New descriptors of the double bonded substituent effect, sEDA() and pEDA(), were constructed based on quantum chemical calculations and NBO methodology. They show to what extent the σ and π electrons are donated to or withdrawn from the substituted system by a double bonded substituent. The new descriptors differ from descriptors of the classical substituent effect for which the pz orbital of the ipso carbon atom is engaged in the π-electron system of the two neighboring atoms in the ring. For double bonded substituents, the pz orbital participates in double bond formation with only one external atom. Moreover, the external double bond forces localization of the double bond system of the ring, significantly changing the core molecule. We demonstrated good agreement between our descriptors and the Weinhold and Landis’ “natural σ and π-electronegativities”: so far only descriptors allowing for evaluation of the substitution effect by a double bonded atom. The equivalency between descriptors constructed for 5- and 6-membered model structures as well as linear dependence/independence of the constructed parameters was discussed. Some interrelations between sEDA() and pEDA() and the other descriptors of (hetero)cyclic systems such as aromaticity and electron density in the ring and bond critical points were also examined.
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Organic & Biomolecular Chemistry
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