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 .
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.
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- 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