Probing cations recognized by a crown ether with the 3D-RISM theory (original) (raw)
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Journal of the American Chemical Society, 1990
Molecular dynamic simulations are used to predict the binding affinity in host-guest systems by the thermodynamic cycleperturbation method. The relative free energy of solvation of Na+ and K+ in methanol (19.8 and 19.1 kcal mol-') and the relative free energy of binding of Na+ and K+ to 18-crown-6 in methanol (-3.8 and-3.0 kcal mol-') are calculated by thermodynamic integration and thermodynamic perturbation, respectively. These results are in reasonable agreement with the experimental values, 17.3 and-2.47 kcal mol-', respectively. In addition, the contributions to the relative free energy from the internal energy and entropy are calculated. Finally, a detailed analysis is made of the structure and its fluctuations in this system to provide additional insight to the selectivity of binding. Since their discovery in 1967 by Pedersen' at Du Pont, crown ethers have been a very important member of the class of molecules known as organic Although synthetic, crown ethers share with enzymes and other biological molecules the capacity of recognizing and selectively binding ions or other molecules. Crown ethers display a wide range of binding specificities, in part because they range in size from a ring containing as few as 9 ring atoms to more than 30.4 In addition, the ring can be modified to decrease its flexibility by replacing the saturated carbon-carbon linking group with aromatic carbon^.^ Substitutions can also be made in which chains are added in such a fashion that they aid in complex formation from above or below the plane of the ring.* Originally, it was proposed that the selectivity of crown ethers depended mainly on the size of the guest relative to the cavity
Ab initio investigation of the structure and alkali metal cation selectivity of 18-crown-6
J Am Chem Soc, 1994
We present an ab initio, quantum mechanical study of 18-crown-6 (18~6) and its interaction with the alkali metal cations Li+, Na+, K+, Rb+, and Cs+. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31+G*) and relativistic effective core potentials. Electron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NEiO) and associated methods. The affinity of 18c6 for the alkali metal cations is quite strong (50-100 kcal mol-', depending on cation type), arising largely from the electrostatic (ionic) interaction of the cation with the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as the electrostatic interaction. Agreement of the calculated binding enthalpies and experimentally determined quantities is rather poor. For example, the binding energy for K+/18c6 (-71.5 kcal mol-') is about 30 kcal mol-' stronger than that determined by experiment, and it is not clear how to reconcile this difference. Our calculations clearly show that solvation effects strongly influence cation selectivity. Gas-phase 18c6 preferentially binds Li+, not K+ as found in aqueous environments. We show, however, that K+ selectivity is recovered when even a few waters of hydration are considered.
An Ab Initio Investigation of the Structure and Alkali Metal Cation Selectivity of 18-Crown-6
Journal of the American Chemical Society, 1994
We present an ab initio, quantum mechanical study of 18-crown-6 (18~6) and its interaction with the alkali metal cations Li+, Na+, K+, Rb+, and Cs+. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31+G*) and relativistic effective core potentials. Electron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NEiO) and associated methods. The affinity of 18c6 for the alkali metal cations is quite strong (50-100 kcal mol-', depending on cation type), arising largely from the electrostatic (ionic) interaction of the cation with the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as the electrostatic interaction. Agreement of the calculated binding enthalpies and experimentally determined quantities is rather poor. For example, the binding energy for K+/18c6 (-71.5 kcal mol-') is about 30 kcal mol-' stronger than that determined by experiment, and it is not clear how to reconcile this difference. Our calculations clearly show that solvation effects strongly influence cation selectivity. Gas-phase 18c6 preferentially binds Li+, not K+ as found in aqueous environments. We show, however, that K+ selectivity is recovered when even a few waters of hydration are considered.
Journal of Molecular Structure: THEOCHEM, 2010
The binding interaction of alkali metal ions (charged guest) within the narrow cavity of crown ethers (neutral host) of different cavity size has been studied using quantum chemical density functional theory. Different conformational structures, binding energies and various thermodynamic parameters of free crown ethers and their metal ion complexes have been determined with the B3LYP functional using a large split valence 6-311++G(d,p) basis set. Geometry optimization was performed using guess structures obtained from semi-empirical PM3 optimized structures without imposing any symmetry restriction. The calculated values of binding enthalpy increase with increase in cavity size, i.e., with increase in donor O atoms and are found to be in good agreement with gas phase experimental results. We have demonstrated the effect of micro-solvation on the binding interaction between the alkali metal ions (Li + and Na +) and the macrocyclic crown ethers by considering micro-solvated metal ions up to six water molecules directly attached to the metal ion. A metal ion exchange reaction involving the replacement of lithium ion in metal ion-crown ether complexes with sodium ion contained within a metal ion-water cluster serves as the basis for modeling binding preferences in solution. An attempt has been made to study the effect of micro-solvation on the binding interaction of metal ions with crown ethers by considering two water molecules attached to metal ion-crown ether complexes. The calculated OAH stretching frequency of H 2 O molecule in micro-solvated metal ion-crown complexes is less blue-shifted in comparison to hydrated metal ions. The calculated IR spectra can be compared with an experimental spectrum to determine the presence of micro-solvated metal ion-crown ether complexes in extractant phase.
Analytical Chemistry, 1990
A simple three-electrode polarographic assembly with the electrolyte dropplng electrode for the study of the Ion transport across a IIquM-Hquid Interface was presented. Transfer of alkall-metal catlons from water to nltrobenzene (NB) or 1,2-dlddwoethane (1,2-DCE) facimatd by complex formation with dlbenzo-18trown-6, dlbenzo-24trown-8, or dlbenzo-30-crown-10 was studied. Experimental current vs potential data were used to clarlfy the mechanism of the ion transport and to evaluate the stablllty constants of crown ether complexes. The stabNlty constants are 105-107 larger in NB and 10'o-10'2 larger In 1,P-DCE than those In water. The selectlvlty sequence changes with the solvent, comparing water (K' > Na' > Rb' > Cs') wlth NB (Na' > K' > Rb' > Cs',
American Journal of Research …, 2013
In this account 12-crown-4 (12c4), 15-crown-5 (15c5) and 18-crown-6 (18c6), their cluster complexes with Li+, Na+, K+ with general chemical formula as [M(crown ether)]+ and the water solvated complexes of Li+, Na+, K+ are theoretically studied. The chemical properties of crown ether complexes of Li, Na and K cations are compared with those of water-solvated complexes of these species. The B3LYP/6-31+G(d,p) level of calculation has been used for obtaining equilibrium geometries and Rho(r) functions (electron density distributions). By the aid of fundamental physical theorems implemented in the Quantum Theory of Atoms in Molecules (QTAIM), the structures and the physical nature of the chemical bonds have been determined for the abovementioned species at the B3LYP/6-31+G(d,p)computational level. These results establish the Metal-oxygen in all complexes in this work as ionic. Also Li+, Na+ and K+ have coordination number of 4 with 12c4 and possess the coordination number of 5 with 15c5. But the Li+ shows the coordination number of 3 with 18c6 crown ether and Na+ and K+ exhibits the coordination number of 6.
Ion Selectivity of Crown Ethers Investigated by UV and IR Spectroscopy in a Cold Ion Trap
The Journal of Physical Chemistry A, 2012
Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M + •B15C5 and M + •B18C6 (M = Li, Na, K, Rb and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na + •B15C5 and K + •B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating the optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are
Electroanalysis, 2002
Lariat crown ethers (LCEs) offer the ability to bind selectively alkali metal ions. The presence of a coordinating functionality in the sidearm chain provides a means to further tune the selectivity of LCEs. A series of four LCEs based on a 15-crown-5 structural motif were synthesized with variations in the coordinating functionality of the sidearm and the substitution at the carbon pivot atom. Poly(vinyl chloride) liquid-membrane ion-selective electrodes were prepared with each of the four ionophores, and their response patterns to various alkali metals were determined. The results of this experimentation revealed that the coordinating ability of the sidearm is more important than the substitution at the geminal group at the pivot atom. It was also demonstrated that the change of a single atom in the sidearm structure, to induce coordinating ability to the sidearm, alters the selectivity between sodium and potassium by roughly five orders of magnitude.
ChemistrySelect, 2021
Novel organomagnesium crown ether molecules have been computationally characterized for the first time using density functional theory (DFT). Monomer units of MgC 6 have been used as building blocks. The potential energy surface of the parent elemental composition, MgC 6 H 2 , has been extensively explored using both DFT and coupled-cluster methods. It is concluded that the seven-membered ring isomer, 1-magnesacyclohept-4-en-2,6-diyne, is the thermodynamically most stable molecule at all levels. Thus, the latter has been used as the building block for organomagnesium crown ethers. Both alkali (Li + , Na + , and K +) and alkaline-earth (Be 2+ , Mg 2+ , and Ca 2+) metal ions selective complexes have been theoretically identified. Binding energies (∆E at 0 K) and thermally corrected Gibbs free energies (∆G at 298.15 K) have been computed for these metal ions with MgC 6-9-crown-3 and MgC 6-12-crown-4 to gauge their binding affinities.