Recognizing chemical bonds

Three representations of the structure of the [Li4(CH3)4] tetramers within the crystals of methyllithium. 

A further example of an alkyl lithium compound with ambiguous chemical bonding that was resolved on the basis of electron density topology considerations is given in the on-line supplementary section for chapter 10. 

There are many other types of contact or weak bonding interaction that cannot always be decided on by distance criteria alone. These include weak dispersive interactions between heavy elements, interactions between aromatic units and hydrogen bonding. Other sources of information from spectroscopy, electron density topology or quantum mechanical calculations have to be used jointly to give a satisfactory chemical explanation. This should always be remembered when you see a crystal structure with a simple line drawn between two atoms. 

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One potential approach extends the idea of chemical amplification introduced in our preceding description of dry-film resists. In 1982, Ito and co-workers (37,38) recognized that if a photosensitizer producing an acidic product is photolyzed in a polymer matrix containing acid-labile groups, the acid will serve as a spatially localized catalyst for the formation or cleavage of chemical bonds. 

Historically the phenazine dyes have played an important part in the dyestuffs industry, although their use has largely been superseded by the more modern, color-fast dyes, in particular those dyes which become chemically bonded to the fibers of the materials being dyed. Amongst the earliest examples of phenazine dyes are those compounds known as the safranines. The discovery of the safranines has been attributed to Greville Williams in 1859 and they were apparently in commercial use shortly after that date, but it was not until 1886 that it was recognized that phenosafranine (138) was indeed a phenazine containing system. 

Enzymes catalyzing the hydrolysis of esters are termed esterases. They belong to a larger group of enzymes termed hydrolases, which can cleave a variety of chemical bonds by hydrolytic attack. In the classification of hydrolases of the International Union of Biochemistry (lUB), the following categories are recognized ... 

The validity (or lack thereof) of the classical Zind formahsm as applied to less polar intermetallics, involving metals along the Zind border, is nicely probed by electron-poof trelides. Seminal work by Corbett [44] and Belin [48] recognized the proclivity of trelides (Ga, In, H) to form cluster-based anion structures. The apparent electron deficiency in the chemical bonding of these cluster com-... 

In recentyears, metal NPs synthesized in ILs were recognized as suitable materials to promote the formation of chemical bonds in reactions other than palladium-catalyzed carbon-carbon cross-coupling reactions. For example, aldehydes and esters... 

A simpler model for ethane recognizes what we already know for methane that each carbon atom is bonded to four other atoms. Given that knowledge, we can now write simply, CH3—CH3, showing only the carbon-carbon bond. Since each carbon atom forms four bonds and since only one is shown (the carbon-carbon bond), it follows that each carbon atom must make three bonds to hydrogen atoms. Even simpler is the model CH3CH3, in which none of the chemical bonds is shown directly. Once we have gained more experience, it will be clear that this simple representation contains all the information that the more detailed one does. Here are two other models for ethane ... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

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