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(1)H, (13)C and (15)N NMR chemical shifts for a variety of novel quinoxalines were determined by different 2D methods and were calculated using the GIAO DFT approach. Comparison with experimental data shows good correlations in the case of (1)H, (13)C and (15)N chemical shifts. Different combinations of basis sets were tested. In non-polar solvents quinoxalines exist as dimers owing to strong hydrogen bonding. Calculations for dimers improve the correlation between experiment and theory. Additive empirical methods for estimating chemical shifts have drawbacks and have to be used with a great care for this type of compound.  相似文献   
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The electrical and magnetic properties of the nanodiamond composites comprising nanodiamond, pyrolytic carbon, and nanosized pores were studied. The composites are p-type semiconductors and their resistance decreases by 12 orders of magnitude as the pyrocarbon-to-diamond ratio γ increases from 0 to 80 wt %. Evidence for paramagnetic properties of the nanodiamond composites was obtained. The observed properties are explained by increased concentration of surface Tamm states. The paramagnetic properties are explained in terms of the electron spins localized on the nanodiamond surface in the composite.  相似文献   
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The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO2) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO2(111) thin films and CeO2 powders, and theoretical calculations of CeO2(111) surfaces with oxygen vacancies (Ov) at the surface and in the bulk. We show that, on a stoichiometric CeO2(111) surface, H2 dissociates and forms surface hydroxyls (OH). On the pre-reduced CeO2−x samples, both films and powders, hydroxyls and hydrides (Ce−H) are formed on the surface as well as in the bulk, accompanied by the Ce3+ ↔ Ce4+ redox reaction. As the Ov concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H2/CeO2 interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria-based catalysts in a hydrogen-rich atmosphere.  相似文献   
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The application of a variety of “surface‐science” techniques to elucidate surface structures and mechanisms of chemical reactions at zeolite surfaces has long been considered as almost impossible because of the poor electrical and thermal conductivity of those materials. Here, we show that the growth of a thin aluminosilicate film on a metal single crystal under controlled conditions results in adequate and well‐defined model systems for zeolite surfaces. In principle, silicate films that contain metals other than Al (e.g. Ti, Fe, etc) may be prepared in a similar way. We believe that this approach opens up a new playground for experimental and theoretical modeling of zeolites, aimed at a fundamental understanding of structure–reactivity relationships in such materials.  相似文献   
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Palladium was vapor deposited on a thin FeO(111) film grown on a Pt(111) substrate. Scanning tunneling microscopy study has revealed that Pd wets the FeO substrate and at elevated temperatures forms extended Pd(111) monolayer islands in contrast to other oxide supports previously studied. For the first time, we have imaged the metal-oxide interface structure with atomic resolution and explained the results on the basis of ab initio calculations.  相似文献   
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