The O–HO, O–HN and C–HO hydrogen bonds in 1,4-dimethylpiperazine mono-betaine monohydrate Z. Dega-Szafran * , A. Katrusiak, M. Szafran Faculty of Chemistry, Adam Mickiewicz University, ul. Grunwaldzka 6, 60-780 Poznan ´ , Poland Received 18 September 2007; received in revised form 24 October 2007; accepted 25 October 2007 Available online 4 November 2007 Abstract 1,4-Dimethylpiperazine mono-betaine (1-carboxymethyl-1,4-dimethylpiperazinium inner salt, MBPZ) crystallizes as monohydrate. The crystals are orthorhombic, space group Pccn. Two MBPZ molecules and two water molecules form a cyclic oligomer, (MBPZH 2 O) 2 . The O–HO and O–HN hydrogen bonds are of 2.769(1) and 2.902(1) A ˚ , respectively. The dimers interact with the neighboring molecules through the C–HO hydrogen bonds of 3.234(1) A ˚ . The piperazine ring assumes a chair conformation with the N(4)–CH 3 and N + (1)–CH 2 COO  groups in the equatorial position and the N + (1)–CH 3 group in the axial one. The FTIR spectrum is compared with that calculated by the B3LYP/6-31G(d,p) level of theory. Ó 2007 Elsevier B.V. All rights reserved. Keywords: 1-Carboxymethyl-1,4-dimethylpiperazinium dipolar ion; O–HO, O–HN and C–HO hydrogen bonds; DFT calculations; FTIR and Raman spectroscopies 1. Introduction Betaines (ammonium alkanoates) are zwitterions (inner salts, dipolar ions) because they possess formally charged ammonium and carboxylate groups separated by one or more sp 3 carbon atoms. A crucial role in the structure, function and activity of betaines is played by the attractive Coulombic interaction between the positive nitrogen atom and an oxygen atom of the COO  group [1,2]. Betaines crystallize as hydrates or/and anhydrous compounds [2–6]. In the hydrates, water molecules contribute to the charge compensation. Recently, we have prepared two betaines of 1,4-dimeth- ylpiperazine: 1-carboxymethyl-1,4-dimethylpiperazinium inner salts (MBPZ, mono-betaine) and 1,4-dicarboxymeth- yl-1,4-dimethylpiperazinium inner salt (DBPZ, di-betaine) [7]. DBPZ crystallizes with two molecules of water, which link betaines into infinite chains through the O–HO hydrogen bonds of 2.771(2) and 2.820(2) A ˚ [6]. The present study on the X-ray diffraction analysis of 1,4-dimethylpi- perazine mono-betaine, MBPZ, has been performed with the aim of determining the molecular features present in the crystal structure, the isolated molecule structure calcu- lated by the B3LYP/6-31G(d,p) method, and the spectro- scopic properties studied by FTIR method. MBPZ has two different proton-acceptor centers, the carboxylate group (pK 1 = 1.48) and the tertiary nitrogen atom (pK 2 = 4.90) [7]. 2. Experimental 1,4-Dimethylpiperazine mono-betaine (MBPZ) was pre- pared according to the method described in Ref. [7], m.p. 218–219 °C. MBPZ is a very hygroscopic compound. Crystals of MBPZ for X-ray diffraction experiment were grown from the water–isopropanol mixture. The crystal structure was determined by X-ray diffraction, measured with a KUMA KM-4 CCD diffractometer [8]. The struc- ture was solved by direct methods using SHELXS-97 [9] 0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.10.032 * Corresponding author. Tel.: +48 61 8291216; fax: +48 61 8291505. E-mail address: degasz@amu.edu.pl (Z. Dega-Szafran). www.elsevier.com/locate/molstruc Available online at www.sciencedirect.com Journal of Molecular Structure 880 (2008) 69–76