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+) (whereAandA+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 groupP21. The dimeric (L-Cys...L-Cys+) cation and the dimeric (F−...H—F) anion are formedviastrong 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.
