A theoretical investigation of benzene-AlX3 and ethene-AlX3 (X = H, F, Cl)interactions

The present report details the results of a high-level theoretical investig ation of benzene-AlX3 and ethene-AlX3 (X = Il, F, CI) interactions. The bin ding energies, preferred modes of interaction of AlX3 with benzene and ethe ne, and vibrational frequencies for these complexes have been evaluated at the MP2 level using basis sets ranging from the 6-31+G* to the much larger (6-311++G(2df,p) + diffuse(d,p)). In the lowest energy conformers of the be nzene complexes, the Al atom is directly placed over one of the benzene car bons, while in the ethene complexes, the Al atom lies above the center of t he pi bond. The binding energies of both the benzene and ethene complexes o f AlX3 an dominated by electrostatic contributions, which is in contrast to the dominance of the electron correlation energy in the benzene-BX3 comple xes. A very sharp increase in the negative charge of the benzene carbon clo sest to the Al atom in the lowest energy conformers of the benzene complexe s from -0.2 to -0.4 an points to an important role of activation of the aro matic ring by the Lewis acid in electrophilic aromatic substitution reactio ns.