Hexafluorosilicates of alanine

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 P2(1). 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.

Amino acid hexafluorosilicates – an overview

Single crystal data on eight new hexafluorosilicate salts of amino acids are presented in this work. This includes crystal and molecular structures of the following salts: 2 β-Ala · H2SiF6 (I), 2 Bet · H2SiF6 (II), 2L-Orn · H2SiF6 (III), 2L-Ser · H2SiF6 (IV), 2L-Leu · H2SiF6 · 2 H2O (V), 2L-Ile · H2SiF6 · 2 H2O (VI), 2L-Met · H2SiF6 · H2O (VII), and 2L-Asp · H2SiF6 · 2 H2O (VIII). In all these crystals, the SiF6-anion is ordered, except for (III), where disorder of the fluorine atoms occurs. An analysis of previously published data on amino acid hexafluorosilicates showed that the amount of hydrogen bonding towards the SiF6-anion is an indication of disorder – we found that all structures with 0.6 or less hydrogen bonds per fluorine atom have disorder.

Reinvestigation of L-tryptophan picrate: establishment of the existence of the L-tryptophan L-tryptophanium dimeric cation

A crystal structure redetermination of the L-tryptophan picrate crystal previously studied by Ishida et al. (Chem. Pharm. Bull. 41 (1993) 433-438) showed that it comprises L-tryptophan L-tryptophanium dimeric cation, one picrate anion and picric acid. The O⋯O distance of the O-H⋯O hydrogen bond in the dimeric cation is equal to 2.470(6) Å. The infrared spectrum of the crystal was registered and analyzed. The infrared spectrum of the crystals contains a broad absorption band centered at ca. 1170 cm(-1), which is assigned to the stretching vibration of the O-H bond.