Bonding adsorption

A common theme is the existence of modified (enantioselective) sites and unmodified (racemic) sites. For the case of the tartaric acid modified Ni, it is postulated that the tartaric acid is adsorbed on the surface and stereodirects (through hydrogen bonding) adsorption of the incoming 3-ketoesters.18 19 Support for this comes from an isotope effect from deuterium labeling.23 Increased enantioselectivities resulting from co-modification with NaBr is believed to result from poisoning the racemic sites.24 A similar technique in... 

Thermal analysis techniques reveal that water is bound in opal in more than one manner. Most of the water is physically held in inclusions or microscopic pores within the opal, that is, in spaces between the microspheres. Water held in this manner can escape through complex systems of microscopic fissures or cracks, induced by temperatures even below 100°C. Some water is held within the opal via chemical bonding ( adsorption ) to the surfaces of the silica microspheres and is retained to temperatures approaching 1000°CJ7J Furthermore, since the microspheres themselves are composed of much smaller silica particles, water is additionally coated on the surfaces of these minute particles. The porous nature of opal and its thermal sensitivity require special care, for dehydration may result in cracking that greatly diminishes the value of this gemstone. 

In principle, electron-withdrawing isocyanides on metal surfaces are more likely to favor the bridge-bonding mode. Since the migration of a terminal isocyanide to a bridging position formally results in the oxidation of the metal (M) atoms, easily-oxidized metals should favor the bridge-bonding adsorption mode. 

Adsorption of Disulfides and Related Compounds Adsorption and further electrode reactions of different disulfides at gold electrodes depend on chemical properties of the adsorbates. It has been evidenced in some papers that at least for some disulfides, adsorption process involves the cleavage of the S—S bond. Adsorption of diphenyl disulfide on a polycrystalline gold electrode has been studied in detail by Borsari etal. [166]. They have proposed the mechanism of adsorption shown in Scheme 1 ... 

Abstract—A discussion is given of the conditions for operation of hydrogen bond adsorption in a variety of processes where vapourized or dissolved organic substances are adsorbed, by inorganic or organic solids. 

From non-aqueous solvents hydrogen bond adsorption of most proton donors occurs on inorganic oxides but on nylon and wool fibres only hydroxylic compounds are strongly adsorbed, either from non-aqueous solvents or in the vapouy phase. 

The role of hydrogen bonds in liquid or vapour-phase adsorption processes has not hitherto been very clearly defined. The present paper is a brief review of some recent investigations in this laboratory, many of them unpublished, which it is believed may help to define the conditions favouring hydrogen bond adsorption especially from dilute solutions, by inorganic and organic substrates. 

Methods used here to identify hydrogen bond adsorption from solution include ... 

Data such as those in Table 1 are a useful gqide in studying the possibility of hydrogen bond adsorption in different systems, but they must be used with eaution. Even if two groups are shown to interact when present in substances in solution it does not always follow that they will do so under adsorption conditions. This will be discussed below. ... 

Hydrogen Bond Adsorption- of Mono-functional, Solutes... 

Fig. 1. H-bonding adsorption of simple memo- and bifunetional solutes by inorganic or organic polar surfaces...

Little information is available upon hydrogen bond reactions in vapour phase adsorption, except in the important reactions of proteins and textile fibres with moisture, which have received widespread study. In these reactions substrate-water hydrogen bonds are undoubtedly responsible for the initial monolayer adsorption of water with increase in vapour pressure the adsorbed water condenses into multilayers by formation of water-water hydrogen bonds. Adsorption of organiq molecules of low molecular weight, in the vapour phase, on nylon or wool is considerably greater when they contain a hydroxylic group than when they do not [8] (Table 5), so that the hydroxy-compounds... 

Electrosorption is a replacement reaction. We have already discussed the role of the solvent in the interphase, in the context of its effect on the double-layer capacitance. It is most important for our present discussion to know that the electrode is always solvated and that the solvent molecules are held to the surface both by electrostatic and by chemical bonds. Adsorption of a molecule on such a surface requires the removal of the appropriate number of solvent molecules, to make place for the new occupant, so to speak. This is electrosorption. In this chapter we shall restrict our discussion to the electrosorption of neutral organic molecules from aqueous solutions, without charge transfer. Using the notation RH for an unspecified organic molecule, we can then represent electrosorption in general by the reaction... 

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

Except for the competitive Langmuir isotherm (Table 10.7), the simple isotherm models cannot consider competition between multiple sorbates for a single sorption site. They are most applicable to physical adsorption of molecular species where the sorbate is held by much weaker hydrogen bonds or residual bonds. Adsorption of ions and complexed species is generally best studied with models that can consider more independent variables. To accurately predict adsorption of ions that compete for adsorption sites and also form important complexes, may require a model that considers as many as a dozen or more variables. 

Even in molecules with no functional groups which are expected to interact with silanol sites on silica gel through hydrogen bonding, adsorption on solid surfaces modifies their electronic states and their photophysical processes but the mechanism of adsorption interaction is not elucidated in detail. 

Support for a neutral hydrogen-bonded adsorption complex comes also from a measurement of the distance between the methanol and the zeolite protons in H-MOR by wide line NMR at 4 K [28],... 

Consequently, the problem arises as to whether the low-frequency absorption observed in such studies as those of Eischens et al. 95, 96), can be interpreted as being due to a bridge bonded adsorption complex. Blyholder 98, 99) prefers to assign these low-frequency bands to linear CO adsorption on edge or corner sites on the metal crystallites of the sample, since he argues that these sites allow a greater back bonding... 

Si(100)/H-(2 X 1) STM (47) ond-layer molecule pulls nearest metal neighbors outward radial expansion parallel to surface of 3-fold site in top metal layer Dangling bond adsorption... 

Most of the chapters discuss local properties of surface atoms and molecules atomic structure, chemical bonding, adsorption, catalysis, and mechanical properties. Transport properties, electron transport, surface magnetism, and optical properties are important subjects of surface science but are not treated here. 

In more complicated cases, in particular if covalent chemical bonding, adsorption induced relaxations of the surface or partially occupied electronic surface bands play a role for the adsorption, an accurate description of weak molecule/surface interactions is still a challenge for the current theoretical methods. The development of DFT functionals capable to account for the Van der Waals interaction as well as of fast wave function based correlation methods (e.g., local correlation approaches) is therefore highly welcome. 

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