Stereochemical descriptors stereodescriptors

The first class uses manually assigned stereodescriptors (e.g., CIP or template-based) as additional attributes of the graph nodes and edges to further refine the symmetry classes. The algorithms then proceed with the selection of canonical numberings as in the non-stereochemical case. This method works as well as the original stereochemical descriptors describe the structure, but note the problems cited above for the CIP system. 

The assignment of descriptors and configuration is sometimes arbitrary, at best, when based on model structures and pseudo-atom coordination numbers. A more explicit stereochemical notation is achieved by using this notation, which states within the stereodescriptor the model structure on which the notation is based. In this notation the pseudo-square pyramidal structure is [5PF-5-14-C(i )]. This structure can be expected to result in geometric isomers when one of the... 

In another approach (see NMR Data Correlation with Chemical Structure),three-dimensional (3D) substructure descriptors are used as a basis for the prediction. In this case the distribution function is simplified and a subsequent analysis is not required. This extension of the HOSE code is based on the ability to count the total number of steric interactions over a three-, four-, and five-bond sphere and additional parameters to describe the axial and equatorial substitution pattern for each atom. In the simple case of cis/trans ksomerism the number of interactions is calculated by means of the display coordinates. More complex situations are described by up/down bonds in this case the calculation of the number of steric interactions for each atom is performed by comparison of the query structure with carefully selected references from a library of ring skeletons. Figure 6 exemplifies the method. The discussion of the stereochemical effects is focused on the two diastereotopic methyl groups at position 4. The experimental shift values are 22.0 and 33.6 ppm. The prediction without stereodescriptors results in 27.4 ppm for both atoms (Figure 6, upper part). If the stereochemical effects are considered, the predicted shifts (21.5 and 33.5 ppm) deviate insignificantly from the experimental values (Figure 6, bottom). 

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