High resolution, vibrationally resolved, near-edge x-ray absorption fine structure ͑NEXAFS͒ spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d 5-pyridine in the gas phase have been recorded. The high resolution of 65 meV ͑150 meV͒ at the C s ͑N 1s͒ ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory ͑DFT͒ calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ϳ2:3 intensity ratio between the two * components in the C 1s spectrum rather than the ϳ3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both theoretical methods yield almost perfect agreement with experiment.

No abstract

We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C6-ring-containing molecules benzene (C6H6), 1,3- and 1,4-cyclohexadiene (C6H8), cyclohexene (C6H10), cyclohexane (C6H12), styrene (C8H8), and ethylbenzene (C8H10) which allow us to examine the gradual development of delocalization of the corresponding pi electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to pi*- and sigma*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s-->1pi* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C6H8) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.

Vanadium oxide contains differently coordinated oxygen sites which can participate in catalytic oxidation reactions of industrial relevance. For a full understanding of corresponding reaction steps an estimate of the relative importance of these oxygen sites for a given reaction at the surface is required. This may be possible using electron spectroscopy together with theoretical methods. We have performed density functional theory cluster calculations to describe the electronic structure at the V2O5(010) surface which possesses singly, doubly, and triply coordinated oxygen sites, O(1–3). For these sites we have evaluated O 1s core level excitation spectra using the transition state method. The analysis of the spectra based on atom projected densities-of-states of the unoccupied orbitals shows that all spectral features are determined by local V–O bonding. The spectra for different oxygen coordination differ enough to allow discrimination between the oxygen sites. In contrast, differences in the calculated O 1s core ionization potentials depend much less on coordination. A comparison of the theoretical O 1s core level excitation spectra with experimental NEXAFS and ELNES spectra yields rather good agreement and assists the interpretation of the latter. Further, the theoretical IP data are consistent with experimental XPS spectra.

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.