Hbonds

Hbonds are non-covalent interactions that form between a hydrogen atom covalently bonded to an electronegative donor atom (such as oxygen, nitrogen, or fluorine) and an electronegative acceptor atom bearing a lone pair. The hydrogen acts as a bridge between the donor and acceptor, resulting in an interaction that is stronger than most van der Waals contacts but weaker than covalent bonds. Geometrically, Hbonds are most favorable when the X–H...Y angle is near linear and the X–H...Y distance falls in about 2.7 to 3.2 Å for typical O–H...O or N–H...N arrangements.

Energetics vary with environment but generally yield bond energies of roughly 4 to 40 kJ per mole. Strength can be increased by cooperative effects or decreased by competitive solvation, such as in water, where extensive hydrogen-bond networks form and continually rearrange.

Hbonds play a central role in the structure and function of many systems. In water, they create a dynamic three-dimensional network that underpins properties like high cohesion, surface tension, and anomalous density. In biology, Hbonds stabilize secondary structures in proteins and the double helix in nucleic acids. For example, guanine–cytosine base pairs form three hydrogen bonds, whereas adenine–thymine pairs form two. Hbonds also mediate enzyme–substrate interactions and the binding of small molecules to macromolecules.

Because Hbonds are sensitive to distance, angle, and the surrounding solvent, they are inherently dynamic. They are studied with spectroscopy and computational methods, and are sometimes distinguished from stronger covalent bonds in discussions of chemical bonding. They are not discrete bonds in the sense of covalent chemistry but rather directional electrostatic interactions that can be cooperative and context-dependent.