Spectrochimica Acta Part A 78 (2011) 128–132 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Glycine glycinium picrate—Reinvestigation of the structure and vibrational spectra V.V. Ghazaryan a , M. Fleck b , A.M. Petrosyan a,∗ a Institute of Applied Problems of Physics, NAS of Armenia, 25 Nersessyan Str., 0014 Yerevan, Armenia b Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria article info Article history: Received 9 May 2010 Received in revised form 9 August 2010 Accepted 8 September 2010 Keywords: Diglycine picrate Crystal structure Vibrational spectra abstract 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The investigation of the reaction between glycine and picric acid has been started with the work of Levene more than a hun- dred years ago [1], who concluded that a compound with 1:1 molar ratio is formed, which logically has been named glycine picrate. Later it has been pointed out [2] that the crystal obtained in [1] in reality is diglycine picrate (DGP) and a probable rea- son of the error committed was indicated. A preliminary X-ray diffraction study of a DGP crystal has been carried out by Guha [3], who established that DGP crystallized as a monoclinic species in the centrosymmetric space group P2 1 /c with Z = 4. However, full structural results have not been published. In 1994, Kai et al. [4], apparently independently of the previous results, prepared a crystal from a mixed solvent of methanol and water contain- ing equimolar quantities of glycine and picric acid. It seems that the authors expected to obtain glycine picrate, but the determi- nation of the crystal structure showed that the obtained crystal consists of two glycine molecules and one picric acid molecule thus confirming the previous results [2,3]. The hydrogen atom of the phenolic hydroxy group in picric acid is formally transferred from picric acid to one glycine molecule, which becomes a cationic glycinium. This molecule forms a strong hydrogen bond with the second glycine molecule which retains its zwitterionic form. Con- sequently, this crystal belongs to the previously known class of amino acids salts with dimeric cations (see references of salts with ∗ Corresponding author. Tel.: +374 10 241106; fax: +374 10 281861. E-mail addresses: apetros@iapp.sci.am, apetros@mail.ru (A.M. Petrosyan). glycine glycinium cations in [5]) and should more correctly be called glycine glycinium picrate. Uma Devi et al. [6] have grown and characterized the crystal of “glycine picrate” (in fact diglycine picrate, see [7]) from an aque- ous solution containing equimolar quantities of glycine and picric acid. The authors of [6] have registered the IR spectrum and inter- preted it assuming the presence of glycinium cations and picrate anions. However, the IR spectrum and interpretation are question- able. The spectrum does not contain any absorption bands in the vicinity of 1700 cm -1 , expected for stretching vibration of C O bond of glycinium cation. Another paper on “glycine picrate”, also obtained from aqueous solution containing equimolar quantities, has been published [8]. To confirm the functional groups present in the grown crystals, the IR and Raman spectra were recorded by the authors. The IR spectrum was not shown, but authors noted that it is very similar to that reported in [6]. The Raman spectrum was registered in the range 100–3500 cm -1 , however, only the part of the spectrum (400–2000 cm -1 ) is shown and peaks were assigned to the picrate anion only. In addition, the authors of [8] report the observation of rather strong powder second harmonic generation (SHG) signals (2.34 times more than that of KDP), which is phys- ically impossible in centrosymmetric crystals (see e.g. [9,10]). In addition, the authors of [11] reported on observation of ferroelec- tricity in “glycine picrate” which as well as SHG is impossible in centrosymmetric crystal (see [12]). As said above, the authors of [4] have determined the structure, although with a rather high value of R = 0.088. In addition, they do not discuss any bond lengths in the glycine and glycinium moieties. The aim of the present study is therefore the more accurate deter- mination of the crystal and molecular structure of diglycine picrate, 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.09.009