Mixed salts of amino acids: Syntheses, crystal structure and vibrational spectra ofL-histidinium(2+) · NO₃· ClO₄andL-histidinium(2+) · NO₃· BF₄

Abstract

Mixed salts / L-histidine nitrate-perchlorate / L-histidine nitrate-tetrafluoroborate / Single crystal structure analysis / X-ray diffraction / Vibrational spectra / Amino acid Abstract. Crystals of L-histidinium(2þ) nitrate-perchlorate (L-His 2þ Á NO 3 À Á ClO 4 À ) and L-histidinium(2þ) nitrate-tetrafluoroborate (L-His 2þ Á NO 3 À Á BF 4 À ) 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 P2 1 2 1 2 1 , Z ¼ 4) and unit cell parameters of both compounds match to some extent. In each case the asymmetric unit contains a doubly charged L-His 2þ cation with the charge counterbalanced by NO 3 À as well as ClO 4 À or BF 4 À anions. The structural differences between both compounds are subtle, mainly expressed in different conformations of the L-histidinium(2þ) cations, which leads to different hydrogen bond networks. The L-His 2þ cations form O--H Á Á Á O hydrogen bonds to the nitrate anions and weak N--H Á Á Á O and N--H Á Á Á F hydrogen bonds. Powder SHG tests confirm considerable second order nonlinear optical activity for L-His 2þ Á NO 3 À Á BF 4 À but not for L-His 2þ Á NO 3 À Á ClO 4 À , this discrepancy being a result of the structural difference.