Characterization of cation-π interactions in aqueous solution using deuterium nuclear magnetic resonance spectroscopy

Chemical interactions of aromatic organic contaminants control their fate, transport, and toxicity in the environment. Recent molecular modeling studies have suggested that strong interactions can occur between the pi electrons of aromatic molecules and metal cations in aqueous solutions and/or on mineral surfaces, and that such interactions may be important in some environmental systems. However, spectroscopic evidence for these so-called cation-pi interactions has been extremely limited to date. In this paper, cation-pi interactions in aqueous salt solutions were characterized via H-2 nuclear magnetic resonance (NMR) spin-lattice relaxation times (T-1) and calculations of molecular correlation times (tau(c)) for a series of perdeuterated (d(6)-benzene) benzene-cation complexes. The T-1 values for d(6)-benzene decreased with increasing concentrations of LiCl, NaCl, KCl, RbCl, CsCl, and AgNO3, with the largest effects observed in the AgNO3 and CsCl solutions. Upon normalizing tau(c) values by solution viscosity effects, an overall affinity trend of Ag+ much greater than Cs+ > K+ > Rb+ > Na+ > Li+ was derived for the d(6)-benzene-cation complexes. The ability of Ag+ to complex d(6)-benzene was significantly reduced upon addition of NH3, which strongly coordinates Ag+ at high pH. Results with d(6)-benzene, d(8)-naphthalene, d(2)-dichloromethane, and d(12)-cyclohexane in 0.1 M methanolic salt solutions confirmed that spin-lattice relaxation rates are characterizing cation-pi interactions. The relatively strong cation-pi bonding observed between Ag+ and aromatic hydrocarbons probably results from covalent interactions between the aromatic pi electrons and the d orbitals of Ag+, in addition to the normal electrostatic interaction.