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 investigation of benzene-AlX₃ and ethene-AlX₃ (X = H, F, Cl) interactions. The binding energies, preferred modes of interaction of AlX₃ with benzene and ethene, 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 benzene complexes, the Al atom is directly placed over one of the benzene carbons, while in the ethene complexes, the Al atom lies above the center of the π bond. The binding energies of both the benzene and ethene complexes of AlX₃ are dominated by electrostatic contributions, which is in contrast to the dominance of the electron correlation energy in the benzene-BX₃ complexes. A very sharp increase in the negative charge of the benzene carbon closest to the Al atom in the lowest energy conformers of the benzene complexes from -0.2 to -0.4 au points to an important role of activation of the aromatic ring by the Lewis acid in electrophilic aromatic substitution reactions.