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Characterization of Cation–π Interactions in Aqueous Solution Using Deuterium Nuclear Magnetic Resonance Spectroscopy
Author(s) -
Zhu Dongqiang,
Herbert Bruce E.,
Schlautman Mark A.,
Carraway Elizabeth R.
Publication year - 2004
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2004.2760
Subject(s) - chemistry , aqueous solution , benzene , mesitylene , magic angle spinning , inorganic chemistry , nuclear magnetic resonance spectroscopy , molecule , naphthalene , crystallography , stereochemistry , organic chemistry
ABSTRACT 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 π 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–π interactions has been extremely limited to date. In this paper, cation–π interactions in aqueous salt solutions were characterized via 2 H nuclear magnetic resonance (NMR) spin–lattice relaxation times ( T 1 ) and calculations of molecular correlation times (τ 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 AgNO 3 , with the largest effects observed in the AgNO 3 and CsCl solutions. Upon normalizing τ c values by solution viscosity effects, an overall affinity trend of Ag + ≫ 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 NH 3 , 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–π interactions. The relatively strong cation–π bonding observed between Ag + and aromatic hydrocarbons probably results from covalent interactions between the aromatic π electrons and the d orbitals of Ag + , in addition to the normal electrostatic interaction.