Influence of Substituents on Cation−π Interactions. 2. Absolute Binding Energies of Alkali Metal Cation−Fluorobenzene Complexes Determined by Threshold Collision-Induced Dissociation and Theoretical Studies

Abstract

Threshold collision-induced dissociation of M + (C 6 H 5 CH 3 ) x with Xe is studied using guided ion beam mass spectrometry. M + include the following alkali metal ions: Li + , Na + , K + , Rb + , and Cs + . Both mono- and bis-complexes are examined (i.e., x = 1 and 2). In all cases, the primary and lowest energy dissociation channel observed is endothermic loss of an intact toluene ligand. Sequential dissociation of a second toluene ligand is observed at elevated energies in the bis-complexes. Minor production of ligand exchange products, M + Xe and M + (C 6 H 5 CH 3 )Xe, is also observed. The cross section thresholds for the primary dissociation channel are interpreted to yield 0 and 298 K bond dissociation energies for (C 6 H 5 CH 3 ) x - 1 M + -C 6 H 5 CH 3 , x = 1-2, after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energies of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical binding energies are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) and B3LYP/6-311+G(2d,2p) levels using the B3LYP/ 6-31G* geometries. Zero-point energy and basis set superposition error corrections are also included. The agreement between theory and experiment is reasonably good when full electron correlation is included (for Li + , Na + , and K + ) but is less satisfactory when effective core potentials are used (for Rb + and Cs + ). In all cases, the experimentally determined bond dissociation energies are greater than the theoretically determined values. In most cases, better agreement is found for the MP2 values than the B3LYP values. The trends in M + (C 6 H 5 CH 3 ) x binding energies are explained in terms of varying magnitudes of electrostatic interactions and ligand-ligand repulsion in the complexes. Comparisons are also made to previous experimental bond dissociation energies of M + (C 6 H 6 ) x to examine the influence of the methyl substituent on the binding, and the factors that control the strength of cation-π interactions.