A theoretical study on the conformations, energetics, and solvation effects on the cation-π interaction between monovalent ions Li+, Na+, and K+ and naphthalene molecules

The conformational structures of the cation-pi interaction between monovalent cations (M+ = Li+, Na+, and K+) and one or two naphthalene molecules are studied at the density functional level of theory with Becke's three parameters (B3LYP). The 6-311G(2d,p) basis sets are used except in the case of potassium, where the contracted [8s4p1d/14s9p1d] basis set was used. Two stationary points are found for M+ (C10H8). One global minimum (GM) has the ion on the near center above one of the two C-6-rings of the naphthalene plane and the other with C-2v symmetry has the ion just on the C-2-axis above naphthalene. The calculated binding energies are 43.3 (Li+), 28.3 (Na+), and 17.5 (K+) kcal/mol, and follow the conventional electrostatic trend (Li+, largest; K+, smallest). The activation energies for the ion transfer between two equal minima GM also follows the same trend: 6.9 (Li+), 2.0 (Na+), and 0.6 (K+) kcal/mol. The most stable isomer (D3) of M+ (C10H8)(2) is a ferrocene type complex having C-2h symmetry, where the two naphthalene molecules shift horizontally to minimize the repulsion of two planes. The second binding energies of Li+(C10H8)(2), Na+(C10H8)(2), and K+(C10H8)(2) are 23.5, 20.8, and 14.1 kcal/mol, respectively. The larger ligand-ligand repulsion in Li+(C10H8)(2) leads to a second naphthalene binding energy, that is about 20 kcal/mol lower than the first, which is more than five times the reduction found for K+. The aqueous solvation effect is examined with the Self-Consistent Reaction Field (SCRF) method based on Tomasi's Polarized Continuum Model (PCM). A partial reordering (Li+ > K+ > Na+) occurs in the relative aqueous solvation free energies, but the full reordering (K+ > Na+ > Li+), obtained by Kumpf and Dougherty in M+(C6H6)(2), does not hold good in the 2 : 1 naphthalene : ion complexes. (C) 1999 Elsevier Science B.V. All rights reserved.