What Is the Structure of the Naphthalene-Benzene Heterodimer Radical Cation? Binding Energy, Charge De localization, and Unexpected Charge-Transfer Interaction in Stacked Dimer and Trimer Radical Cations

The binding energy of the naphthalene(+center dot)(benzene) heterodimer cation has been determined to be 7.9 +/- 1 kcal/mol for C10H8+center dot(C6H6) and 8.1 +/- 1 kcal/mol for C10H8+center dot(C6D6) by equilibrium thermochemical measurements using the mass-selected drift cell technique. A second benzene molecule binds to the C10H8+center dot(C6D6) dimer with essentially the same energy (8.4 +/- 1 kcal/mol), suggesting that the two benzene molecules are stacked on opposite sides of the naphthalene cation in the (C6D6)C10H8+center dot(C6D6) heterotrimer. The lowest-energy isomers of the C10H8+center dot(C6D6) and (C6D6)C10H8+center dot(C6D6) dimer and trimer calculated using the M11/cc-pVTZ method have parallel stacked structures with enthalpies of binding (-Delta H degrees) of 8.4 and 9.0 kcal/mol, respectively, in excellent agreement with the experimental values. The stacked face-to-face class of isomers is calculated to have substantial charge-transfer stabilization of about 45% of the total interaction energy despite the large difference between the ionization energies of benzene and naphthalene. Similarly, significant delocalization of the positive charge is found among all three fragments of the (C6D6)C10H8+center dot(C6D6) heterotrimer, thus leaving only 46% of the total charge on the central naphthalene moiety. This unexpectedly high charge-transfer component results in activating two benzene molecules in the naphthalene(+center dot)(benzene)(2) heterotrimer cation to associate with a third benzene molecule at 219 K to form a benzene trimer cation and a neutral naphthalene molecule. The global minimum of the C10H8+center dot(C6H6)(2) heterotrimer is found to be the one where the naphthalene cation is sandwiched between two benzene molecules. It is remarkable, and rather unusual, that the binding energy of the second benzene molecule is essentially the same as that of the first. This is attributed to the enhanced charge-transfer interaction in the stacked trimer radical cation.