Topographic Indices and Configuration of Annulenes

Balaban, following the method first used by Gordon, used the three possible orientations of a planar honeycomb (hexagonal) lattice with some of the bonds in vertical position to define (1) the boundary code of polycyclic aromatic hydrocarbons 

For distinguishing cis/trans isomers of conjugated polyenes or of alkenes, Randic et and then Pogliani started from the observation that in an s-cis butadi- 

Here c and t stand for cis and trans, respectively, n = 4 for the virtual ring, and X is Randid s first-order molecular connectivity  

For trans double bonds X. = 0 and X., = X, but for cis double bonds the index X,. is smaller than X, achieving thereby a differentiation between TIs of cis/trans diastereomers. Index X, is linearly related to Randic s XX index  

For a series of physical properties such as boiling points, refractive index, density, and molarrefractivity, X,., scores better than other indices such as the GAI index.  

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