Open-Resonance-Assisted Hydrogen Bonds and Competing Quasiaromaticity

The delocalization of electron density upon tautomerization of a proton across a conjugated bridge can alter the strength of hydrogen bonds. This effect has been dubbed resonance-assisted hydrogen bonding (RAHB) and plays a major role in the energetics of the tautomeric equilibrium. The goal of this work was to investigate the role that π-delocalization plays in the stability of RAHBs by engaging other isomerization processes. Similarly, acid–base chemistry has received little experimental attention in studies of RAHB, and we address the role that acid–base effects play in the tautomeric equilibrium. We find that π-delocalization and the disruption of adjacent aromatic rings is the dominant effect in determining the stability of a RAHB.