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) Šseems 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.

ß-Alaninium tetrafluoroborate: a new salt with di-ß-alaninium dimeric cation.

ß-Alaninium tetrafluoroborate crystallizes in the monoclinic system (space group P21/n, Z=4). The asymmetric unit contains one ß-alaninium cation and one tetrafluoroborate anion, in which the fluorine atoms are disordered. All ß-alaninium cations are connected with the symmetry related cations via an inversion center, thus forming dimeric centrosymmetric ß-Ala(+)···ß-Ala(+) cations. The O···O distance (2.698(2)Å) in the dimeric cation is relatively long compared to known distances of previously reported salts with A(+)···A(+) type dimeric cations. The infrared and Raman spectra are studied.

L-Methioninium picrate.

Single crystals of l-methioninium picrate were obtained by evaporation of aqueous solution containing equimolar quantities of each component. l-Methioninium picrate crystallizes in the monoclinic system (space group P21, Z=2). The asymmetric unit contains one l-methioninium cation and one picrate anion. The carboxyl group of the cation forms an O-H⋯O hydrogen bond with the phenoxy oxygen atom and has an O⋯O distance of 2.669(3)Å. The protonated amino group of the l-methioninium cation forms N-H⋯O hydrogen bonds with the phenoxy oxygen atom of the picrate anion and the carbonyl oxygen atom of the symmetry related cation. The infrared and Raman spectra are recorded and discussed.

L-tryptophan L-tryptophanium chloride.

L-tryptophan L-tryptophanium chloride is a new salt with (A⋯A(+)) type dimeric cation. It crystallizes in the monoclinic system (space group P2(1), Z=2). The asymmetric unit contains one zwitterionic L-tryptophan molecule, one L-tryptophanium cation and one chloride anion. The dimeric cation is formed by a O-H⋯O hydrogen bond with the O⋯O distance equal to 2.5556(18) Å. The infrared and Raman spectra of the crystal are studied and compared with the spectra of L-tryptophanium chloride.

On the existence of "L-threonine formate", "L-alanine lithium chloride" and "bis L-alanine lithium chloride" crystals.

We argue that the recently reported crystals "L-threonine formate" as well as "L-alanine lithium chloride" and "bis L-alanine lithium chloride" actually are the well-known crystals L-threonine and L-alanine, respectively.

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.

On the infrared spectrum of L-lysinium(2+) sulfate.

It is argued that the infrared spectrum of L-lysine sulfate (L-Lys2+·SO4(2-)) published by Krishnakumar et al. [V. Krishnakumar, S. Manohar, R. Nagalakshmi, Spectrochim. Acta A75 (2010) 1394-1397] cannot belong to this crystal species. To confirm this, crystals (L-Lys2+·SO4(2-)) were synthesized and identified by XRD, and ATR FTIR and Raman spectra of L-Lys2+·SO4(2-) were registered and are discussed.

Glycine glycinium picrate--reinvestigation of the structure and vibrational spectra.

The crystal of diglycine picrate (glycine glycinum picrate) has been obtained from an aqueous solution containing stoichiometric quantities of the components. The species crystallizes in the monoclinic system (space group P2(1)/c). The crystal structure was determined with high accuracy, IR and Raman spectra are discussed and compared with previous results, and the molecular structure is presented. It was shown that crystals of diglycine picrate obtained from the solution containing equimolar quantities may contain picric acid as impurity, which is the reason for the previously reported observation of second harmonic generation in this centrosymmetric crystal. With this example we want to point out the risk of misinterpretation of SHG signals in general.