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

Abstract

The conformational structures of the cation– π 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 10H 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 10H 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 10H 8) 2, Na +(C 10H 8) 2, and K +(C 10H 8) 2 are 23.5, 20.8, and 14.1 kcal/mol, respectively. The larger ligand–ligand repulsion in Li +(C 10H 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 hold good in the 2 : 1 naphthalene : ion complexes.