THE COMPARATIVE BASICITIES,Li+ANDNa+CATION AFFINITIES WITH A SERIES OF HETEROCYCLIC MOLECULES: A DFT STUDY

Ground state gas phase proton affinities, alkali metal cation (Li+, Na+) affinities and basicities of pyrrole, furan, thiophene and pyridine have been calculated computationally with the help of DFT /B3LYP method of calculation at hybrid triple zeta 6-311G(d,p) basis set level. Different binding sites of pyrrole, furan and thiophene for protonation are observed. Proton affinity (PA) value of Cα–H+ complexes of pyrrole, furan and thiophene are found to be higher compared to Cβ–H and X–H+ complexes (X= N, O, S). In case of pyridine, protonation is found to occur at heteroatom (N) and the most stable protonated complex is formed. Results obtained in this calculation shows good agreement with the experimental values. Alkali metal cation (Li+, Na+) affinity and basicity of the same molecules have been calculated at the same level of theory. Pyridine exhibits the highest affinity for Li+ and Na+ cation. The electronic properties of the complexes indicate that polar co-valent sigma bond is formed by a proton with the binding site of the corresponding molecule whereas alkali metal cation (Li+,Na+) –free molecule interactions are predominantly of an ion-dipole attraction and the ion-induced dipole interaction as well rather than a covalent interaction.Calculated proton and metal cation affinities are sought to be correlated with some of the computed system parameters like the calculated net charge on the binding atom of the free molecules and with the net charge on proton, Li+ and Na+ of the protonated, lithium and sodium complexes.