Low-barrier hydrogen bond
Standard hydrogen bonds are longer (e.g. 2.8 Å for a O···O h-bond), and the hydrogen ion clearly belongs to one of the heteroatoms. When pKa of the heteroatoms is closely matched, a LBHB becomes possible at a shorter distance (~2.55 Å). When the distance further decreases (< 2.29 Å) the bond is characterized as a short-strong hydrogen bond or SSHB.
Low-barrier hydrogen bonds are especially relevant to enzyme catalysis because when they form in an transition state they can significantly accelerate otherwise difficult reactions. Such a transition state has been captured by X-ray crystallography and shows a low-barrier hydrogen bond between the catalytic Serine and a stabilizing Lysine with a distance of 2.53 Å. 
- Day, Victor W.; Hossain, Md. Alamgir; Kang, Sung Ok; Powell, Douglas; Lushington, Gerald; Bowman-James, Kristin (2007). "Encircled Proton". J. Am. Chem. Soc. 129 (28): 8692–3. doi:10.1021/ja0724745.
- Schiøtt B, Iversen BB, Madsen GK, Larsen FK, Bruice TC (October 1998). "On the electronic nature of low-barrier hydrogen bonds in enzymatic reactions". Proc. Natl. Acad. Sci. U.S.A. 95 (22): 12799–802. Bibcode:1998PNAS...9512799S. doi:10.1073/pnas.95.22.12799. PMC 23598. PMID 9788994.
- Cleland WW, Kreevoy MM (24 June 1994). "Low-barrier hydrogen bonds and enzymic catalysis". Science 264 (5167): 1887–90. Bibcode:1994Sci...264.1887C. doi:10.1126/science.8009219. PMID 8009219.
- Nichols DA, Hargis JC, Sanishvili R, Jaishankar P, Defrees K, Smith EW, Wang KK, Prati F, Renslo AR, Woodcock HL, Chen Y (9 June 2015). "Ligand-Induced Proton Transfer and Low-Barrier Hydrogen Bond Revealed by X-ray Crystallography". Journal of the American Chemical Society. doi:10.1021/jacs.5b00749.