The role of conjugative interactions in acidic and basic character of five membered aromatic heterocyclics

The trends in protonation and deprotonation enthalpies for the different potential sites in five membered aromatic heterocyclic molecules have been analysed employing molecular orbital calculations at B3LYP/6311++G∗∗//B3LYP/6-31+G∗, MP2/6-311++G∗∗//MP2/6-31+G∗ and G2MP2 theoretical levels. NBO analysis has been also carried out for the twelve heterocyclic molecules and their protonated and deprotonated species. The reasons for the most basic and acidic site in the molecules under study have been explored in terms of conjugative interactions, electronegativity and charge holding ability of the heteroatoms.