Hydrogen-Bonding Interactions in Selected Super-molecular Systems: Electron Density Point of View Tapan K. Ghanty* and Swapan K. Ghosh* Theoretical Chemistry Section, RC & CD DiVision, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed: May 5, 2003; In Final Form: June 19, 2003 Ab initio and density functional theoretical calculations have been performed to quantify the hydrogen-bonding interactions for selected supermolecular systems, experimental investigations on which have been reported very recently (Angew. Chem., Int. Ed. 2001, 40, 3240). An analysis and rationalization of the nature of pairwise interactions in different hydrogen bonds involved in these ternary supermolecular systems is presented that uses the frameworks of Morokuma energy decomposition as well as Bader’s topological theory of atoms in molecules involving the electron density F(r), its Laplacian ∇ 2 F(r), and also other related quantities at the bond critical points. The pK a values of the aromatic acids, which have been used earlier to rationalize the specific intermolecular interactions between aromatic acids (hydrogen-bond donor) and isonicotinamide (hydrogen-bond acceptor as well as donor), are, however, found not to show any regular trend with the calculated binary interaction energy values or the electron density-based bonding parameters using experimental geometries. The calculated quantities corresponding to the computationally optimized geometries of the molecular species, however, do show some regular trends with the corresponding pK a parameters. 1. Introduction Among the weak intermolecular interactions, hydrogen bond- ing has been the subject of increasing research activities 1-5 in recent years due to its importance in many chemical and biological systems and processes. The essence of the physical interactions that contribute to hydrogen bonding has been the subject of numerous discussions in the literature, and even the nature of interactions involved in an O-H‚‚‚O hydrogen bond sometimes appears to be controversial. 6,7 While the key feature in inorganic supramolecular systems 8 is mostly the metal-ion coordination, it is the hydrogen bonding that plays 9,10 an important role for assembling 11-13 organic molecules through crystal engineering or for stabilizing supramolecular aggregates in water. Although the procedures for synthesizing covalently bonded molecular species with desired structures and properties have more or less been standardized, in contrast, the supramo- lecular synthesis involving noncovalent interactions such as hydrogen bonding (through crystal engineering) has yet to attain the same level of sophistication. Higher levels of refinement and versatility in crystal engineering require identification of reliable supramolecular synthons 14 and synthetic strategies for building desired multicomponent structures. In recent years, many systematic studies have been reported demonstrating the synthesis of molecular assemblies with increasing complexity and dimensionality. Studies involving ternary supermolecules reporting 15,16 1:1:1 ternary cocrystal-containing supermolecules, which consist of three different components, have been rather recent. The recently reported synthetic strategy of Aakero ¨y et al. 15 has been based on the rule of thumb that in a system with various hydrogen-bonding functionalities “the best hydrogen- bond donor and the best hydrogen-bond acceptor will prefer- entially form hydrogen bonds to one another” and the second- best donor will form a hydrogen bond to the second-best acceptor and so on. It has also been assumed that a small number of specific intermolecular interactions can provide a large part of the stabilization energy of molecular crystals. For this purpose, different derivatives of benzoic acids (H-bond donor) and isonicotinamide (H-bond acceptor) molecules have been considered and an attempt has been made to rationalize the nature of specific intermolecular interactions intuitively by using the pK a values of the constituent carboxylic acids. Although it is sometimes possible to qualitatively follow the synthetic strategies to construct supermolecular systems by considering the possible interactions judged intuitively through the param- eters obtained from conventional chemical concepts such as pK a values, as considered recently, an alternative elegant way to do this would be to quantify 17-19 the intermolecular interactions by use of ab initio theoretical techniques for a set of binary or ternary systems. Although the intermolecular interaction energy provides the gross features about the nature and quantification of an interaction, more precise and detailed qualitative and quantitative information and insight can be retrieved through consideration of other aspects of the electron density distribution. One such tool that has been quite valuable in providing insight into various aspects of the structure and bonding in molecules involves the topological analysis 20 of the electron density F(r), which has proved to be highly successful. Recently this approach has been extensively used 21-26 for the studies of interesting and unusual bonding aspects in several molecular systems including (H 2 O) 2 + and (H 2 S) 2 + . The interactions underlying the design strategy of supermolecules can be rationalized through detailed studies of the topological aspects of their electron distribution. We thus propose to employ the topological theory of atoms in molecules (AIM), 20 which has been known to provide a rigorous procedure to partition a molecular system into its atomic fragments defined by the gradient vector field ∇F(r) and discuss chemical bonding through the bond path and bond critical point * Corresponding authors: e-mail: skghosh@magnum.barc.ernet.in and tapang@apsara.barc.ernet.in. 7062 J. Phys. Chem. A 2003, 107, 7062-7067 10.1021/jp035208w CCC: $25.00 © 2003 American Chemical Society Published on Web 08/05/2003