Quantum chemistry study of the interactions of Li+, Cl-, and I- ions with model ethers

The geometries and energies of complexes of Li+, Cl-, and I- with methane and model ether molecules have been studied using ab initio electronic structure calculations. For Li+, a [5s3p2d] basis set which accurately describes core electrons was derived. For Cl-, basis sets as large as [8s7p4d1f] were considered, while for I- an 2sp1d ECP basis set augmented by a set of diffuse functions as large as [5sp4d1f] was employed. Calculations were performed at the SCF and MP2 levels of theory, and the effects of basis set superposition error on binding energies were considered. For the methane and ether molecules both D95** and cc-pVTZ basis sets with additional diffuse s and p functions were employed. The binding energies of Li+ to methane, dimethyl ether, and ttt 1,2-dimethoxyethane (DME) are found to be around 10, 40, and 40 kcal/mol, respectively. The binding energy of Li+ to tgt DME is approximately 60 kcal/mol due to the favorable interaction of Li+ with both DME oxygen atoms. The binding of Cl- and I- to dimethyl ether is much weaker, around 5-7 kcal/mol. A simple atomistic force field with two-body potential functions representing polarization effects is found to reproduce the ab initio complex energies quite well for the single ligand complexes. Polarization effects contribute significantly to the binding of Li+ to the neutral molecules, while the polarization effects in Cl- and I- complexes with dimethyl ether are relatively weak. The two-body force field accounts only partially for the decrease in binding per ligand in Li+-[O(CH3)(2)](n) complexes with the number of ligands as observed in the quantum chemistry calculations.