ORIGINAL PAPER Crystallographic and Computational Analysis of the Supramolecular Structure of 2,5-Dimethylpyrazolo [1,5-a]pyrimidin-7(4H)-one Oleg Ya Borbulevych Received: 9 May 2009 / Accepted: 3 December 2009 / Published online: 23 December 2009 Ó Springer Science+Business Media, LLC 2009 Abstract The structure of 5-dimethylpyrazolo[1,5-a]pyr- imidin-7(4H)-one 1 has been determined by X-ray crys- tallography. The compound 1 crystallizes in the space group P-1 with two molecules in the asymmetric unit solvated with one molecule of the acetic acid The unit cell parameters are a = 8.796(1) A ˚ , b = 10.531(1) A ˚ , c = 10.680(1) A ˚ , a = 96.901(2)8, b = 98.135(2)8, c = 107.248(2)8. In the crystal molecules A and B are linked into R 2 2 (9) hydrogen bonded rings and these rings form chains parallel to [101] Overall, the supramolecular structure 1 consists of two hydrogen bonded and two p–p stacking dimers. An energetic interplay between these dimers has been studied using B3LYP DFT calculations. In addition p–p interactions are analyzed using the Bader’s atoms in molecules’ (AIM) theory. Keywords 5-dimethylpyrazolo[1,5-a]pyrimidin-7(4H)- one Á Supramolecular structure Á Density functional theory (DFT) calculations Á AIM theory Á Bader’s topological theory Á X-ray structure Á p–p Stacking interactions Introduction Heterocyclic compounds containing imidazole or pyrazole rings have potential pharmaceutical applications [1]. Some of them exhibit a broad spectrum of pharmacological actions, ranging from anthelmintic activity [2] to anticancer therapy [3]. Particularly, pyrazolo[1,5-a] pyrimidin deriv- atives are interesting as potential and effective antitricho- monal and anti-inflammatory drugs [4–6]. Structurally, they have the functional groups favoring the formation of intermolecular N–HÁÁÁO H-bonds and possess an essentially planar fused heterocyclic system, which is favorable for a p–p stacking arrangement in the crystal phase and in solution. Recently it was demonstrated that supramolecular assemblies of molecules (e.g. via stacking) are important for action mechanisms of some drugs [7]. Supramolecular arrangement is generally dominated by conventional H-bonds but other types of H-bonding and p–p stacking interactions play a significant role in crystal packing and molecular recognition in various biological and chemical systems [8]. However, the fundamental question of crystal engineering addresses the understanding of the interplay and relationship between several supramolecular synthons in the formation of a particular crystal structure [9]. The elucidation of details of a supramolecular aggregation par- ticularly relies on a combination of experimental structural methods and theoretical computational approaches [10–12], which provides a profound insight into energetic and geo- metrical supramolecular synthon determinants. The present paper reports the crystallographic and the- oretical study of supramolecular structure in the crystal of 2,5-dimethylpyrazolo[1,5-a]pyrimidin-7(4H)-one 1. Electronic supplementary material The online version of this article (doi:10.1007/s10870-009-9669-y) contains supplementary material, which is available to authorized users. O. Y. Borbulevych (&) Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, 251 Nieuwland Science Hall, Indiana 46556, USA e-mail: oborbulevych@yahoo.com 123 J Chem Crystallogr (2010) 40:412–416 DOI 10.1007/s10870-009-9669-y