Hydrogen-bonding interactions in selected super-molecular systems: electron density point of view

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 ρ (r), its Laplacian ∇2ρ (r), and also other related quantities at the bond critical points. The pKa 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 pKa parameters