Ab initio molecular orbital study of cation-π binding between the alkali-metal cations and benzene

We present optimized geometries and binding energies for alkali-metal cation complexes with benzene. Results are obtained for Li+ through Cs+ at the RHF/6-311G* and MP2/6-311+G* levels of theory with K+, Rb+, and Cs+ represented by relativistic ECPs and associated valence basis sets. RHF/6-311G* frequencies are used to verify the optimized geometries are minima and used to calculate binding enthalpies. The effects of basis-set superposition error (BSSE) are estimated at both the RHF and MP2 levels. We obtain BSSE-corrected MP2/6-311+G* binding enthalpies (in kcal/mol) of 35.0 (Li+), 21.0 (Na+), 16.0 (K+), 13.3 (Rb+), and 11.6 (Cs+). The distances (Angstrom) between the center of the benzene ring and the cation are 1.865 (Li+), 2.476 (Na+), 2.894 (K+), 3.165 (Rb+), and 3.414 (Cs+). Additional single-point CCSD(T)/6-311+G*//MP2/6-311+G* and CCSD(T)/6-311+G(2d,2p)//MP2/6-311+G* calculations indicate that the MP2/6-311+G* results are well converged with regard the extent of electron correlation, whereas small changes in binding energy are still observed when larger basis sets are used. Additional calculations using local and nonlocal density functional theory are included for comparison.