A theoretical study on the conformations and energetics on the cation-pi interaction between monovalent ions (M+ = Li+, Na+, and K+) and anthracene and phenanthrene molecules

The conformational structures of the cation-pi interaction between monovale nt cations (M+ = Li+, Na+, and K+) and anthracene and phenanthrene molecule s are surveyed at the hybrid density functional level of theory with the Be cks's three parameters (B3LYP). The 6-31G* basis sets were used except in t he case of potassium ion-complexes, where the 3-21G* basis sets were applie d. Three stationary points (two isomers and one transition state) were foun d in the ion-anthracene complexes, but five stationary points (three isomer s and two transition states) in the ion-phenanthrene complexes. The relativ e stability ladders were calculated at the B3LYP level of theory with the 6 -311G(2d,p) basis sets except in the case of potassium, where the contracte d [8s4p1d/14s9p1d] basis set was applied. The global minimum isomer (Ib: C- s) has the ion (Li+ or Na+) on the near center above the terminal Cs-ring o f the anthracene plane, whereas the global minimum isomer (Ia: D2v) has the K+ ion just on the D-2h-axis above anthracene. The calculated binding ener gies are 43.5 (Li+), 29.1 (Na+), and 17.1 (K+) kcal/mol, following the conv entional electrostatic trend (Li+, largest; K+, smallest). The activation e nergies for the ion transfer (Ib -> Ia both for Li+ and Na+ and Ib <- Ia fo r K+) also follow the same trend: 6.8 (Li+), 2.2 (Na+), and 0.7 (K+) kcal/m ol. The global minimum isomer of ion-phenanthrene complexes (IIb: C-1) has the ion (Li+ and Na+) on the near center above the terminal C-6-ring of phe nanthrene plane. The calculated binding energy of ion-phenanthrene complexe s agrees well with that of the corresponding ion-anthracene complex. The po tential along the direction of K+ ion transfer is really shallow, suggestin g that the K+ ion freely moves over the molecular plane. (C) 2000 Elsevier Science B.V. All rights reserved.