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.