Canonical ring description

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

It is a wide-spread assumption that the electronic structure of I can be rationalized by using either Forster-Coulson-Mofitt orbitals23 24 or Walsh orbitals25, which are considered to represent equivalent orbital sets and to lead to similar descriptions of bonding in the C3 ring of 1. The Walsh orbitals have attracted special attention, in particular among organic chemists, since they seem to be close to the canonical (delocalized) SCF MOs of 1 and seem to explain... 

The canonical vector representation allows an easy as well as exact description of the ring conformations of these species see Table II. Each conformer is characterized by its projection on to idealized Chair (C), Boat (B), and Skew boat (S) conformations... 

To register, look up, or otherwise access a ring template, a canonical (i.e., unique and atom-numbering-independent) description of the RS is first obtained (Figure 54). The hash function reduces this to a short code, which acts as an address in the hash table. The hash table eventually yields the address of a ring data record where the ring is stored. 

Regardless of whether a canonical description of a complete structure is available at the time of redrawing, it is not useful because the acyclic portions destroy the symmetry that would otherwise be present between different ring atoms. The first step, therefore, is to create a scratch structure to which only the ring system is copied. Heteroelements are converted to carbon charges and all stereochemistry are removed and all bond orders are set to 1. Other types of connection table may require further reduction. The second step is to canonical-ize the reduced connection table, either with the same algorithm used for regular molecules,or with a specialized version that is more efficient with reduced connection tables. 

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