Quantitative evaluation of resonance interaction: monosubstituted 1,3-butadienes

Relative resonance energy of various groups X was expressed as the reaction energy of the isodesmic homodesmotic reaction in which this group is transferred from an aliphatic compound, 1-substituted butane or pentane, to 1-substituted butadiene or benzene. Energies of 18 (E)-1-substituted 1,3-butadienes with ap or sp conformation on the central bond, and of the same number of substituted benzenes were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d,p). The reaction energies are very similar for all reaction series; in the case of ap-butadienes the reaction energy can be suggested as the most trustworthy standard scale of the substituent conjugative power (resonance). This scale differ significantly from the traditional scales of resonance effects, σR, σR+ or σR−, derived essentially from acid-base equilibria; the reason is that the resonance scales are influenced by the presence of charge. Relation of our model to the classical term of resonance was confirmed both from the C2-C3 bond lengths and from the Mulliken charges on the CH2 group. The C2-C3 bond is shortened by all substituents, the charges are changed in the opposite direction by acceptors and donors. However, there is no direct proportionality between the substituents effects on the energy, on geometry and the charges.