How resonance assists hydrogen bonding interactions: an energy decomposition analysis

Area of Science:

• Quantum chemistry
• Computational chemistry
• Molecular interactions

Background:

• Resonance-assisted hydrogen bonds (RAHBs) are proposed to enhance interactions through pi delocalization.
• The precise nature and origin of RAHBs, particularly their covalent character, remain debated.

Purpose of the Study:

• To investigate the nature of resonance-assisted hydrogen bonds (RAHBs).
• To explore the synergistic interplay between pi delocalization and hydrogen bonding.
• To re-evaluate the prevailing understanding of RAHB mechanisms.

Main Methods:

• Utilized the block-localized wave function (BLW) method, a variant of ab initio valence bond (VB) theory.
• Analyzed intermolecular interaction energies and decomposed them into electrostatic, polarization, charge transfer, and correlation components.
• Examined various molecular systems including formic acid dimers, beta-diketone enols, and related model compounds.

Main Results:

• Enhanced interactions in RAHBs predominantly stem from electrostatic attraction, not increased covalency.
• Resonance redistributes electron density, increasing monomer dipole moments and thus dipole-dipole interactions.
• The covalent character of the hydrogen bonds showed minimal change, contradicting the notion of strong covalent enhancement.

Conclusions:

• RAHBs are primarily driven by electrostatic forces, with resonance amplifying dipole moments.
• The contribution of covalent character to RAHB stabilization is minor.
• Proposed the term resonance-assisted binding (RAB) to emphasize the electrostatic nature over electron transfer.