin states. Clear-cut diagnostic features in the spectra of the acetophenone, anisole, and aniline complexes showed the sites of Cr+ attachment to be the carbonyl oxygen site for acetophenone (bis-complex) and the ring-pi site for anisole and aniline (both mono- and bis-complexes). The bis-complexes of aniline and anisole are low-spin (probably doublet) states, while the mono-complexes of these same ligands are high-spin (sextet) states. The dimethyl aniline complex gave a cluttered spectrum in poor agreement with calculations, which may reflect a mixture of binding-site isomers in this case.
TL;DR: In this article, the synthesis of homogeneously distributed carbon nanofibers with well-defined morphology on vapor-grown carbon nanometers was achieved by a sequence of gas-phase steps, thus fully avoiding wet chemistry.
Abstract: The synthesis of homogeneously distributed carbon nanofibers with well-defined morphology on vapor-grown carbon nanofibers was achieved by a sequence of gas-phase steps, thus fully avoiding wet chemistry: first, carbon nanofibers were exposed to oxygen plasma to introduce oxygen-containing functional groups. Then, the chemical vapor deposition of ferrocene was carried out under oxidizing conditions, yielding nanofiber-supported iron oxide nanoparticles. Secondary carbon nanofibers with diameters in the range from 10 to 20 nm were subsequently grown from cyclohexane catalyzed by the sintered metallic iron nanoparticles under reducing conditions. XPS was applied to monitor the chemical changes of the surface composition and the sintering of the metallic iron particles in hydrogen. The morphology and the height distribution of the sintered iron oxide nanoparticles was derived by a unique combined application of scanning electron microscopy and scanning tunneling microscopy. The specific surface area of the ...
TL;DR: The results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions.
Abstract: The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d5/2 core levels of the two topmost Ru layers. The evolution of the Ru 3d5/2 spectra with oxygen exposure at 475 K and the corresponding O2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface “oxide”, which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher ca...
TL;DR: Using density-functional theory calculations, the ground state energy and vibrational spectra for a large number of stable and metastable geometries of charged vanadium clusters with sizes of 3-15 atoms are calculated.
Abstract: The far-infrared vibrational spectra for charged vanadium clusters with sizes of 3–15 atoms have been measured using infrared multiple photon dissociation of Vn+Ar→Vn++Ar. Using density-functional theory calculations, we calculated the ground state energy and vibrational spectra for a large number of stable and metastable geometries of such clusters. Comparison of the calculated vibrational spectra with those obtained in the experiment allows us to deduce the cluster size specific atomic structures. In several cases, a unique atomic structure can be identified, while in other cases our calculations suggest the presence of multiple isomers.
TL;DR: The vibrational far-infrared (IR) spectra of isolated metal clusters in the gas phase can be measured by performing photo dissociation spectroscopy of their rare gas complexes as discussed by the authors.
Abstract: The vibrational far-infrared (IR) spectra of isolated metal clusters in the gas phase can be measured by performing photo dissociation spectroscopy of their rare gas complexes For these experiments an intense and widely tunable source of far-IR radiation is required, and the Free Electron Laser for Infrared eXperiments (FELIX) is ideally suited for this Vibrational spectra are obtained for vanadium cluster cations as well as for neutral and cationic niobium clusters The comparison of the experimental vibrational spectra with theoretically calculated spectra allows for the determination of the structure of the metal clusters
TL;DR: In this article, the thermal and native oxidation of CuGaSe2 thin films was studied by in-situ X-ray photoelectron spectroscopy (XPS) in ultra high vacuum (UHV) environment.
TL;DR: It is suggested that this restructuring may be responsible for the activation process by facilitating the formation of an active oxygen layer on the PdO surface.
Abstract: The progressive activation of a Pd(2 wt %)/gamma-alumina catalyst under the reaction conditions of catalytic combustion of methane (CCM) was studied. The reasons of this activation were investigated by XPS, CO-chemisorption, and HR-TEM. The removal of carbon from the surface cannot explain the observed activation process. Sintering of the palladium particles was detected but this parameter alone does not fully explain the activation process of the catalyst. HR-TEM imaging evidences (i) that PdO is present both in the fresh and the active catalyst and (ii) that the PdO nanoparticles sinter and restructure (surface roughening) during the reaction. Development of preferential faces was not observed. It is suggested that this restructuring may be responsible for the activation process by facilitating the formation of an active oxygen layer on the PdO surface. CCM on Pd/gamma-Al(2)O(3) depends on the thermal history of the catalyst and is a structure-sensitive reaction.
TL;DR: The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K, and the rate-controlling character of the different elementary steps of the reaction system is discussed.
Abstract: Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state condit...
TL;DR: The interactions of sodium dodecyl sulfate with the triblock copolymer L64 and hexaethylene glycol mono-n-dodecyl ether and C12EO6 were studied to evaluate an interaction parameter using regular solution theory and to determine the structure of the monomeric polymer/micellar SDS complex.
Abstract: The interactions of sodium dodecyl sulfate (SDS) with the triblock copolymer L64 (EO13−PO30−EO13) and hexaethylene glycol mono-n-dodecyl ether (C12EO6) were studied using electromotive force, isothermal titration microcalorimetry, differential scanning micocalorimetry, and surface tension measurements. In certain regions of binding, mixed micelles are formed, and here we could evaluate an interaction parameter using regular solution theory. The mixed micelles of L64 with both SDS and C12EO6 exhibit synergy. When L64 is present in its nonassociated state, it forms polymer/micellar SDS complexes at SDS concentrations above the critical aggregation concentration (cac). The cac is well below the critical micellar concentration (cmc) of pure SDS, and a model suggesting how bound micelles are formed at the cac in the presence of a polymer is described. The interaction of nonassociated L64 with C12EO6 is a very rare example of strong binding between a nonionic surfactant and a nonionic polymer, and C12EO6/L64 mi...
TL;DR: In this paper, the electronic structure of high-quality CrO2(100) films was investigated by means of x-ray absorption and resonant photoemission spectroscopy at the 2p-3d excitation threshold.
Abstract: The electronic structure of high-quality CrO2(100) films was investigated by means of x-ray absorption and resonant photoemission spectroscopy at the 2p-3d excitation threshold. The obtained binding energy of the occupied Cr 3d states is in agreement with the results predicted within the local spin-density approximation using the dynamical mean-field theory [L. Craco , Phys. Rev. Lett. 90, 237203 (2003)]. The reported data support a model of CrO2 as a half-metallic ferromagnet with strong electron-correlation effects.
TL;DR: Using a single silica precursor, Rhodamine 6G organic dye molecules have been entrapped in silica particles resulting into core-shell particles of ∼500-nm diameter.
TL;DR: A new class of colored heterometallic waveguides developed using a nanocolloidal sol-gel route combined with thermal treatment under ammonia atmosphere is reported in this article, which are photostable and degrade the cationic pigment methylene blue under visible light irradiation.
Abstract: A new class of colored heterometallic ZnxTiyOw–zNz waveguides developed using a nanocolloidal sol–gel route combined with thermal treatment under ammonia atmosphere is reported. The resulting mesoporous layers are photostable and degrade the cationic pigment methylene blue under visible light irradiation. Semiconductor photocatalysis is currently being explored in environmental sciences, solar cell technologies, and preparative “green” technology.
TL;DR: In this paper, the adsorption of a bi-functional organic molecule like phenol on Si(1.0.0)2.2.1 was investigated by synchrotron radiation-induced photoemission in the valence band.
TL;DR: In this paper, the structure of thin niobia films grown on a Cu 3 Au(1/0/0) substrate was studied using low energy electron diffraction, scanning tunneling microscopy, angular resolved photoelectron spectroscopy using synchrotron radiation and density functional theory calculations.
TL;DR: It was found that by adsorption of H(2) at 77 K or of n-butane at room temperature, the promoters neither enhance the Lewis nor the Brønsted acid strength nor facilitate alkane activation via oxidative dehydrogenation.
Abstract: Sulfated zirconia (SZ) and sulfated zirconia promoted with 2 wt % manganese (MnSZ) or iron (FeSZ), all active in n-butane isomerization, were investigated using diffuse reflectance Fourier transform IR spectroscopy (DRIFTS). By adsorption of H(2) at 77 K or of n-butane at room temperature, it was found that the promoters neither enhance the Lewis nor the Bronsted acid strength. SZ and promoted SZ do not exhibit higher acid strength than zeolites. In a batch experiment using 70 hPa of H(2), SZ did not react at 473 K. Reaction of H(2) with MnSZ produced water (band at 5242 cm(-1)) and a decrease in the sulfate groups (multiple bands). Heating of SZ in 10 hPa n-butane to 573 K caused total reduction of sulfate to H(2)S (2583, 2570 cm(-1)) and partial and total oxidation of butane to olefinic species (3062 cm(-1)), CO(2), and water. MnSZ and FeSZ reacted with n-butane already at 373 K; products of skeletal isomerization (methyne CH vibration at 2910 cm(-1)) were detected and sulfate groups were consumed. Rather than increasing the acidity, the promoters enhance the oxidation potential of sulfate and facilitate alkane activation via oxidative dehydrogenation.
TL;DR: The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument and the rate-limiting character of the different elementary steps of the reaction system is discussed.
Abstract: The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient (s(NO)) were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument. NO dissociation and N2 productions were observed in the transient state from 425 K and above on Pd(111) surfaces with selective nitrogen production. Maximum nitrogen production was observed between 475 and 500 K. It was found that, at low temperatures, between 300 and 350 K, molecular adsorption occurs with a constant initial s(NO) of 0.5 until the Pd(111) surface is covered to about 70-80% by NO. Then s(NO) rapidly decreases with further increasing NO coverage, indicating typical precursor kinetics. The dynamic adsorption - desorption equilibrium on Pd(111) was probed in modulated beam experiments below 500 K. CO titration experiments after NO dosing indicate the diffusion of oxygen into the subsurface regions and beginning surface oxidation at > or = 475 K. Finally, we discuss the results with respect to the rate-limiting character of the different elementary steps of the reaction system.
TL;DR: Taking advantage of the intrinsically higher epoxidation selectivity of Cu over Ag under vacuum conditions, three phenylpropene structural isomers are used to examine the correlation between adsorption geometry and oxidation chemistry and it is found that under comparable conditions alpha-methylstyrene, trans-methyl Styrene, and allylbenzene behave very differently on the oxygenated Cu(111) surface.
Abstract: It has long been conjectured that the difficulty of heterogeneously epoxidizing higher alkenes such as propene is due to the presence in the molecule of "allylic" H atoms that are readily stripped off by the oxygenated surface of the metal catalyst resulting in combustion. Here, taking advantage of the intrinsically higher epoxidation selectivity of Cu over Ag under vacuum conditions, we have used three phenylpropene structural isomers to examine the correlation between adsorption geometry and oxidation chemistry. It is found that under comparable conditions alpha-methylstyrene, trans-methylstyrene, and allylbenzene behave very differently on the oxygenated Cu(111) surface: the first undergoes extensive epoxidation accompanied by relatively little decomposition of the alkene; the second leads to some epoxide formation and extensive alkene decomposition; and the third is almost inert with respect to both reaction pathways. This reactive behavior is understandable in terms of the corresponding molecular conformations determined by near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations. The proximity to the surface of the C=C function and of the allylic H atoms is critically important in determining reaction selectivity. This demonstrates the importance of adsorption geometry and confirms that allylic H stripping is indeed a key process that limits epoxidation selectivity in such cases.