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.