Charge transfer in the presence of dopants is relevant for the adorption and activation of small molecules, such as O2 . Scanning tunneling microscopy and DFT calculations provide evidence for the formation of strongly bound superoxo species on chemically inert, Mo-doped CaO films. This oxygen surface species shows a high propensity to dissociate. Dopants could also be important for the activation of hydrocarbons on inert oxides.
Ladungstransfer in Gegenwart von Dotanden ist auch für die Adsorption und Aktivierung kleiner Moleküle wie O2 relevant. Rastertunnelmikrokopie und DFT‐Rechnungen lieferten Hinweise auf die Bildung stark gebundener Superoxo‐Spezies auf chemisch inerten, Mo‐dotierten CaO‐Filmen (siehe Bild). Diese Sauerstoff‐Oberflächenspezies zeigt eine hohe Tendenz zur Dissoziation. Dotanden könnten auch bei der Aktivierung von Kohlenwasserstoffen an inerten Oxiden wichtig sein.
Quantum-chemical calculations employing different theoretical methods and basis sets have been performed on borabenzene (C 5 H 5 B) as well as on its adducts to dinitrogen (N 2 ) and the rare gases Ne, Ar, and Kr. In agreement with previous calculations, the ground state of borabenzene was found to be a planar singlet with six electrons in molecular orbitals of p symmetry and a wide C-B-C bond angle (142.28). Depending on the method (PUMP2, SAC-CI, CASPT2 (8,8)), the lowest triplet state was found to be 28 to 46 kcal mol À1 (1 kcal mol À1 ¼ 4.186 kJ mol À1 ) higher in energy. The energies associated with the formation of the adducts with N 2 , Ne, Ar, and Kr were calculated as À14.9, À0.5, À1.4, and À3.5 kcal mol À1 respectively. Our calculated spectrum of the normal modes as well as the electronic excitation spectrum of the N 2 adduct reproduce qualitatively the characteristic features of the IR and the UV-vis spectra described by experimentalists. The corresponding calculated spectra (normal modes, UV-vis) of the rare gas adducts were found to be very similar to those of free borabenzene. A Enthalpy change associated with the reaction 3 Á ethylene þ borabenzene-2 Á trans-butadiene þ 2-borabutadiene.
1 Quantum-chemical ab initio and time-dependent density functional theory (TD-DFT) calculations employing various basis sets were used to elucidate the spatial as well as the electronic structure of C5H5Al () and C5H5Ga (2) (ala- and galabenzene). The lowest closed shell singlet states of both compounds were found to have a non-planar structure of CS symmetry with C-X-C bond angles of about 116° (MP2/6-311++G**) and 125° (CCSD/aug-cc-pVDZ). At approximately 103°, the corresponding angles of the lowest triplets are significantly smaller. The lowest triplet state of alabenzene is also non-planar (CS) at the MP2 level while optimization with the CCSD and the CASPT2 method resulted in planar structures with C2v symmetry. The corresponding state of galabenzene has C2v symmetry at all levels of optimization. The relative stability of the lowest closed shell singlet and the lowest triplet (ΔE(T1-S0)) state is small and its sign even strongly method-dependent. However, according to the highest levels of theory applied in this study the singlet states of both molecules are slightly lower in energy than the corresponding triplets with singlet/triplet gaps between about 0.5 and 5.8 kcal/mol in favour of the singlet states. Most of the applied methods give a slightly smaller splitting for ala- than for galabenzene. Independent of the applied method (TD-DFT/CAM-B3LYP/6-311++G(3df,3pd)//MP2/6- 311++G** or SAC-CI/6-31++G(3df,3pd)//MP2/6-311++G**), the general shape of the calculated UV/VIS spectral curves are quite similar for the lowest singlet states of ala- and galabenzene, and the same applies to the spectra of the normal modes. The calculated UV/VIS spectra of C5H5Al and C5H5Ga are featured by long wavelength bands of moderate intensity around 900 nm at the TD-DFT and between 1300 and 1500 nm at the SAC-CI level. According to both methods these bands are predominantly due to HOMO(π)→LUMO(σ*) transitions. The results of isodesmic bond separation reactions for the singlet states indicate some degree of stabilization due to delocalization in both of the title compounds. With our best values between 29 and 32 kcal/mol this stabilization appears to be only slightly less than the previously reported value for borabenzene (∼38 kcal/mol).
Density functional theory (DFT) is used to systematically investigate the electronic structure of platinum clusters grown on different graphene substrates. Platinum clusters with 1 to 10 atoms and graphene vacancy defect supports with 0 to 5 missing C atoms are investigated. Calculations show that Pt clusters bind more strongly as the vacancy size increases. For a given defect size, increasing the cluster size leads to more endothermic energy of formation, suggesting a templating effect that limits cluster growth. The opposite trend is observed for defect‐free graphene where the formation energy becomes more exothermic with increasing cluster size. Calculations show that oxidation of the defect weakens binding of the Pt cluster, hence it is suggested that oxygen‐free graphene supports are critical for successful attachment of Pt to carbon‐based substrates. However, once the combined material is formed, oxygen adsorption is more favorable on the cluster than on the support, indicating resistance to oxidative support degradation. Finally, while highly‐symmetric defects are found to encourage formation of symmetric Pt clusters, calculations also reveal that cluster stability in this size range mostly depends on the number of and ratio between PtC, PtPt, and PtO bonds; the actual cluster geometry seems secondary.
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