Unusual hydrogen bonding in L‐cysteine hydrogen fluoride

L‐Cysteine hydrogen fluoride, or bis(L‐cysteinium) difluoride–L‐cysteine–hydrogen fluoride (1/1/1), 2C3H8NO2S+·2F−·C3H7NO2S·HF or L‐Cys+(L‐Cys...L‐Cys+)F−(F−...H—F), provides the first example of a structure with cations of the `triglycine sulfate' type, i.e.A+(A...A+) (where A and A+ are the zwitterionic and cationic states of an amino acid, respectively), without a doubly charged counter‐ion. The salt crystallizes in the monoclinic system with the space group P21. The dimeric (L‐Cys...L‐Cys+) cation and the dimeric (F−...H—F) anion are formed via strong O—H...O or F—H...F hydrogen bonds, respectively, with very short O...O [2.4438 (19) Å] and F...F distances [2.2676 (17) Å]. The F...F distance is significantly shorter than in solid hydrogen fluoride. Additionally, there is another very short hydrogen bond, of O—H...F type, formed by a L‐cysteinium cation and a fluoride ion. The corresponding O...F distance of 2.3412 (19) Å seems to be the shortest among O—H...F and F—H...O hydrogen bonds known to date. The single‐crystal X‐ray diffraction study was complemented by IR spectroscopy. Of special interest was the spectral region of vibrations related to the above‐mentioned hydrogen bonds.