A topological analysis of charge density in complexes between derivatives of squaric acid and ammonium cation David Qui ~ nonero, Antonio Frontera * , Pau Ballester, Carolina Garau, Antoni Costa, Pere M. Dey a * Departament de Qu  õmica, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain Received 18 December 2000; in ®nal form 15 March 2001 Abstract A topological analysis of the electron charge density in complexes between derivatives of squaric acid and ammo- nium cation has been carried out. There are excellent relationships between either the charge density or its associated Laplacianattheringcriticalpointoriginateduponcomplexationandbothgeometricandenergeticparameters.Thereis a remarkable in¯uence of the substituents attached to the four-membered ring on the charge density at the ring critical point. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Supramolecular chemistry bears on the orga- nized entities of higher complexity that results from the association of two or more chemical species held together by intermolecular forces [1]. Molecular receptors are organic structures held mainly by covalent bonds, which are able to bind selectively ionic or molecular substrates or both) by means of various intermolecular interactions, leading to an assembly formed by two or more species [2]. Molecular recognition is de®ned by the energy involved in the binding of a substrate by a given receptor molecule. It implies a pattern recognition process through a structurally well- de®ned set of intermolecular interactions. Molecular modeling techniques based on high- level ab initio calculations have become incipient and incisive tools that provide insight into the molecular recognition process for simple molecu- lar receptors. These techniques help us in getting a deeper understanding of the intermolecular forces involvedinthebindingprocessthat®nallyaccount for the molecular recognition features of the re- ceptor. In non-polar organic solvents, the so-called strong or conventional hydrogen bond [3] O; N±H O; N, an electrostatic force with some covalent character, is probably the most important directional intermolecular force used for supramolecular construction. The hydrogen bond can be successfully studied by means of the topological properties of the electron charge den- sity q using the theory of `atoms in molecules' [4,5].Althoughtherearesomeexampleswherethis method has not given a correct interpretation of the bond pattern [6,7], it has been satisfactorily 18 May 2001 Chemical Physics Letters 339 2001) 369±374 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +34-011-34-971. E-mail address: tonif@soller.uib.es A. Frontera). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)00395-5