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A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations.

Many DFT studies of O(2) dissociation on Pt(111) give conflicting information on preferred paths and final states. Here we report large p(4 × 4) unit cell minimum energy pathway evaluations and compare O(2) adsorption and dissociated states on Pt(111). Calculations reveal how the pathways for O(2) dissociation starting from top-fcc-bridge, top-hcp-bridge, and top-bridge-top sites are interconnected. They also provide a direct reaction pathway for the dissociation of an O(2) molecule from a top-fcc-bridge into an hcp and an fcc site, which is consistent with low temperature scanning tunneling microscope experiments. Such a pathway is shown to be considerably perturbed by the presence of co-adsorbed oxygen atoms. We quantify the coverage dependence through the construction of a Brønsted-Evans-Polanyi relationship relating the O(2) dissociation activation energies to the binding energies of the dissociated O atoms. We also show that all pathways starting from a top-fcc-bridge site give the smallest barriers for O(2) dissociation.

We report a first-principles, periodic supercell analysis of oxygen adsorption, diffusion, and dissociation at the kinked Pt(321) surface. Binding energies and binding site preferences of isolated oxygen atoms and molecules have been determined, and we show that both atomic and molecular oxygen prefer binding in bridge sites involving coordinatively unsaturated kink Pt atoms. Binding energies of atomic and molecular oxygen in different sites correlate well with the average metallic Pt coordination number of Pt atoms forming each site, although differences exist between adsorbates in symmetrically similar sites due to the inherent chirality of the surface. Atomic O in the strongest binding bridge sites experiences relatively small energy barriers for diffusion to neighboring sites compared to O on Pt(111). However, due to the structure of the surface, O diffusion is only rapid between different sites around the kink Pt atom, whereas the effective long-range tracer diffusion, as determined from a simple course-grain model, is shown to be anisotropic and slower than on the Pt(111) surface. Four dissociation pathways for O(2) at low coverage are also reported and found to be in agreement with experimental observations of facile dissociation, even at low temperature., (© 2011 American Chemical Society)

Bulk rutile TiO2 and its (110) surface have been investigated with a computationally efficient semiempirical tight binding method: self-consistent-charge density functional tight binding (SCC-DFTB). Comparisons of energetic, mechanical, and electronic properties are made to density functional theory (DFT) and to experiment to characterize the accuracy of SCC-DFTB for bulk rutile TiO2 and TiO2(110). Despite the fact that the SCC-DFTB parameters for Ti, Ti-Ti, and Ti-O were developed in the context of small biologically relevant Ti containing compounds, SCC-DFTB predicts many properties of bulk TiO2 and the TiO2(110) surface with accuracy similar to local and gradient-corrected DFT. In particular, SCC-DFTB predicts a direct band gap of TiO2 of 2.46 eV, which is in better agreement with experiment, 3.06 eV, than DFT utilizing the local density approximation (LDA), 2.0 eV. SCC-DFTB also performs similar in terms of accuracy as LDA-DFT for the phonon frequencies of the bulk lattice and for the relaxed geometry of the TiO2(110) surface. SCC-DFTB does, however, overestimate the surface energy of TiO2(110) compared to LDA-DFT. Nevertheless, the overall accuracy of SCC-DFTB, which is substantially more computationally efficient than DFT, is encouraging for bulk rutile TiO2 and TiO2(110).

Periodic DFT calculations are used to study the effect of a homogeneous electric field applied perpendicular to a Pt(111) surface on the bond distances, binding energies, and vibrational frequencies of atop- and fcc-adsorbed CO at various coverages. The observed structural and energetic modifications can be understood in terms of modest field-induced charge transfer between charged metal surface and adsorbate and are well-described by classical first and second-order Stark models. Electronic differences between atop and fcc adsorption cause CO in these sites to respond differently to applied fields. After correcting for the GGA site preference error, CO adsorption is predicted to shift from atop to fcc at potentials <-0.19 V A(-1). The results are in qualitative agreement with previously reported cluster-based DFT models but differ quantitatively due to difference in modeled coverage, surface relaxation, and finite size effects. The calculated 44.4 cm(-1) V(-1) A shift in C-O stretch frequency with electric field (Stark tuning rate) compares favorably with UHV experiments but is significantly lower than the value obtained in electrochemical measurements, highlighting the importance of adsorbate environment on the magnitude of the tuning rate. The calculated coverage dependence of the tuning rate is in good agreement with previous UHV experiments.

The effects of adsorbate coverage on catalytic surface reactions are not well understood. Here, we contrast the rates of O2 and NO2 dissociations, two competing reactions in NO oxidation catalysis, versus oxygen coverage at a Pt(111) surface. In situ x-ray photoelectron spectroscopy experiments show that the NO2 dissociation rate is less sensitive to O coverage than is O2. Density-functional theory simulations reveal an NO2 reaction pathway that is more adaptable to an increasingly crowded surface than is O2 dissociation. While the rates are comparable at low coverage, NO2 dissociation is many orders of magnitude faster at O coverages typical of NO oxidation catalysis.

FTIR smog chamber techniques were used to measure k(Cl+n-C3H7OH) = (1.74 +/- 0.15) x 10-10 and k(Cl+CH2ClCH2CH2OH) = (7.54 +/- 0.73) x 10-11 cm3 molecule-1 s-1 in 700 Torr of N2 at 296 K. The reaction of Cl with n-C3H7OH gives CH3CH2CHOH, CH3CHCH2OH, and CH2CH2CH2OH radicals in yields of 60 +/- 5, 25 +/- 8, and 15 +/- 3%, respectively. Neither CH3CH2CHClOH nor CH3CHClCH2OH is available commercially, and infrared spectra for the three chlorides CH3CH2CHClOH, CH3CHClCH2OH, and CH2ClCH2CH2OH were calibrated experimentally. MP2/6-31G(d,p) calculations were used to corroborate the experimental vibrational assignments. Analysis reveals that each geometric isomer possesses several structurally and spectroscopically distinct conformers arising from intramolecular hydrogen bonding and, in the case of CH3CH2CHClOH, negative hyperconjugation. These conformers interchange slowly enough to be distinguished within the room-temperature vibrational spectrum. The experimentally observed vibrational spectra are well described by a Boltzmann-weighted superposition of the conformer spectra. As is typical of alpha-halogenated alcohols, CH3CH2CHClOH readily decomposes heterogeneously to propanal and HCl.

FTIR smog chamber techniques and ab initio calculations have been used to investigate the kinetics and mechanism of the reaction of Cl atoms with i-propanol in 700 Torr of N(2) at 296 K. The reaction is observed to proceed with a rate constant of k(1) = (8.28 +/- 0.97) x 10(-11) cm(3) molecule(-1) s(-1) and gives CH(3)C(OH)CH(3) and CH(3)CH(OH)CH(2) radicals in yields of 85 +/- 7 and 15 +/- 7%, respectively. Calculations indicate that abstraction of the secondary H can proceed through a lower energy pathway than the primary. Rapid decomposition of the chlorination product CH(3)CCl(OH)CH(3) complicates its direct detection, likely due to heterogeneous chemistry. IR spectra for the chlorides CH(3)CCl(OH)CH(3) and CH(3)CH(OH)CH(2)Cl were inferred experimentally and assignments confirmed via comparison with ab initio computed spectra.

From data of 400 five-generation families living in the Federal Republic of Germany different ways of interaction between the generations are demonstrated. These data point to a very high degree of integration of the very old family members and reject the thesis of a more or less disintegration of the older generation. The interaction between the 5th and the 4th generation is closer than these between the 5th and and the 3rd or 2nd generation; this may be explained by the different cohort-specific socialization-histories and by factors of secular change.