The quantum-chemical analysis of donor–acceptor properties of the conjugated heterocyclic compounds is performed. For quantitative estimation of donor/acceptor ability, the topological index φ₀ is proposed; it connects with the positions of frontier levels and points on the shifting of the energy gap relative to the reference electron balanced system, for example, polyenes or polyacenes with the balanced donor and acceptor abilities. This index enables to the investigate the donor/acceptor properties of the heterocyclic compounds more detailed, taking into consideration the number of the cycles; presence, number and position of the heteroatoms as well as their type. The examples of the 5-, 6- and 7-membered heterocyclic compounds are studied, as well as the effects of the modification of the chemical constitution by replacing of the CH-fragments by atoms O, S and NH groups or by condensation with benzene. Proposed parameter could be useful for an analysis of the stability of the intermolecular stack’s complex, for example, biological active complex.

Method of quantitative estimation of the π-electron affinity for conjugated molecules, which is directly related to donor/acceptor properties is presented. The index φ₀ is proposed, it is determined by disposition of the frontier levels in respect to the non-bonding π-level (Fermi level). It is shown that this index/parameter can be estimated not only by quantum-chemical calculation, but also experimentally using the ionization potentials and electron affinity or/and the electron transition energy. The non-bonding level energy can be estimated basing on the parity theorem: in the alternant conjugated molecules, the occupied and vacant levels (including the highest occupied and lowest vacant levels) are disposed symmetrically in respect to the non-bonding levels. The proposed parameter is convenient for quantitative analysis of the donor/acceptor properties as well as the affinity of any conjugated molecule.