Cation-π interactions in competition with cation microhydration: a theoretical study of alkali metal cation-pyrene complexes

Cation-pi interactions were systematically investigated for the adsorption of H+ and alkali metal cations M+ to pyrene by means of Moller-Plesset perturbation theory (MP2) and density functional theory (DFT). The main aims were to determine the preferred adsorption sites and how the microhydration shell influences the adsorption process. The preferred adsorption sites were characterized in terms of structural parameters and energetic stability. Stability analysis of the M+-pyrene complexes revealed that the binding strength and the barrier to transitions between neighboring sites generally decreased with increasing cation size from Li+ to Cs+. Such transitions were practically barrierless (<< 1 kcal/mol) for the large Rb+ and Cs+ ions. Further, the influence of the first hydration shell on the adsorption behavior was investigated for Li+ and K+ as representatives of small and large (alkali metal) cations, respectively. While the isolated complexes possessed only one minimum, two minima-corresponding to an inner and an outer complex-were observed for microhydrated complexes. The small Li+ ion formed a stable hydration shell and preferentially interacted with water rather than pyrene. In contrast, K+ favored cation-pi over cation- water interactions. It was found that the mechanism for complex formation depends on the balance between cation-p interactions, cation-water complexation, and the hydrogen bonding of water to the pi-system.