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Carbocation−π Interaction: Computational Study of Complexation of Methyl Cation with Benzene and Comparisons with Related Systems

1998; American Chemical Society; Volume: 120; Issue: 40 Linguagem: Inglês

10.1021/ja980505d

1943-2984

Paul C. Miklis, R. Ditchfield, Thomas A. Spencer,

Protein Structure and Dynamics

An investigation of the interaction of carbocations with aromatic rings has been initiated by a computational study of complexation of methyl cation with benzene to determine if this is appropriately included as an example of η6 cation−π interaction. Specifically, electronic structure calculations for three types of π complex of methyl cation with benzene, 1(η6), 2(η2), 3(η1), and the Wheland σ complex 4 have been obtained at several theoretical levels. The results indicate that inclusion of electron correlation is required for accurate calculation of intermolecular distances and binding energies and that the B3LYP/6-31G* level of theory provides a practical, reliable approach for the study of carbocation−π interactions. Although none of the π complexes is an energy minimum, the maximum binding energy of CH3+ above the periphery of the ring is more than twice as large as that of optimum binding above the ring centroid. In addition, the association energies in η2 and η1 complexes are ca. 80% of the binding energy calculated for the equilibrium σ complex. The comparison of results for C6H6- -CH3+, C6H6- -SiH3+, and C6H6- -Na+ with earlier theoretical work and with experiment confirms the reliability of the B3LYP/6-31G* method. An examination of the dependence of binding in complexes 1−4 on intermolecular separation was also conducted. The results indicate that at distances >2 Å, a "π approach" toward 2 or 3 has a binding energy which is competitive with the approach to σ complex (4) formation. This work also shows clearly that, in contrast to complexes of coordinatively saturated cations with benzene, at intermolecular distances 3.5 Å, binding at the periphery of the aromatic system is comparable in energy to η6 binding. With respect to the postulated stabilization of carbocation intermediates in biochemical reactions via π interactions with aromatic residues, the results show that very substantial stabilization can be afforded to carbocations positioned appropriately over any portion of a benzene ring and at distances considerably greater than typical covalent bonding distances. Thus, an enforced separation between carbocation and aromatic amino acid side chain residue to avoid unwanted covalent bond formation between protein and substrate would still allow substantial stabilization via carbocation−π interaction.