Conjugation Effects and Aromaticity in Intramolecular Resonance-Assisted Hydrogen Bonds (4) Proton Transfer and Antiaromaticity

Abstract

In order to evaluate changes in the hydrogen-bond effects from the singlet state to the triplet state on aromaticity of the pi-conjugated system involving intramolecular so-called resonance-assisted hydrogen bonds DFT and MP2 calculations were carried out on 2-hydroxycyclohexa-2,5-dienones fused with m-membered (m=3,5,7) pi-conjugated rings. The m-dependence of the aromaticity indices, HOMA derived from the bond alternation and NICS(1) on the basis of the NMR chemical shifts, for the m-membered rings inverted from the singlet state to the triplet state. Especially the character of the 5-membered ring was anti-aromatic in the singlet state, but became aromatic in the triplet state. The (anti)-aromatic characters were enhanced with the presence of the hydrogen bonds, particularly in the transition state for the proton transfer along the hydrogen bonds. The potential energy curve for the proton transfer of the compound with the 5-membered ring in the singlet state crossed that in the triplet state, suggesting the existence of the low-energy path for the proton transfer.