Structural topological isomerism

Thiocyanic acid (a), is tautomeric, existing essentially (95 per cent) in the iso form. Esters of both types, RSCN and SCNR, are known, and in some metal-coordination compounds the SCN ligand is attached to the metal through S and in others through N. On the other hand, the oxygen analogue cyanic acid exists entirely in the iso form and forms only one type of ester, RNCO. Both isomers (b) 

More complex examples of isomerism include that of carboranes, in which C atoms are introduced into polyhedral frameworks of B atoms, and of compounds such as S (NH)8 n These form cyclic molecules similar to the Sg molecule of elementary sulphur but with some S atoms replaced by NH groups. Three of the four possible isomers of S6(NH)2 have been characterized, namely the 1,3, 1,4, and 1,5 isomers. 

Coordination compounds provide numerous examples of structural isomerism, of which the following are a selection  

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Two Co(ll) coordination polymers with formulas [Co(oda)(H20)2 H20]n and [Co(oda)(H20)-H20]n (H2oda=oxydiacetic acid) were characterized by single crystal XRD and TG. [Co(oda)(H20)2 H20]n has a covalently linked 1-D chain structure, while [Co(oda)(H20) H20]ii has a covalently linked 3-D chiral network with channels. The structures showed an unusual example of topological isomerism, and the structural interconversion between [Co(oda)(H20)2 H20]n and [Co(oda)(H20)-H20]n revealed that self-assembly in the synthesis and interconversion of crystalline solids is a thermodynamically controlled process [135]. 

H. L. Frisch and D. Klempner, Topological Isomerism and Macromolecules, Adv. Macromol. Chem. 2, 149 (1970). Macromolecular catenanes. Review of IPN structures lENs. 

The earlier sections have only considered the way atoms are bonded to each other in a molecule (topology) and how this is translated into a computer-readable form. Chemists define this arrangement of the bonds as the constitution of a molecule. The example in Figure 2-39, Section 2.5.2.1, shows that molecules with a given empirical formula, e.g., C H O, can have several different structures, which are called isomers [lOOj. Isomeric structures can be divided into constitutional isomers and stereoisomers (see Figure 2-67). 

In the case of n-butene isomerization it was demonstrated (Figure 2) that the ideal micro-pore topology led to retardation of the C8 dimer intermediate and that the catalyst based on the ferrierite structure was close to optimal in this respect [1). For selective isodewaxing a one-dimensional pore structure which constrained the skeletal isomerization transition state and thereby minimized multiple branching such as the SAPO-11 structure was found to meet these criteria. Clearly, these are ideal systems in which to apply computational chemistry where the reactant and product molecules are relatively simple and the micro-porous structures are ordered and known in detail. 

The study of chemical reactions requires the definition of simple concepts associated with the properties ofthe system. Topological approaches of bonding, based on the analysis of the gradient field of well-defined local functions, evaluated from any quantum mechanical method are close to chemists intuition and experience and provide method-independent techniques [4-7]. In this work, we have used the concepts developed in the Bonding Evolution Theory [8] (BET, see Appendix B), applied to the Electron Localization Function (ELF, see Appendix A) [9]. This method has been applied successfully to proton transfer mechanism [10,11] as well as isomerization reaction [12]. The latter approach focuses on the evolution of chemical properties by assuming an isomorphism between chemical structures and the molecular graph defined in Appendix C. 

Valuable applications of this principle have been described by Paquette for the synthesis of topologically spheric molecules. Coates used this reductive fragmentation for the synthesis of bicy-clo[6.3.0]undecenecarboxylates, compounds which bear a structural resemblance to the a-b rings of the fusicoccin and ophiobolane natural products. Reduction of the unsaturated keto ester (164) affords an isomeric mixture of esters, the equilibration of which effects conversion to the single ester (165 Scheme 56). 

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