The influence of substituents on cation-π interactions.: 4.: Absolute binding energies of alkali metal cation -: Phenol complexes determined by threshold collision-induced dissociation and theoretical studies

The kinetic energy dependence of the collision-induced dissociation of M+(C6H5OH) and M+(C6H5OH)(2) with Xe is studied using guided ion beam mass spectrometry. M+ includes the following alkali metal cations: Li+, Na+, K+, Rb+, and Cs+. The primary dissociation channel observed in all complexes is endothermic loss of an intact phenol ligand. Sequential dissociation of a second phenol ligand is observed at elevated energies in the bis-complexes. The cross section thresholds for the primary dissociation channel are interpreted to yield 0 and 298 K bond dissociation energies for (C6H5OH)(x-1)M+-C6H5OH, x = 1-2, after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energies of the reactants, and dissociation lifetimes. Ab initio and density functional theory calculations at the MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* level of theory are used to determine the structures, molecular constants, and theoretical binding energies of these complexes. The agreement between theory and experiment is good when full electron correlation is included, except for the Li+(C6H5OH) complex, and somewhat less satisfactory when effective core potentials are used. The experimental value for Na+-C6H5OH bond energy determined here is within experimental error of the value previously reported. The trends in M+(C6H5OH)(x) binding energies are explained in terms of varying magnitudes of electrostatic interactions and ligand-ligand repulsion in the complexes. Comparisons are also made with the other cation-pi complexes to benzene to examine the influence of the hydroxyl substituent on the binding, and the factors that control the strength of cation-pi interactions.