Mixed salts of amino acids: Syntheses, crystal structure and vibrational spectra of L-histidinium(2+) · NO₃ · ClO₄ and L-histidinium(2+) · NO₃ · BF₄

Abstract

Crystals of L-histidinium(2+) nitrate-perchlorate (L-His²⁺ · NO₃^– · ClO₄^–) and L-histidinium(2+) nitrate-tetrafluoroborate (L-His²⁺ · NO₃^– · BF₄^–) have been obtained by crystallization from aqueous solutions. For both compounds, single-crystal XRD analysis as well as ATR FTIR and Raman spectroscopy was performed to determine and investigate the crystal structures, which turned out to be closely related, albeit not isotypic, although symmetry (space group P2₁2₁2₁, Z = 4) and unit cell pa rameters of both compounds match to some extent. In each case the asymmetric unit contains a doubly charged L-His²⁺ cation with the charge counterbalanced by NO₃^– as well as ClO₄^– or BF₄^– anions. The structural differences between both compounds are subtle, mainly expressed in different conformations of the L-histidinium(2+) cations, which leads to different hydrogen bond networks. The L-His²⁺ cations form O—H···O hydrogen bonds to the nitrate anions and weak N—H···O and N—H···F hydrogen bonds. Powder SHG tests confirm considerable second order nonlinear optical activity for L-His²⁺ · NO₃^– · BF₄^– but not for L-His²⁺ · NO₃^– · ClO₄^–, this discrepancy being a result of the structural difference.