Orthorhombic-to-monoclinic temperature-dependent phase transition of hexamethylenetetraminium-3,5-dinitrobenzoate-3,5-dinitrobenzoic acid monohydrate crystal Suchada Chantrapromma a , Hoong-Kun Fun b, * , Anwar Usman b a Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand b X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia Received 1 December 2005; accepted 11 December 2005 Available online 30 January 2006 Abstract The crystal of hexamethylenetetraminium-3,5-dinitrobenzote-3,5-dinitrobenzoic acid monohydrate undergoes a temperature-dependent phase transition from orthorhombic P bca at temperature higher than 220 K to monoclinic P2 1 /c at temperature lower than 220 K. The crystal structure analyses for data collected at 300 and 203 K show that in both temperature structures, there are two conventional intermolecular hydrogen bonds between the hexamethylenetetraminium and water molecule and between 3,5-dinitrobenzote and 3,5-dinitrobenzoic acid in the asymmetric unit. Another conventional intermolecular O–H/N hydrogen bonds link the hexamethylenetetraminium and water molecules into chains parallel to the a-direction and stabilized by the 3,5-dinitrobenzoate-3,5-dinitrobenzoic acids. There are also another six weak intermolecular interactions interconnecting the molecules into a three dimensional network. The shorter donor-acceptor distances of the prevailing hydrogen bonds and the two extra weak intermolecular C–H/O interactions interconnecting the hexamethylenetetraminium and 3,5-dinitrobenzoate-3,5-dinitrobenzoic acids in the low temperature monoclinic structure are attributable to the reversible temperature-dependent phase transition. q 2006 Elsevier B.V. All rights reserved. Keywords: X-ray crystallography; Phase-transition; Phenols–amines adducts 1. Introduction Interactions between phenols or organic acids with amine bases in solid phase have been of interest. They are widely used to study hydrogen bonds since the two components are generally linked by intramolecular or intermolecular O–H/O, O–H/N or N–H/O types of hydrogen bonds which are among the most robust and versatile synthons in crystal engineering [1,2]. Among the organic acids, 3,5- dinitrobenzoic acid is of interest since it readily forms 3,5- dinitrobenzoate anion when co-crystallized with amine bases by transferring a H atom. This H-transfer process leads to the formation of salt-like adducts consisting of 3,5-dinitrobenzoate anion and amine-based cation. In these types of adducts, the anion is expected to act as a multiple acceptor of hydrogen bonds via the nitro and carboxylate groups. Owing to this interest, we have recently prepared crystalline forms of 3,5-dinitrobenzoic acid with various amines including quinuclidine and hexamethylenetetramine (HMT). The 3,5- dinitrobenzoic acid with quinuclidine forms two types of adducts; quinuclidinium-3,5-dinitrobenzoate and quinuclidi- nium-3,5-dinitrobenzoate-3,5-dinitrobenzoic acid [3], whereas with hexamethylenetetramine, it forms only one type of adduct; hexamethylenetetraminium-3,5-dinitrobenzote-3,5- dinitrobenzoic acid monohydrate (adduct I). The H-transfer process occurs in these adducts. With changing temperature, the adduct I undergo distortive reversible phase-transitions from orthorhombic to monoclinic which theoretically have been described macroscopically by Landau ferroelastic theory [4] and microscopically using the pseudo-spin approach [5] in our previous studies. In this paper, we report the preparation, temperature-dependent phase transition and the X-ray struc- tural analyses of the adduct I (Scheme 1). 2. Experimental The adduct I was prepared by thoroughly mixing of 0.70 g (5 mmol) hexamethylenetetramine and 1.06 g (5 mmol) 3,5- Journal of Molecular Structure 789 (2006) 30–36 www.elsevier.com/locate/molstruc 0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.12.013 * Corresponding author. Tel.: C60 4 6533652; fax: C60 4 6579150. E-mail address: hkfun@usm.my (H.-K. Fun).