Absolute binding energies of alkali-metal cation complexes with benzene determined by threshold collision-induced dissociation experiments and ab initio theory

The sequential bond dissociation energies (BDEs) of the mono- and bis-benze ne complexes with alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) are det ermined experimentally:by collision-induced dissociation (CID) with Xe in a guided ion beam mass spectrometer and theoretically by ab initio calculati ons. The kinetic energy dependence of the CID cross sections are analyzed t o yield 0 and 298 K bond energies for (C6H6)(x-1)M+-C6H6 (x = 1-2) after ac counting for the effects of the internal energies of the reactant ions, the multiple collisions of the ions with xenon, and the dissociation lifetimes of the ionic complexes. Ab initio binding energies are calculated at the M P2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* level and corrected for zero-poin t energies (ZPE) and basis set superposition errors (BSSE). The theoretical BDEs are in reasonably good agreement with the experimentally determined 0 K bond energies when full electron correlation is included (for Li+, Na+, and K+) but differ appreciably when effective core potentials (ECPs) are us ed for the K+, Rb+, and Cs+ metal ions. The trends in M+(C6H6)(x) binding e nergies are explained in terms of varying magnitudes of electrostatic inter actions and ligand-ligand repulsions in the complexes. Agreement between ou r BDEs and the few previous experimental M+(C6H6)(x) BDEs is found to be go od in most cases. Comparisons are also made to previous theoretical M+(C6H6 )(x) BDEs in the literature and to the experimental BDEs of alkali-metal io n-water and alkali-metal ion-dimethyl ether complexes.