2-(Ammoniummethyl) pyridinium dihydrogen squarate hydrate: An experimental and theoretical study Ufuk Korkmaz ⇑ , Ahmet Bulut Department of Physics, Faculty of Arts and Sciences, Ondokuzmayis University, Kurupelit, 55139 Samsun, Turkey highlights  IR–UV spectroscopy and X-ray single crystal diffraction.  Squaric acid, Ab initio calculations.  CAHB interactions are dominant in the crystal packing.  This research is investigation of supramolecular architectures of title structures. article info Article history: Received 14 April 2013 Received in revised form 15 July 2013 Accepted 16 July 2013 Available online 23 July 2013 Keywords: Squaric acid X-ray diffraction Strong hydrogen bonding Vibrational spectra UV–Vis Quantum chemical calculations abstract The experimental and theoretical investigation results of newly synthesized squarate salt of 2-(ammoniummethyl) pyridinium dihydrogen squarate hydrate (1), 2(C 4 HO 4 )C 6 H 10 N 2 H 2 O, were reported in this study. The crystal structure of the title compound was found to crystallize in the triclinic P–1 space group. Each of two squaric acid molecule in the asymmetric unit has donated one H atom to the nitrogen atom of pyridine part and one to the nitrogen atom of methyl part of a 2-(aminomethyl) pyri- dine molecule, forming the (1) salt. The X-ray analysis clearly indicated that the crystal packing has shown the hydrogen bonding ring pattern of R 4 3 ð14Þ through OAHO and NAHO together with R 4 2 ð14Þ pattern through NAHO interactions. Ab initio calculations were performed on the compound at DFT/B3LYP/6-31++G(d,p) (2) and HF/ 6-31++G(d,p) (3) level of theory. The results of the optimized molecular structure and vibrational properties for the 1 obtained on the basis of two models are pre- sented and compared with the experimental data. The molecular electrostatic potential (MEP), frontier molecular orbitals (FMOs) and conformational flexibility of the title compound were also studied at the 2 level. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The most interesting topic of molecular crystal engineering is the arrangement of molecules and ions as building blocks [1]. One of the most fundamental conditions for producing stable crystals requires the detailed information about the forces that forms the crystal structure. Of these, hydrogen bonding is one of the most important forces in terms of both the energy and the direction [2–4]. Among hydrogen bonding types, hetero-molecular hydrogen bonding is stronger than homo-molecular hydrogen bonding [5]. Determining the hydrogen bonding pattern is also crucially important concept for supramolecular structure in crystal engineering [6–7]. In this case, choosing proper hydrogen atom, donor and acceptor sites, one can form stable and desired geomet- rical hydrogen bonding patterns. This leads to form completely a novel structure that has unique physical and chemical properties [8–10]. It should be mentioned here that the hydrogen bonding is being used to design two and three dimensional supramolecular assemblies [11–13]. Apart from that hydrogen-bonded systems generated from organic cations and anions are of special interest because they would be expected to show stronger hydrogen bonds than neutral molecules [14–18]. It is also emphasized by many workers that proton-transfer plays a crucial role for hydrogen bonding features in the crystal structure of a salt [18–20]. In the present work, we have selected squaric acid and its an- ions because of their potential in forming supramolecular structure as well as that they are flat and rigid systems. Squaric acid (H 2 SQ) can be found in three forms uncharged H 2 SQ, the HSQ  monoanion and SQ 2 dianions on deprotonating by amines [17–22]. In the squarate anion form, the anions are generally linked to amines by NAHO hydrogen bonds [21–26]. We report here the synthesis, structural characterization of 1 (2(C 4 HO 4 )C 6 H 10 N 2 H 2 O) molecule (Fig. 1). The aim of this research is investigation of 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.07.026 ⇑ Corresponding author. Tel.: +90 536 5291998; fax: +90 362 4576091. E-mail address: ukkorkmaz@gmail.com (U. Korkmaz). Journal of Molecular Structure 1050 (2013) 61–68 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc