Glycine hydrogen fluoride: Remarkable hydrogen bonding in the dimeric glycine glycinium cation M. Fleck a , V.V. Ghazaryan b , A.M. Petrosyan b,⇑ a Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria b Institute of Applied Problems of Physics, NAS of Armenia, 25 Nersessyan Str., 0014 Yerevan, Armenia article info Article history: Received 13 June 2010 Received in revised form 9 September 2010 Accepted 10 September 2010 Available online 17 September 2010 Keywords: Crystal structure Vibrational spectra Salts of amino acids Glycine hydrogen fluoride abstract Crystals of glycine hydrogen fluoride (GlyHF) were prepared from an aqueous solution containing stoi- chiometric quantities of the components. The crystal structure of GlyHF was determined, IR and Raman spectra were registered and are discussed. GlyHF crystallizes in the orthorhombic space group Pbca with Z = 32. The most remarkable feature of the structure is the existence of symmetric dimeric glycine–gly- cinium cations with short hydrogen bonds (OO distance of 2.446 Å), charge-counterbalanced by hydro- gen bifluoride (FAHF)  anions – in addition to the expected glycinium cations and fluoride anions. These results were compared with previously published data on crystals grown in the system glycine– HF–H 2 O. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction During our on-going study dealing with the search of new mixed salts of amino acids, we have recently studied the systems glycine + HBF 4 (HClO 4 )+H 2 O and glycine + HF + H 2 O. The results on the systems glycine + HBF 4 +H 2 O as well as glycine + HClO 4 + H 2 O have already been reported [1], whereas the present paper deals with the system glycine + HF + H 2 O. This system was studied more than 60 years ago by Frost [2], who prepared diglycine hydrogen chloride (2GlyHCl) as well as the respective bromide and iodide compounds (2GlyHBr and 2GlyHI), and also tried to prepare the likewise fluoride salt (2GlyHF). However, this salt proved elusive as Frost was not able to obtain any such product. Glycine hydrogen fluoride (GlyHF) along with fluorides of other amino acids are used as ingredient of cariostatic compositions [3], although its crystal structure has not been determined up to date. Recently some papers devoted to this system were published. Khandpekar [4] declared that he had succeeded in the growth of diglycine hydrogen fluoride crystal by slow evaporation at room temperature from an aqueous solution with a stoichiometric ratio of components. However, he did not present any physical or chemical data that would have confirmed the composition and structure. In addition, Selvaraju et al. [5] reported the growth of glycine hydrogen fluoride (GlyHF) from an aqueous solution with an equimolar ratio of components by slow evaporation. Again, no crystal structure was reported, but at least the parameters of an orthorhombic unit cell, an IR spectrum and TG and DTA curves were reported. Moreover, second harmonic generation was de- tected. The same crystal was obtained and characterized again by Vijayan et al. [6]. Cell parameters, IR spectrum and TG, DTA data are in good agreement with the respective data of [5]. The SHG efficiency measured by the powder technique proved rather high (6.3 times higher than KDP) nonlinear optical activity. How- ever, the results presented in [5,6] do not allow to elucidate the structure of the crystal. (Certainly, the determination of only unit cell dimensions by no means proves composition or structure of any crystal.) In both papers, the authors discussing the IR spectra mention the carboxylate group, which excludes the existence of glycinium cations. Moreover, the IR spectra shown in [5,6] corre- spond well to the spectrum of a-glycine. Therefore at least for us it remains unclear what crystal species was actually obtained in [5,6]. It is worth to mention also that authors of [7] speaking about diglycine hydrofluoride in their Introduction quoted a reference (N. Vijayan, G. Bhagavannarayana, S. Kumararaman, Mater. Lett. 60 (2006) 2848), which is a strange combination of [5,6]. The pa- pers [5,6], as mentioned above, are devoted to glycine hydrofluo- ride. Furthermore, they did not quote Ref. [4] in which the 2GlyHF crystal allegedly was obtained. Recently, we emphasized the importance of structure determination of new crystals to avoid such situations [8], and these examples underline this statement. In this contribution we present the results on structure determination and show IR and Raman spectra of glycine hydrogen fluoride, crystallized from an aqueous solution. 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.09.010 ⇑ Corresponding author. Tel.: +374 10 241106; fax: +374 10 281861. E-mail address: apetros@iapp.sci.am (A.M. Petrosyan). Journal of Molecular Structure 984 (2010) 83–88 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc