Geometric difference

The term resonance has also been applied in valency. The general idea of resonance in this sense is that if the valency electrons in a molecule are capable of several alternative arrangements which differ by only a small amount in energy and have no geometrical differences, then the actual arrangement will be a hybrid of these various alternatives. See mesomerism. The stabilization of such a system over the non-resonating forms is the resonance energy. 

Usually, two conformations are regarded as geometrically different if their minimized RMS deviation is equal to or larger than 0.3 A in Cartesian space RMSx z)> or 30 " in torsion angle space (RMSja). respectively. 

The Diels-Alder reaction of a diene with a substituted olefinic dienophile, e.g. 2, 4, 8, or 12, can go through two geometrically different transition states. With a diene that bears a substituent as a stereochemical marker (any substituent other than hydrogen deuterium will suffice ) at C-1 (e.g. 11a) or substituents at C-1 and C-4 (e.g. 5, 6, 7), the two different transition states lead to diastereomeric products, which differ in the relative configuration at the stereogenic centers connected by the newly formed cr-bonds. The respective transition state as well as the resulting product is termed with the prefix endo or exo. For example, when cyclopentadiene 5 is treated with acrylic acid 15, the cw fo-product 16 and the exo-product 17 can be formed. Formation of the cw fo-product 16 is kinetically favored by secondary orbital interactions (endo rule or Alder rule) Under kinetically controlled conditions it is the major product, and the thermodynamically more stable cxo-product 17 is formed in minor amounts only. 

The charge distribution determined within clusters by CNDO has been reported for only a few cases. Let us consider only one cluster, the 13-atom fee cluster with only two geometrically different types of atom. There is a center atom with 12 nearest neighbors, and there are 12 surface atoms each with 4 nearest neighbors. At the equilibrium bond length (0.34 nm) the center atom has a net positive charge, but this situation is reversed at the bulk experimental distance (0.288 nm). 

Molecular geometrical differences between the amides (225) and (226), the latter having the more pyramidal arrangement around nitrogen, have been correlated with the known differences in their solvolytic behaviour. 

There exist two geometrically different varieties of these sites, which are referred to as B5 sites because both can be made to accommodate a nitrogen molecule, which is then coordinated by five atoms. They occur at steps on the (100) and (111) planes, and particularly on (110), (311), and other high-index planes. A later paper by van Hardeveld and van Montfoort (10) contains additional evidence showing that the B5 sites are indeed responsible for the infrared-active form of nitrogen adsorption, and also that the number of B5 sites in the sample can be estimated with fair accuracy from the intensity of the 2200 cm-1 band. This means that infrared study of nitrogen adsorption can give valuable information about the structure of the surface of metal particles. 

As experimental tools, a large variety of conceptually and geometrically different M-B-M junctions has been proposed to the scientific community. To name only a... 

The extent of dissociation into ions is a function of the structure of that part of the molecule that becomes the carbonium ion as well as the often quite different structure of the ion itself. There are always geometrical differences between the carbonium ion part of the... 

Interesting geometrical differences between helical conformations in isotactic and syndiotactic polymers have recently been found.25 27... 

Geometrical Different spatial distribution of atoms or groups around central atom. Cis trans in square planar and octahedral complexes. 

Reactions on synthetic polymers often mimic similar reactions involving small molecules where size is the main difference. There are some exceptions where nearby groups may hinder or assist in the reaction in which differences occur. Here, the main differences are often kinetic, though some geometrical differences are found for specific cases. Reactions in which the rate of reaction in polymers is enhanced by the presence of neighboring groups are called anchimeric assistance. 

It is thus a higher form of molecular "behaviour than selective com-plexation alone and involves two stages of information input. Enzyme reactions are examples of such processes, as well as, for instance, drug-receptor interactions. Two substrates could, in principle, display very similar thermodynamic and kinetic complexation behaviour (no selection) but still only one of them may be able to undergo a specific reaction (because of geometrical differences, for instance) and thus be recognized. 

The factor of 1.2 in Eqs. 24 and 25 differs for that given in Eq. 6. This follows from the geometrical difference between cubic and sphere. 

The assumption that PBBs and PBDEs share many toxicological characteristics with PCBs also does not consider geometrical differences due to the higher atomic weight and considerably larger molecular volume of bromine compare to chlorine (Hardy 2000, 2002). These differences contribute to dissimilar physical/chemical properties that can influence the relative toxicokinetics and toxicities of the chemicals. 

As for cis- and trans- l-(9-anthryl)-2-phenylethylenes (9-styrylanthra-cenes) 68a and 69a, differences in their ground state molecular geometry in terms of angles between the planes of the three jt-systems, viz. anthracene, ethylene, and phenyl, as revealed by X-ray diffraction analyses, are relatively small (see Table 14). The major geometrical difference between the two isomers is found in the angle between the anthracene and ethylene ir-systems. 

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