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

摘要

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~+ (C_(10)H_8). 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~+ (C_(10)H_8)_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~+ (C_(10)H_8)_2, Na~+ (C_(10)H_8)_2, and K~+(C_(10)H_8)_2 are 23.5, 20.8, and 14.1 kcal/mol, respectively. The larger ligand-ligand repulsion in Li~+(C_(10)H_8)_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~+(C_6H_6)_2, does not hole good in the 2: 1 naphthalene: ion complexes.