Structural isomers, definition

Another common theme in FP studies is the richness of possible surface phases -in some cases, dozens of structural isomers are computed to be thermodynamically accessible at room temperature. This has led to speculation that many oxide surfaces are more dynamic than previously thought, but definitive conclusions will only be possible once the processes of surface diffusion are identified and their activation energies are computed. This is perhaps the next frontier in FP oxide simulation. Meanwhile, the flexible surface model for active sites on metals [5] is finding some application in explaining the apparently facile diffusion of interstitial ions in non-stoichiometric oxides, despite the rigidity of the oxide lattice [26]. 

B-adducts involve the -carbon atom endo/exo notation, as usual, defines the di-astereoisomeric relation of the C-N-0 nitrone-group with respect to the substi-tuent(s). The isomer definition is made clearer in the transition structures which are taken to be "concerted according to the extant literature. One can see that all the adducts allowed are present in the reaction mixture with variable, but non negligible fractions. The same is true for the reactions of 3,4-dihydro-isoquinoline-N-oxide and C-phenyl-N-methylnitrone,3 whereas the reactions of N-H-nitrone (H-nitrone), N-Me-nitrone and N-(t)-Bu-nitrone3t>.c lead to a single adduct, which, in our notation, is the B-regioadduct (Scheme 3). The experimental trends extracted from Tables 1-2 and from the cited literature can be summarised as follows ... 

Cis and trans isomers are stereoisomers—compounds that have identical connectivities (i.e., their atoms are attached in the same sequence) but differ in the arrangement of their atoms in space. They are distinct from constitutional or structural isomers (Sections 1-9 and 2-5), which are compounds with differing connectivities among atoms. Conformations (Sections 2-8 and 2-9) also are stereoisomers by this definition. However, unlike cis and trans isomers, which can be interconverted only by breaking bonds (try it on your models), conformers are readily equilibrated by rotation about bonds. Stereoisomerism will be discussed in more detail in Chapter 5. 

This method of diene formation with definite E and Z structures has wide synthetic applications [518], particularly for the syntheses of natural products with conjugated polyene structures. Bombykol and its isomers (650 and 651) have been prepared by this method[5l9]. The synthesis of chlorothricolide is... 

Recently, Nesmeyanov and co-workers have published definitive evidence that dwaZ reactivity (the formation of derivatives of two different structural formulas) extends beyond tautomerism (isomers in equilibrium or reversible isomeric transformation). A single molecular species can form two series of derivatives, in one of which a transfer of the reaction center occurs in the reaction. 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

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