Progress in understanding the stability of <?A3B2 pi6?>strong hydrogen-bonded systems

Strong hydrogen-bonded systems, characterized by high interaction energies, short interaction distances, and pronounced directionality, play a central role in modulating molecular structures, tuning condensed-phase properties, and sustaining biological functions. The nature of strong hydrogen bonds (SHBs) encompasses not only classical noncovalent interactions dominated by electrostatics, but also non-negligible covalent contributions arising from orbital overlap and charge transfer; moreover, their stability is often coupled to variations in chemical composition, spatial structure, and external environments. Here, we systematically survey the fundamental theories, structural and energetic characteristics, governing factors, investigative approaches, and representative applications of strong hydrogen-bonded systems. Our goal is to summarize key advances and outstanding issues in the field, and to provide guidance for the rational design and performance tuning of strong hydrogen-bonded systems in materials science, energy science, and biomolecular engineering.