Extensive experimental and theoretical studies have been made in the past decade to reveal the cation-π interaction and its role in biomacromolecules1. Now most of the π systems in biomacromolecules are nitrogen heterocyclics. Hence we choose pyrrole, imidazole, pyridine and indole as key molecules whose interaction is to be monitored. Now, clays especially 2:1 dioctahedral smectites are layered aluminosilicates. The octahedral aluminum layer is sandwiched between tetrahedral silicon layers. The Substitution of a divalent metal ion for octahedral Al results in a net negative charge and the interaction with positive ions (exchangeable cation) to form an interlayer phase. In case of smectites the activity solely lies on the orientation of the hydroxyl hydrogen attached to the octahedral aluminum present in 2:1 dioctahedral smectite2. Now, the hard soft acid-base (HSAB) principles classify the interaction between acids and bases in terms of global softness. In the present study we monitor the adsorption complex formed either by cation-π interaction or the H-bond interaction. The feasibility is monitored using Fukui function and enthalpy calculation using DFT.

　　 In the present study, all cluster calculations have been carried out with DFT using DMOL code of MSI Inc. BLYP exchange correlation functional and DNP basis set was used through out the calculation. The Fukui function is defined by as follows:

The function f is the local quantity, which has different values at different points in the species, N is the total number of electrons, μ is the chemical potential, and v is the potential acting on an electron due to all nuclei present. The gross atomic charges were evaluated by using the technique of electrostatic potential (ESP) driven charges. The Al2Si6O24H18 cluster was generated from the clay structure as shown in Fig. 1(a). Now for the local softness calculations a smaller cluster with formula AlSi4O16H10 is chosen.

The complex chosen are shown in Fig. 1 (a) and (b).

First from the Fukui function values the active site of the interacting molecule and clay clusters were monitored. This is followed by the interaction energy calculations to validate the results obtained from the reactivity indices. Both cation-π and H-bond interactions were compared. Although the calculated binding strength is similar for both the interactions, but the enthalpy change of the whole reaction process suggested that the formation of H-bond complex is favorable to the stability of the whole system. The results also suggest that that all the H-atoms of the protonated heterocyclic are likely to be used as hydrogen bond donor.

1. J.C. Ma and D. A. Daugherty, Chem. Rev. 1997, 97 (5) 1303.
2. A. Chatterjee, T. Iwasaki and T. Ebina, J. Phys. Chem. A. 2000, 104, 8216.