Editing Hydrogen bond (section)

==Symmetric hydrogen bond==
A [[symmetric hydrogen bond]] is a special type of hydrogen bond in which the proton is spaced exactly halfway between two identical atoms. The strength of the bond to each of those atoms is equal. It is an example of a [[three-center four-electron bond]]. This type of bond is much stronger than a "normal" hydrogen bond. The effective bond order is 0.5, so its strength is comparable to a covalent bond. It is seen in ice at high pressure, and also in the solid phase of many anhydrous acids such as [[hydrofluoric acid]] and [[formic acid]] at high pressure. It is also seen in the [[bifluoride]] ion {{chem2|[F***H***F]-}}. Due to severe steric constraint, the protonated form of Proton Sponge (1,8-bis(dimethylamino)naphthalene) and its derivatives also have symmetric hydrogen bonds ({{chem2|[N***H***N]+}}),<ref>Khashayar Rajabimoghadam Yousef Darwish Umyeena Bashir Dylan Pitman Sidney Eichelberger Maxime A. Siegler Marcel Swart Isaac Garcia-Bosch Aerobic Oxidation of Alcohols by Copper Complexes Bearing Redox-Active Ligands with Tunable H-Bonding https://doi.org/10.1021/jacs.8b08748</ref> although in the case of protonated Proton Sponge, the assembly is bent.<ref>{{Cite journal|last1=Ozeryanskii|first1=Valery A.|last2=Pozharskii|first2=Alexander F.|last3=Bieńko|first3=Agnieszka J.|last4=Sawka-Dobrowolska|first4=Wanda|last5=Sobczyk|first5=Lucjan|date=2005-03-01|title=[NHN]+ Hydrogen Bonding in Protonated 1,8-Bis(dimethylamino)-2,7-dimethoxynaphthalene. X-ray Diffraction, Infrared, and Theoretical ab Initio and DFT Studies|journal=The Journal of Physical Chemistry A|volume=109|issue=8|pages=1637–1642|doi=10.1021/jp040618l|pmid=16833488|bibcode=2005JPCA..109.1637O|issn=1089-5639}}</ref>