Conjugation Effects and Aromaticity in Intramolecular Resonance-Assisted Hydrogen Bonds (5) Configuration Dependence in the Triplet State

Abstract

In order to examine the inversion of aromaticity between the singlet and the triplet states for the pi-conjugated intramolecular resonance-assisted hydrogen-bond system DFT calculations were carried out on 2-hydroxycyclohexadienones fused position-isomerically with m-membered (m=3,5,7) pi-conjugated rings. The HOMA indices on the basis of the bond alternation in the outer pi-conjugated rings for the acene-type compounds depended on m reversely to those in the singlet state, and were very sensitive to the positional isomerism. Those for the helicene-type compounds depended on m reversely to those for the acene-type ones, though their sensitivity to the positional isomerism was not apparent. The inversion of aromaticity in the triplet state compared to that in the singlet state was obviously elucidated with the NICS values on the basis of the NMR chemical shifts particularly for the 6-memebered rings. The positional isomerism for the NICS values was unfortunately indefinite. The HOMA indices and the NICS values for the present system indicated the enhancement of not only aromaticity but anti-aromaticity for the hydrogen-bond forms compared to the corresponding non-hydrogen-bond forms.