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···O distance is at least 2.8 Ångström) the hydrogen ion clearly belongs to one of the heteroatoms (a heteroatom is an atom that is not carbon or hydrogen so how is hydrogen a heteroatom). 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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