Mechanism and thermodynamics of ion selectivity in aqueous solutions of 18-crown-6 ether. A molecular dynamics study

Journal Article · Wed Jul 05 04:00:00 EDT 1995 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja00131a018· OSTI ID:81697

Dang, L X ^[1]

---

1. Pacific Northwest Lab., Richland, WA (United States)

We have performed extensive classical molecular dynamics simulations to examine the mechanism and thermodynamics for ion selectivity of 18-crown-6 ether in aqueous solutions. We have computed the free energy profiles or potentials of mean force and the corresponding binding free energies for M{sup +}:18-crown-6 (M{sup +} = K{sup +}, Na{sup +}, Rb{sup +}, Cs{sup +}) complexation in water. To the best of our knowledge, the present work is the first to employ the potential of mean force approach to the evaluation of crown ether selectivity in an aqueous solution. The resultant potentials of mean force indicate that minima free energy surfaces for K{sup +} and Na{sup +} are located at the crown ether center-of-mass. A well-defined second minimum is also observed in the potential of mean force of the Na{sup +}:18-crown-6 complex in water. It appears that K{sup +} is selected over Na{sup +} because of the greater free energy penalty associated with displacing water molecules from Na{sup +} as it approaches the crown ether. This leads to a substantial increase in the activation free energy and a decrease in the corresponding binding free energy for Na{sup +}:18-crown-6 complexation as compared to K{sup +}:18-crown-6. The selection of K{sup +} over Rb{sup +} and Cs{sup +} is due to the size of the cation relative to that of the crown ether cavity. 26 refs., 11 figs., 2 tabs.

• Sponsoring Organization: USDOE
• DOE Contract Number: AC06-76RL01830
• OSTI ID: 81697
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 26 Vol. 117; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
44 INSTRUMENTATION
INCLUDING NUCLEAR AND PARTICLE DETECTORS
66 PHYSICS
99 GENERAL AND MISCELLANEOUS
AQUEOUS SOLUTIONS
BINDING ENERGY
CHEMICAL REACTIONS
COMPLEXES
CROWN ETHERS
FREE ENERGY
ION SELECTIVE ELECTRODE ANALYSIS
POTENTIALS
THERMODYNAMIC MOLECULAR MODEL
THERMODYNAMICS