Application of the partitioning of energy in the mndo method to the study of the basicity of imidazole,pyrazole, oxazole,and isoxazole

The technique of energy partitioning of Fischer and Kollmar has been extended to the MNDO SCF-MO method and used in an effort to ascertain the main factors which determine the greater basicity of imidazole and oxazole relative to pyrazole and isoxazole, respectively. The importance of the different degree of destabilization of the five-membered ring π bonding in these azoles upon protonation is specially emphasized in relation to their relative basicity. Furthermore, it is found that the degree of destabilization of the π bonding can be ascribed to the amount of π electron charge density localized on the heteroatoms after protonation. An interpretation for this result is given in terms of the mutual electrostatic field effect due to the cores of the nearest-neighbor atoms.