The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Francesc Illas - One of the best experts on this subject based on the ideXlab platform.
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Bonding Geometry and mechanism of no adsorbed on cu2o 111 no activation by cu cations
Journal of Chemical Physics, 1994Co-Authors: Marcos Fernandezgarcia, P. S. Bagus, José C. Conesa, Jaime Rubio, Francesc IllasAbstract:An ab initio molecular cluster model approach has been used to investigate the adsorption Geometry and the nature of the interaction of the NO molecule with the Cu2O(111) surface. The two possible NO orientations, N‐ and O‐down, have been studied for adsorption on both onefold and threefold surface positions. We show that, in all cases, the most important contribution to the Bonding is the electrostatic interaction, with negligible or small (depending on the orientation and position) contributions from chemical effects. In the monocoordinated position it is found that the two NO orientations exhibit opposite vibrational frequency shift with respect to the free NO molecule. For the three‐coordinated site the vibrational shift is found to be nearly zero. In the first case, when NO is bound through the O atom, its shift is negative, as observed experimentally on Cu2O surfaces. Therefore, the present model calculations suggest that the species which is experimentally observed is O‐down chemisorbed NO on a one...
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Bonding Geometry and mechanism of NO adsorbed on Cu2O(111): NO activation by Cu+ cations
The Journal of Chemical Physics, 1994Co-Authors: Marcos Fernández-garcía, José C. Conesa, P. S. Bagus, Jaime Rubio, Francesc IllasAbstract:An ab initio molecular cluster model approach has been used to investigate the adsorption Geometry and the nature of the interaction of the NO molecule with the Cu2O(111) surface. The two possible NO orientations, N‐ and O‐down, have been studied for adsorption on both onefold and threefold surface positions. We show that, in all cases, the most important contribution to the Bonding is the electrostatic interaction, with negligible or small (depending on the orientation and position) contributions from chemical effects. In the monocoordinated position it is found that the two NO orientations exhibit opposite vibrational frequency shift with respect to the free NO molecule. For the three‐coordinated site the vibrational shift is found to be nearly zero. In the first case, when NO is bound through the O atom, its shift is negative, as observed experimentally on Cu2O surfaces. Therefore, the present model calculations suggest that the species which is experimentally observed is O‐down chemisorbed NO on a one...
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Bonding Geometry and Bonding character of thiocyanate adsorbed on a Ag(100) surface
The Journal of Chemical Physics, 1991Co-Authors: Gianfranco Pacchioni, Francesc Illas, Michael R. Philpott, Paul S. BagusAbstract:The adsorption Geometry and the nature of the interaction of the SCN molecule at an on‐top site of the Ag(100) surface have been investigated using ab initio cluster model wave functions. The SCN anion, SCN−, is a bidentate ligand. If the SCN–Ag bond is ionic, we could expect, by analogy with thiocynate–metal complexes, that SCN could be bound to a metal surface through either the N end or the S end. We show that the chemisorption bond for SCN/Ag is very ionic and that the interaction between chemisorbed SCN and the Ag surface is largely electrostatic. The most important Bonding mechanism is the polarization of the Ag surface due to the presence of SCN−. However, we do find that there is a small, but non‐negligible, covalent interaction. There is a very small energetic cost to change the angle of SCN with respect to the surface between a perpendicular and a parallel orientation. We contrast this with the case of a covalently bonded adsorbate, CO/Ag(100), where the π bond strongly favors orientations near ...
Peter G Jones - One of the best experts on this subject based on the ideXlab platform.
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packing principles for urea and thiourea solvates structures of urea morpholine 1 1 urea 1 4 dioxane 1 1 thiourea morpholine 4 3 and thiourea 1 4 dioxane 4 1
CrystEngComm, 2013Co-Authors: Christina Taouss, Lena Thomas, Peter G JonesAbstract:The solvents 1,4-dioxane and morpholine have been employed to synthesize solvates of urea and thiourea. The structures confirm the tendency of urea to form more rigid systems of hydrogen bonds in the plane of the N2CO moiety, thus forming layer structures with close complementarity of the donors and acceptors, whereas the more flexible sulfur acceptor of thiourea can also accept hydrogen bonds from donors that lie far from the N2CS plane, forming three-dimensional packing patterns with much more variable parameters. A database investigation confirms these tendencies. The solvate urea:morpholine (1:1) crystallizes in Pbcm with Z = 4. The complete urea molecule lies in the mirror plane, as do the heteroatoms of the morpholine molecule. The molecular packing is a layer structure. The solvate urea:1,4-dioxane (1:1) crystallizes in P2/c with Z = 2. The CO bond of the urea molecule lies along a twofold axis, whereas the dioxane molecule lies across an inversion centre. The molecules form a layer structure analogous to that of the morpholine solvate. The thiourea solvates are more complex, and both involve a more irregular hydrogen Bonding Geometry at sulfur. The solvate thiourea:morpholine (4:3) crystallizes in P21/c with Z = 2. The asymmetric unit contains two independent molecules of thiourea, one morpholine on a general position, and one morpholine disordered over an inversion centre. The thiourea molecules combine to form an open framework with a series of channels, in which the morpholine molecules are attached. The solvate thiourea:1,4-dioxane (4:1) crystallizes in P21/n with Z = 2. The asymmetric unit contains two independent molecules of thiourea and one molecule of dioxane across an inversion centre. One thiourea molecule and the dioxane combine to form a layer structure. The second thiourea molecule links these layers in the third dimension.
Yves J. Chabal - One of the best experts on this subject based on the ideXlab platform.
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Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.
Journal of the American Chemical Society, 2012Co-Authors: Peter Thissen, Tatiana Peixoto, Roberto C. Longo, Weina Peng, Wolf Gero Schmidt, Kyeongjae Cho, Yves J. ChabalAbstract:Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the Bonding Geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity an...
I. David Brown - One of the best experts on this subject based on the ideXlab platform.
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A step closer to predicting the Bonding Geometry of crystals
American Mineralogist, 2013Co-Authors: I. David BrownAbstract:A recent paper in this journal by Bickmore et al. (2013, Amer. Min., 98, 340–349) describes an important extension of the bond-valence model that allows us to predict the size of the distortions found in the environment of atoms with electronic anisotropies, paving the way for a quantitative prediction of bond lengths and angles around these atoms.
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Bond Valences in Education
Structure and Bonding, 2013Co-Authors: I. David BrownAbstract:The bond valence theory’s simplicity, its rigor, and predictive power make it ideally suited for introducing the concept of the chemical bond in introductory courses. This chapter suggests how the theory might be presented in a classroom. It starts with a critique of the bond models currently used and then shows how different aspects of the theory might be introduced, beginning with the simple exercise of plotting the lines of field for an array of charges like those found in the ionic model. The generation of stable bonds using the valence matching rule leads naturally to a discussion of Bonding Geometry and the influence of lone pairs. Deriving the classic ball-and-stick model from bond valence theory gives an opportunity to discuss how one might define a covalent bond if this were thought to be useful. The chapter ends showing how the theory can be applied to aspects of chemical reactivity.
Petra Reinke - One of the best experts on this subject based on the ideXlab platform.
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Bonding Geometry of mn wires on the si 100 2 1 surface
Surface Science, 2011Co-Authors: C.a. Nolph, Hui Liu, Petra ReinkeAbstract:Abstract The Bonding Geometry of monoatomic Mn-wires, which form on the reconstructed Si(100)(2 × 1) surface at room temperature, was investigated with scanning tunneling microscopy (STM). The Mn-wire structures are always perpendicular to the Si-dimer rows and the images exhibit a strong modulation of their apparent height as a function of bias voltage. The Mn-wire structures appear as depressions in the empty state images for bias voltages around 0.7 V, and as protrusions for all other bias voltages. It is suggested that the wire-images are defined by mixed Mn-Si states, either through a hybridization between the Mn d-states and the Si-p states, or backBonding from Mn-d electrons into the broken Si-dimer bond. The dominant Bonding Geometry shows that the Mn-wire maxima are positioned in between the Si-dimer rows, and a small percentage of about 20% is in registry with the Si-dimer rows, and might be described as defective wires. The experimental STM images cannot currently be described in a satisfactory manner with theoretical Bonding models from the literature.
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Bonding Geometry of Mn-wires on the Si(100)(2 × 1) surface
Surface Science, 2011Co-Authors: C.a. Nolph, Hui Liu, Petra ReinkeAbstract:Abstract The Bonding Geometry of monoatomic Mn-wires, which form on the reconstructed Si(100)(2 × 1) surface at room temperature, was investigated with scanning tunneling microscopy (STM). The Mn-wire structures are always perpendicular to the Si-dimer rows and the images exhibit a strong modulation of their apparent height as a function of bias voltage. The Mn-wire structures appear as depressions in the empty state images for bias voltages around 0.7 V, and as protrusions for all other bias voltages. It is suggested that the wire-images are defined by mixed Mn-Si states, either through a hybridization between the Mn d-states and the Si-p states, or backBonding from Mn-d electrons into the broken Si-dimer bond. The dominant Bonding Geometry shows that the Mn-wire maxima are positioned in between the Si-dimer rows, and a small percentage of about 20% is in registry with the Si-dimer rows, and might be described as defective wires. The experimental STM images cannot currently be described in a satisfactory manner with theoretical Bonding models from the literature.