Strength of Intramolecular Hydrogen Bonds

The concept of resonance-assisted hydrogen bonds (RAHBs) highlights the synergistic interplay between the pi-resonance and hydrogen bonding interactions. This concept has been well-accepted in academia and is widely used in practice. However, it has been argued that the seemingly enhanced intramolecular hydrogen bonding (IMHB) in unsaturated compounds may simply be a result of the constraints imposed by the sigma-skeleton framework. Thus, it is crucial to estimate the strength of IMHBs. In this work, we used two approaches to probe the resonance effect and estimate the strength of the IMHBs in the two exemplary cases of the enol forms of acetylacetone and o-hydroxyacetophenone. One approach is the block-localized wavefunction (BLW) method, which is a variant of the ab initio valence bond (VB) theory. Using this approach, it is possible to derive the geometries and energetics with resonance shut down. The other approach is Edmiston's truncated localized molecular orbital (TLMO) technique, which monitors the energy changes by removing the delocalization tails from localized molecular orbitals. The integrated BLW and TLMO studies confirmed that the hydrogen bonding in these two molecules is indeed enhanced by pi-resonance, and that this enhancement is not a result of sigma constraints.