Formation of specific chemical bond

For the mechanism of asymmetric catalysis on metal-quartz catalysts there is no unifying point of view. There are the concepts of "contact activation", "specific adsorption", and "formation of specific chemical bonds" It has been proposed that on the surface of the catalyst intermediate surface diastereomers are formed which are decomposed at different rates. The data on the action of quartz-catalysts have been rather thoroughly discussed in addition positive estimations of data were given by Bonner and Harada... 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Chemical adsorption (known as chemisorptioti) often, but not invariably, involves the formation of a chemical bond (i.e., the transfer of electrons) between the gas and the solid. In other words, a specific chemical compound one layer thick... 

Chemical covalent bonding. The formation of covalent chemical bonds between elements at an interface may be an important factor. Such direct chemical bonding would greatly enhance interfacial adhesion, but specific chemical functional groups are required for the reactions to occur. 

Although the occupied orbitals are of main importance, since they are directly involved in the formation of the chemical bond, the unoccupied states also provide complementary information. In X-ray absorption spectroscopy (XAS), often denoted Near Edge X-ray Absorption Fine Structure (NEXAFS), we excite a core electron to the empty states above the Fermi level [3,4,13]. There is a close connection between XES and XAS where the former gives information on the occupied orbitals while the latter relates to the character and symmetry of the unoccupied levels. Both are governed by the dipole selection rule and the localized character of the core orbitals allows a simple atom-specific projection of the electronic structure the major difference is in the final states. In XAS the empty electronic states are probed... 

Adsorption is conveniently considered as either physical adsorption or chemisorption, depending on the nature and strength of the surface forces. Chemisorption can be considered as the formation of a chemical bond between the sorbate and the solid surface. Such interactions are strong, highly specific, and often not easily reversible. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the formation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often useful for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intermolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

In many cases, adsorptive accumulation of organic compounds at the electrode surface proved useful to enhance the efficiency of analysis. Accumulation is based on specific adsorption much more than on physical adsorption. The formation of partial chemical bonds is the reason that heating can exhibit a positive effect. With physisorption alone, a negative effect would be suspected since surface coverage tends to decrease with increased temperature. 

D. N. Shigorin (Moscow) 1. The nature of the hydrogen bond has its own specific. Nevertheless, the nature of the forces which condition the formation of the hydrogen bond does not basically differ from the nature of the forces which constitute ordinary chemical bonds. 

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