LBHBs

Low-barrier hydrogen bonds (LBHBs) are a proposed class of hydrogen bonds in which the proton is shared nearly equally between two electronegative atoms, resulting in an unusually short donor–acceptor distance and a shallow barrier for proton transfer. In theory, this arrangement strengthens the interaction and can stabilize charged states or transition states more effectively than conventional hydrogen bonds.

Characteristics commonly cited for LBHBs include short O…O or N…N distances around 2.4–2.5 Å, near-linear hydrogen-bond geometries, and signs of significant covalent or quantum delocalization in the proton’s position. The concept suggests that when the pKa values of the two participating groups are closely matched, the proton can reside in a shared, low-energy well, lowering the energy required for transfer.

LBHBs have been proposed as contributing factors in enzyme catalysis and other chemical systems, where precise positioning of donors and acceptors could in principle provide extra stabilization of transition states. However, the existence and functional importance of LBHBs are subjects of debate. Critics argue that many reported instances can be explained by very strong conventional hydrogen bonds, electrostatic interactions, or crystal-structure artifacts, and that definitive proof requires direct observation of the shared proton.

Detection relies on a combination of neutron diffraction (which can locate protons), advanced spectroscopic methods (such as NMR and infrared), isotope effects, and high-level computational studies. The current view is nuanced: LBHBs may occur in specific systems under particular conditions, but they are not universally present or universally essential for catalysis.