A Critical Check for the Role of Resonance in Intramolecular Hydrogen Bonding.

Although resonance-assisted H-bonds (RAHBs) are well recognized, the role of π resonance in RAHBs is controversial, as the seemingly enhanced H-bonds in unsaturated compounds may result from the constraints imposed by the σ skeleton. Herein the block-localized wave function (BLW) method, which can derive optimal yet resonance-quenched structures with related physiochemical properties, was employed to examine the correlation between π resonance and the strength of intramolecular RAHBs. Examination of a series of paradigmatic molecules with RAHBs and their saturated analogues showed that it is inappropriate to compare a conjugated system with its saturated counterpart, as they may have quite different σ frameworks. Nevertheless, comparison between a conjugated system and its resonance-quenched (i.e., electron-localized) state, which have identical σ skeletons, shows that in all studied cases, π resonance unanimously reduces the bonding distance by 0.111-0.477 Å, strengthens the bonding by 40-56 %, and redshifts the D-H vibrational frequency by 104-628 cm-1 . Furthermore, there is an excellent correlation between hydrogen-bonding strength and the classical Coulomb attraction between the hydrogen-bond donor and the acceptor, which suggests that the dominant role of the electrostatic interaction in H-bonds and RAHBs originates from the charge flow from H-bond donors to acceptors through π conjugation.