Ab initio study of benzene-BX3 (X = H, F, Cl) interactions

Quantum mechanical ab initio calculations at the MP2/6-311++G** level of th eory have been used to predict the binding energies and geometries of benze ne-BX3 and ethenc-BX3 (X= H, F, Cl) complexes. Single point calculations at a much higher level of correlation (MP4) and larger basis sets (6-311++G(2 df,p) + diffuse(d,p)) have also been carried out. The calculations reveal i nteresting trends in their binding energies and geometries. The binding ene rgies indicate that all of them are weakly bound van der Waals complexes wi th the exception of the C2H4-BH3 complex. While complexes involving BF3 are the weakest (binding energies) in cases of both ethene and benzene, there is a reversal in the relative order of the binding energies as one moves fr om ethene to benzene. Thus C6H6-BCl3 is more tightly bound than C6H6-BH3 Th e geometry exhibited by the lowest energy conformer in cases of complexes i nvolving benzene is different from those involving ethene. In contrast to m ost weak van der Waals interactions involving benzene, H-pi, and aromatic-a romatic, the boron atom lies directly over one of the benzene carbons. This observation has been explained by comparing the geometries obtained in com plexes involving both benzene and ethene. More importantly, there is strong evidence of an unusual increase in the nucleophilicity of one of the benze ne carbons in the lowest energy conformer of systems involving benzene, whi ch implies that Lewis acid-aromatic ring interactions have an important rol e in electrophilic aromatic substitution reactions.