How Resonance Modulates Multiple Hydrogen Bonding in Self-Assembled Systems

The secondary electrostatic interaction (SEI) has been regarded as the fundamental cause for the relative strengths of multiple hydrogen bonds for decades, though recent studies challenged its validation. Here, we used our developed block-localized wave function (BLW) method, which is a variant of ab initio valence bond (VB) theory and can self-consistently derive the wave function for a strictly electron-localized state, to study a series of exemplary multiply hydrogen-bonded complexes and critically examine the role of SEI in the binding. Our computations show that the multiple hydrogen bond in self-assembled complexes is a kind of resonance-assisted hydrogen bond (RAHB) in nature, and the π resonance which moves electron density from the hydrogen bond donor to the acceptor is the true origin of the different hydrogen bond strengths. By quenching the π resonance effect, the hydrogen bond strengths become nearly identical for various neutral doubly, triply, and quadruply hydrogen-bonded dimers where in general the SEI model works. In other words, the SEI plays only a minor role in multiply hydrogen-bonded complexes, and the π resonance, which changes not only electron densities but also molecular polarities (dipole moments), is the major force.