ROLE OF INTERMOLECULAR HYDROGEN-BONDING IN SOME SUPRAMOLECULES - AN AM1 STUDY OF THE BINDING-ENERGIES AND HYPERPOLARIZABILITIES

Semiempirical calculations using the AM1 approximation to MO theory ha ve been carried out on some donor and acceptor (DA) substituted aromat ics containing carboxylic acid, amide, nitro amino and nitro dimethyla mino hydrogen-bonded dimers. Intermolecular binding energies and molec ular hyperpolarizabilities, beta and gamma, have been calculated. Hydr ogen-bonding energies have not varied appreciably with the DA strength s in the acid and amide dimers; however, these energies increase in th e other two dimers. These studies have also indicated that the hydroge n-bonding interactions in acid and amide dimers may not contribute to any significant enhancement of the hyperpolarizabilities, while the mu tually induced polarization enhances the beta value in the two nitro d imers. However the relative rise in gamma is not so significant in all the four dimers. As there is no substantial charge transfer across th e hydrogen bonds in these four dimers, the magnitude of beta and gamma is not in the order observed in stilbenes and tolans substituted with similar DA groups. Hydrogen bonding supplemented by a large mutual po larization could be useful in generating efficient second harmonic gen erating (SHG) materials based on supramolecules.