Conjugation Effects and Aromaticity in Intramolecular Resonance-Assisted Hydrogen Bonds (2) Chain-length Dependence

Abstract

In order to evaluate pi-conjugation effects on intramolecular resonance-assisted hydrogen bonds, DFT calculations were carried out on 1-carbonyl and w-hydroxyl substituted polyacenes fused with 3-, 5-, or 7-membered pi-conjugated rings. The pi-conjugation-length dependence of the stabilization energies due to the intramolecular hydrogen bonds and the barrier heights for proton transfer along the hydrogen bonds for the acenes with the fused 3- and 7-membered rings differs significantly from those for the acenes with the fused 5-membered ring. The O-H bond lengths in the intramolecular hydrogen-bonding conformation show similar, notable pi-conjugation-length dependence, whereas those in the non-hydrogen-bonding conformation are almost constant against the pi-conjugation length. The electron densities at the bond critical points of the O-H bonds indicate also similar behavior. The parameters for aromaticity, HOMA for the outer rings and NICS(1) at the 3-, 5-, and 7-membered rings both in the hydrogen bonding and non-hydrogen bonding conformations vary similarly and noticeably against the pi-conjugation lengths. These results were discussed on the basis of the intramolecular resonance-assisted hydrogen bonds with (4N+2) and 4N pi-conjugated electrons.