Triglycine sulfate (TGS), known since 1956 [1], is a technologically very important ferroelectric crystal. Two more its analogs, triglycine selenate (TGSe) [1] and triglycine tetrafluoroberyllate (TGBeF₄) [2] are known too. Their molecular structure can be presented as AH(A···AH)Y, where A is amino acid (in present case glycine) in zwitter-ionic form, AH is protonated cationic amino acid, (A···AH) is dimeric cation, where A and AH are connected by short hydrogen bond, and Y is divalent (SO₄^2-, SeO₄^2-, BeF₄^2-) anion. Later new analogs of TGS, namely tri-L-valine sulfate [3] and tri-L-valine selenate [4], were discovered.

Recently we started a systematic investigation of salts of amino acids with hexafluorosilicate anion (SiF₆^2-) [5]. In the course of this work we discovered four more new analogs of TGS formed by the same mechanism: GlyH(Gly···GlyH)SiF₆ (I), SarH(Sar···SarH)SiF₆·2H₂O (II), L-AlaH(L-Ala···AlaH)SiF₆·H₂O (III) and L-ProH(L-Pro···L-ProH)SiF₆·H₂O (IV). Our results on the investigation of crystals of salts of L-alanine (III) and sarcosine (II) were recently published [6,7]. In the present work we report our results on the investigation of crystals of salts of glycine (I) and L-proline (IV) in comparison with salts of sarcosine (II), L-alanine (III) and TGS.

The molecular structures of (I) and (IV) are shown in Fig. 1

Table 1. Some crystallographic data of GlyH(Gly···GlyH)SiF₆ (I), SarH(Sar···SarH)SiF₆·2H₂O (II), L-AlaH(L-Ala···AlaH)SiF₆·H₂O (III) and L-ProH(L-Pro···L-ProH)SiF₆·H₂O (IV).

Fig.1. Molecular structure of GlyH(Gly···GlyH)SiF₆ (I, left) and L-ProH(L-Pro···L-ProH)SiF₆·H₂O (IV, right). Probability ellipsoids are shown at the 50% level.

Crystals of salts of optically active amino acids (III, IV) necessarily crystallize in noncentrosymmetric structures (species III has even polar symmetry), while crystals of salts of achiral amino acids (I, II) often crystallize in centrosymmetric structures (see Table 1). The O···O distances in the structures of (I, II, III) are close to the respective distance in TGS (2.470(9) Å) [8], while in the case of (IV) this distance is significantly shorter (see Table 1). As observed for TGS, TGSe and TGBeF₄, phase transitions are possible in related crystals as well. A phase transition was observed in the structure of tri-L-valine selenate at 138K [4], also a structural centrosymmetric-noncentrosymmetric phase transition was observed in (II) near 180K. The work in this direction is in progress. Crystallization conditions of all four crystals (I-IV) will be discussed as well as their infrared and Raman spectra.

References

[1] B.T. Matthias, C.E. Miller, J.P. Remeika, Ferroelectricity of glycine sulfate. Phys. Rev. 104(3), 849-850(1956).

[2] S. Hoshino, T. Mitsui, F. Jona, R. Pepinsky, Dielectric and thermal study of tri-glycine sulfate and tri-glycine fluoberyllate. Phys. Rev. 107(5), 1255-1258(1957).

[3] I. Němec, Preparation and Study of Addition Compounds of Aminoacids with Inorganic Oxyacids. PhD thesis, Charles University in Prague, Faculty of Science (1998).

[4] I. Němec, Z. Mička, Tri-L-valine selenate-study of vibrational spectra and structural phase transition, J. Molecular Structure 563-564, 289-294(2001).

[5] V.V. Ghazaryan, M. Fleck, A.M. Petrosyan, Salts of amino acids with hexafluorosilicate anion, J. Crystal Growth 362, 162-166(2013).

[6] V.V. Ghazaryan, M. Fleck, A.M. Petrosyan, Hexafluorosilicates of alanine, Z. Kristallographie 227(9) 646-655(2012).

[7] M. Fleck, V.V. Ghazaryan, A.M. Petrosyan, Hexafluorosilicates of sarcosine, Solid State Sciences 14, 952-963(2012).

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1. New mixed salts of L-histidinium(2+) comprising hexafluorosilicate anions
2. Two crystalline forms of sarcosine sarcosinium hydrogen L-tartrate