Low-barrier hydrogen bond

A Low-barrier hydrogen bond or LBHB is a special type of hydrogen bond. This type of bond is especially strong because the distance between acceptor and donor is especially short. In regular hydrogen bonds (for example the O-H...O distance is at least 2.8 Ångström) the hydrogen ion clearly belongs to one of the heteroatoms. When the distance decreases to about 2.55 Å the proton is free to move between the two atoms (hence the low-barrier) and the LBHB forms. When the distance decreases even further (< 2.29 Å) the bond is characterized as a short-strong hydrogen bond or SSHB [1].

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 [2].

A low-barrier hydrogen bond was found inside a certain aza crown-type compound depicted below [3]:

Encircled proton Day 2007

In this compound a proton sits comfortably between two amide carbonyl oxygens separated by a distance of 2.45 Å. It would not be expected there because the macrocyclic ring already has two positively charged quaternary ammonium units.
References

1. ^ On the electronic nature of low-barrier hydrogen bonds in enzymatic reactions Birgit Schiøtt, Bo Brummerstedt Iversen, Georg Kent Hellerup Madsen, Finn Krebs Larsen, and Thomas C. Bruice Proc. Natl. Acad. Sci. USA Vol. 95, pp. 12799–12802, October 1998 Article link
2. ^ Low-barrier hydrogen bonds and enzymic catalysis WW Cleland and MM Kreevoy Science 24 June 1994: Vol. 264. no. 5167, pp. 1887 - 1890 doi:10.1126/science.8009219
3. ^ Encircled Proton Victor W. Day, Md. Alamgir Hossain, Sung Ok Kang, Douglas Powell, Gerald Lushington, and Kristin Bowman-James J. Am. Chem. Soc.; 2007; 129(28) pp 8692 - 8693; (Communication) doi:10.1021/ja0724745