Cation-π interactions with a model for an extended π network -: Absolute binding energies of alkali metal cation-naphthalene complexes determined by threshold collision-induced dissociation and theoretical studies

Threshold collision-induced dissociation (CID) techniques are employed to determine the bond dissociation energies (BDEs) of both mono and bis cation-pi complexes of the alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) with naphthalene (C10H8). The primary and lowest energy dissociation channel observed in all cases is endothermic loss of an intact naphthalene ligand. Sequential dissociation of a second naphthalene ligand is observed at elevated energies in the bis complexes. The ligand exchange products, M+Xe and M+(C10H8)Xe, are also observed in minor yield. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures, vibrational frequencies, and rotational constants of these complexes and their primary dissociation products. Theoretical binding energies are determined from single point energy calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31G* geometries. The agreement between theory and experiment is reasonably good for the Li+, Na+, and K+ complexes where full electron correlation is included, except for the Li+(C10H8) complex. Somewhat less satisfactory agreement is found for the Rb+ and Cs+ complexes where effective core potentials (ECPs) are used. The trends in the BDEs of these complexes to naphthalene as well as those to other pi ligands previously examined, aniline, anisole, benzene, fluorobenzene, phenol, and toluene, confirm the noncovalent nature of the bonding in such cation-pi complexes. Comparisons amongst these pi ligands are made to examine the influence of the extended pi network on the binding and the factors that control the strength of cation-pi interactions. (C) 2003 Elsevier Science B.V. All rights reserved.