Stereochemistry at Double Bonds

However, the descriptors cannot be considered independently as there is no free rotation around the double bond, In order to take account of this rigidity, the descriptors of the two units have to be multiplied to fix a descriptor of the complete stereoisomer. 

In this case, two transpositions have to be performed, resulting in a permutation descriptor of (+ 1) for the right-hand subunit. 

The complete descriptor of this isomer is again obtained by multiplication of the descriptors of the two subunits. In this case, we obtain a value of (-r 1), the opposite of the value for the cis isomer, as desired. 

However, as the descriptor oFthc eutirc steveoisoraer is obtained by multiplicatiou of the individual descriptors, again a value of (-(-1 is obtained. Thus, as desired, the stereocenter of a double bond does not change through rotation of a molecule. 

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A similar approach can be applied for treating the stereochemistry at double bonds. 

Using simple rules, a structure is represented by a string of characters unique to that structure. It can also be used to specify stereochemistry at double bonds and chiral centres. SMARTS is a further extension that allows substructure searching. 

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