Influence of substituents on cation-π interactions -: 5.: Absolute binding energies of alkali metal cation-anisole complexes determined by threshold collision-induced dissociation and theoretical studies

Threshold collision-induced dissociation (CID) techniques are employed to determine the bond dissociation energies of cation-pi complexes of anisole and the alkali metal cations. Both mono and sandwich complexes to Li+, Na+, K+, Rb+, and Cs+ are examined. In every complex, the primary and lowest energy dissociation pathway observed is endothermic loss of an intact anisole ligand. Sequential dissociation of a second anisole ligand is observed at elevated energies in the sandwich complexes. Ligand exchange reactions to produce M+Xe and M+(C6H5OCH3)Xe are also observed as minor reaction pathways. The molecular constants necessary for the thermodynamic analysis of the experimental data as well as the structures of these complexes are determined from B3LYP/6-31G* calculations. Theoretical binding energies are determined from single point calculations at the MP2(full)/6-311+G(2d, 2p) level using the B3LYP/6-31G* optimized geometries. The agreement between theory and experiment is very good in all cases except for the Li+(C6H5OCH3) complex. The trends in the bond dissociation energies of these complexes to anisole as well as those to other pi ligands previously studied, aniline, benzene, fluorobenzene, phenol, and toluene, confirm that these cation-pi complexes are noncovalently bound. Comparisons amongst these pi ligands are made to examine the influence of the substituent on the binding, and the factors that control the strength of cation-pi interactions.