Specific solute-solvent interactions: The ethene⁺...Ar complex

A molecule-based description of charge delocalization is introduced to describe interactions which depend upon intermolecular orbital overlap in order to investigate the origin of stabilization in specific solute-solvent complexes. Ionization potentials of aromatic "solute" molecules are shifted to lower energy in rare gas clusters, which has been attributed previously to charge-induced dipole interactions; however, the present work reveals that a charge "delocalization" mechanism may be operative in certain systems. This is due primarily to charge-transfer (CT) effects. A relationship between this interaction and the difference between solute and solvent ionization potentials is derived. The ethene⁺...Ar complex is examined as a specific case. We report the results of ab initio molecular orbital (MO), localized molecular orbital (LMO), and valence-bond (VB) studies of the C₂H₄⁺...Ar complex to provide a VB rationalization for the origin of the stability of the complex. The advantage of the VB treatment employed in the present work is that it allows a natural separation between polarization and CT terms, so it could be shown that the CT interaction provides a key contribution to the stabilization of the C₂H₄⁺...Ar complex. These results suggest that intermolecular charge-transfer resonances may to play a significant role in delocalizing charge among a charged ("solute") molecule and suitably proximate neutral ("solvent") molecules in a cluster.