Unusual hydrogen bonding in L-cysteine hydrogen fluoride

L-Cysteine hydrogen fluoride, or bis(L-cysteinium) difluoride-L-cysteine-hydrogen fluoride (1/1/1), 2C(3)H(8)NO(2)S+center dot 2F(-)center dot C3H7NO2S center dot HF or L-Cys(+)(L-Cys center dot center dot center dot L-Cys(+))F-(F-center dot center dot center dot H-F), provides the first example of a structure with cations of the 'triglycine sulfate' type, i.e. A(+)(A center dot center dot center dot 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 P2(1). The dimeric (L-Cys center dot center dot center dot L-Cys(+)) cation and the dimeric (F-center dot center dot center dot H-F) anion are formed via strong O-H center dot center dot center dot O or F-H center dot center dot center dot F hydrogen bonds, respectively, with very short O center dot center dot center dot O [2.4438 (19) angstrom] and F center dot center dot center dot F distances [2.2676 (17) angstrom]. The F center dot center dot center dot F distance is significantly shorter than in solid hydrogen fluoride. Additionally, there is another very short hydrogen bond, of O-H center dot center dot center dot F type, formed by a L-cysteinium cation and a fluoride ion. The corresponding O center dot center dot center dot F distance of 2.3412 (19) angstrom seems to be the shortest among O-H center dot center dot center dot F and F-H center dot center dot center dot 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.