The generality of uncooperative and cooperative effects in elementary hydrogen-bonded systems

TL;DR

This study uses high-precision ab initio methods to analyze cooperative and uncooperative effects in hydrogen-bonded systems, revealing how bond lengths and interaction strengths vary with intermolecular distance and atom types.

Contribution

It demonstrates the conditions under which hydrogen bonds exhibit cooperative or uncooperative effects and links these effects to measurable indices like CVB and binding energies.

Findings

Cooperative effects cause D-H bond length to first increase then decrease with distance.

Uncooperative systems show larger CVB index and lower binding energies.

Uncooperative effects are linked to non-H-bonding interactions.

Abstract

The cooperative effect plays a significant role in understanding the intermolecular donor-acceptor interactions of hydrogen bonds (H-bonds, D-H...A). Herein, using the benchmark method of high-precision ab initio, the well-known cooperative effect is reproduced in elementary H-bonded systems with different D and A atoms. That is, with the decreasing of intermolecular distance, the D-H bond length first increases and then decreases, while the H...A bond length decreases. On the contrary, when D and A are the same, as the intermolecular distance decreases, the D-H bond length decreases without increasing, which is referred to as the uncooperative effect. Further analyses conclude that compared to cooperative H-bonded systems, uncooperative systems at their respective equilibrium position have a larger core-valence bifurcation (CVB) index (>0.022) and lower binding energies (<0.25 eV), showing a clear linear inverse relationship related to H-bond strength. Therefore, the intermolecular non-H-bonding interactions are predicted to reflect the uncooperative characteristics, which is confirmed by high-precision ab initio calculations. These findings provide a direction for the comprehensive understanding of H-bonds.