Substitutional atom

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

Homogeneous alloys of metals with atoms of similar radius are substitutional alloys. For example, in brass, zinc atoms readily replace copper atoms in the crystalline lattice, because they are nearly the same size (Fig. 16.41). However, the presence of the substituted atoms changes the lattice parameters and distorts the local electronic structure. This distortion lowers the electrical and thermal conductivity of the host metal, but it also increases hardness and strength. Coinage alloys are usually substitutional alloys. They are selected for durability—a coin must last for at least 3 years—and electrical resistance so that genuine coins can be identified by vending machines. 

The Baeyer-Villiger oxidation (p T 226) of (12) will occur with migration of the more substituted atom. The structure of (14) must therefore be ... 

In cyclopropane, the C,C bond between the atom bearing the nitro group and the most substituted atom of the ring is cleaved. 

As a further step currently under investigation, the relationship between local polarizability and local softness is studied with the aim to substitute atom-in-molecule polarizabilities by atom-condensed softness values. In this way, conceptual DFT could be exploited in a computational strategy, an ansatz rarely used until now, the best known example being the electronegativity equalization method [101]. 

Note in comparing reactions 10.1 and 10.3 the isotope effect should also be called primary, since the bond to the isotopically substituted atom is being broken, just as it was in Equation 10.2. However in Equation 10.3 the magnitude of the isotope effect... 

In reaction (10.11) the deuterium isotope effect is a secondary isotope effect, that is one in which the bonding to the isotopically substituted atom is not broken or formed during the course of the reaction. Secondary deuterium isotope effects are generally much smaller than primary ones. 

Secondary hydrogen kinetic isotope effects are further classified as alpha, beta, etc. depending on the distance of the isotopically substituted atom from the bond(s) that is (are) being made or broken (a = 1 bond, 3 = 2 bonds, etc.). Consider the simple Sn2 reaction between hypochlorite anion and ethyl chloride ... 

Halogenated extinguishing agents are hydrocarbons where one or more hydrogen atom is replaced by fluorine, chlorine, bromine, or iodine atoms. The substituted atom is not only rendered nonflammable, but it acts as a very efficient... 

Such halogenated compounds are called organic halides or alkyl halides. The substituted atoms may be fluorine, chlorine, bromine, iodine, or any combination of these elements. 

For the case of para-equilibrium, where M is a substitutional atom, x- is calculated from the equation... 

This is presumably due to the apical position of the substituted atom in QP, which causes a higher perturbation on the dz2(ai) orbital in Csy symmetry 63 b). 

Since vibrational frequencies depend on the masses of moving atoms, the substitution of an atom in a molecule by an isotope of different mass will alter the frequencies of those modes in which the substituted atom moves significantly. This technique may be useful in distinguishing between two possibilities for the assignment of a vibrational band. For example, the vibrational... 

Despite a possible wide significance of such a topic, there is only one reported study of adatom-substitutional impurity atom interaction, where the interaction of a W adatom with substitutional Re atoms in a W lattice is studied by using a W-3% Re alloy as the substrate.182 The planes used in FIM studies of adatom behavior are usually quite small containing only a few hundred atoms. Thus a plane of a W-3% Re alloy is likely to contain a few Re substitutional atoms. The perturbation to the overall electronic and elastic properties of the substrate lattice should still be relatively small. Therefore the interaction of a single substitutional impurity atom with a diffusing adatom can be investigated. 

This type of constraint will be absent in amorphous materials because any of the Nc components can be added (or removed) anywhere in the material without exchanging with any other components. The dNi will also be independent for interstitial solutes in crystalline materials that lie in the interstices between larger substitutional atoms, as, for example, carbon atoms in body-centered cubic (b.c.c.) Fe, as illustrated in Fig. 8.8. In such a system, carbon atoms can be added or removed independently in a dilute solution. 

In this process, the net flux of substitutional atoms across the interface plane results in local volume changes (i.e., as a crystal plane is removed by climb, the crystal contracts in a direction normal to the plane). However, free expansion in directions parallel to the interface plane is constrained by the specimen ends, where significant diffusion has not occurred, and by the coherence of the interface between the expanding and contracting regions. Therefore, dimensional changes parallel to the interface (i.e., normal to the diffusion direction) are restricted, and in-plane compatibility stresses are generated. No out-of-plane compatibility stresses develop because the diffusion couple can expand freely in the diffusion direction. 

A substitutional atom (indicated by shading in Fig. 8.1) may jump and replace an adjacent nearest-neighbor substitutional atom. In the ring mechanism, the substitutional atom exchanges places with a neighboring atom by a cooperative ringlike rotational movement. 

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