ENHANCED AL+ BINDING-ENERGIES OF SOME AZOLES - A THEORETICAL-STUDY OF AZOLE-X+ (X = NA, K, AL) COMPLEXES

Hartree-Fock calculations with the 3-21G(*) and 6-31G* basis have been performed to investigate the structure and energetics of Na+-, K+-, and Al+-azole complexes. Structures have been fully optimized at the 3-21G(*) level. The structures of azole-Al+ complexes resemble closely those of the corresponding protonated species while those of Na+ and K+ complexes are similar to those found upon Li+ association. Na+- and K+-bridged structures, when the azolic system presents two neighbor nitrogens having lone pair electrons, are particularly stable with respect to Li+-bridged systems. This implies that Li+ vs Na+ (or K+) binding energies follow two different linear correlations. For those cases where Na+ and K+ are single coordinated a good linear correlation between their binding energies and the proton affinities is found. A similar behavior is observed when Al+ binding energies are considered. The former correlation obeys a quite simple electrostatic model, which is not fulfilled by Al+ association energies. Our topological analysis of the complexes' charge density shows that the Al+-azole interaction has a nonnegligible covalent character, which involves the low-lying empty p orbitals of Al+. These interactions are responsible for the enhanced stability of some Al+ complexes, as for instance, Al+-imidazole, which presents a stability very close to that of the Li+-imidazole system.