Parity Descriptors

Fig. 3. CC-Matrix of 6. Parity descriptors used for the diagonal entries are described in Section 5.6...

Configurations of Ruch s class A occur pairwise, e.g., asymmetric C-atoms. The parities of their permutation descriptors can be used as their -parity descriptors. A permutation containing an even number of transpositionsP has a parity of +1, conversely, odd permutations have parities of — 1. Since a-atom RASI (see Section 5.4.1) correspond in most casses to the CIP sequences, an assignment of +1 to an asymmetric C-atom is equivalent to an R-configuration, and — 1 to the S, if the skeleton is indexed as shown in 25. 

In the treatment of stereochemical aspects for many chemical problems such as synthetic design representation of tri- and tetracoordinate monocentric configurations by their parity descriptors suffices. The conformational aspect as well as higher coordinate and polycentric configurations can be neglected. Then, it is possible to reduce the CC-matrices to parity vectors Pn whose components +1,0, — 1 represent the configurational features. 

The procedure for converting substances described as modified stereoparents is based on the use of stereospecific templates, one for each stereoparent. The template will have nomenclature locant numbers and parity descriptors for all stereocentres. The procedure for adding atom/bond specific stereodescriptors to the connection table, in simplified form, consists of the following steps ... 

The third stage of conversion will translate the nomenclature stereodescriptor to an atom/bond specific parity descriptor. This process is straightforward for R S, R fS and E/Z because only a single stereocentre is involved. Translating the relative ring descriptors (e.g., cis, a or ejco) is somewhat more complex because it requires identifying the ring plane and the substituents referred to by the relative descriptor. 

ASC represents a structure count exclusive of structures which do not contribute to stabilizing resonance interactions. However, the concept of parity and the derived ASC descriptor do not work in non-altemant systems with three odd-membered rings [Randic and Trinajstic, 1993a]. 

Kiimey and Wiruth developed a risk assessment system based parity on work in 1971 by William Fine, also from the US Navy. The system gives us some w s of describing the likehhood of a hazard giving rise to an unwanted event. The descriptors of likelihood are ... 

The registry software also computes wherever possible a CIP descriptor for each asymmetric atom, steric double bond and chirahty axis. These descriptors are stored in the BRCT and are available for display, but are not used for the purpose of registration. They are utilised however to assist in the interpretation of CIP descriptors used during structure input. A wide variety of stereochemical descriptors may be used at input, and the registry software attempts to interpret these in terms of a graphical representation from which parity codes can be computed for the purpose of registration. The simple example in Figure 8 illustrates this situation. 

This finishes the specification of the atoms which constitute a compound. Again, the definition was simplified, and the specification of both atom symbol and atom attribute was not further elaborated. For atom symbol, the terms should be self-explaining. The atom attributes allow a very precise specification of the properties of an atom. The attributes atom stereo descriptor, coordination, and atom parity flag are part of the stereochemistry description. The different counts and the valency are just positive integers, the cartesian coordinates are provided for a graphical display of a structure with a suitable program. 

Data). It allows the specification of tetrahedral stereocenters, cis/trans double bonds, and allene type stereochemistry by specifying parities associated with the ligands of a stereocenter (tetrahedral) or stereopair (cis/trans or allene). Each of these descriptors is assigned to a stereogroup which can be absolute ( 0 ), relative (uppercase letter), or racemic (lowercase letter). Figure 10 shows a stereocenter block describing atom 3 of molecule 21. 

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