- Computational and NMR Studies on the Complexation of Lithium Ion to 8-Crown-4
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Lithium ion selective crown ethers have been the subject of much research for a multitude of applications. Current research is aimed at structurally rigidifying crown ethers, as restructuring of the crown ether ring upon ion binding is energetically unfavorable. In this work, the lithium ion binding ability of the relatively rigid 8-crown-4 was investigated both computationally by density functional theory calculations and experimentally by 1H and 7Li NMR spectroscopy. Although both computational and experimental results showed 8-crown-4 to bind lithium ion, this binding was found to be weak compared to larger crown ethers. The computational analysis revealed that the complexation is driven by enthalpy rather than entropy, illustrating that rigidity is only of nominal importance. To elucidate the origin of the favorable interaction of lithium ion with crown ethers, activation strain analyses and energy decomposition analyses were performed pointing to the favorable interaction being mainly electrostatic in nature. 8-crown-4 presents the smallest crown ether reported to date capable of binding lithium ion, possessing two distinct conformations from which it is able to do so.
- van der Ham, Alex,Hansen, Thomas,Lodder, Gerrit,Codée, Jeroen D. C.,Hamlin, Trevor A.,Filippov, Dmitri V.
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- Dynamic NMR Study of 1,3,5,7-Tetraoxacyclooctane in Liquid and Liquid Crystalline Solutions and in the Solid State
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Deuterium, proton, and carbon-13 NMR spectra of 1,3,5,7-tetraoxacyclooctane (tetroxocane) were recorded in liquid crystalline and isotropic solutions as well as in the solid state.In solution, tetroxocane exists as a mixture of the boat-chair (BC) and crown (Cr) conformers.Pseudorotation and inversion is extremely fast in the BC form, while inversion in the crown conformer is relatively slow (on the NMR time scale).Analysis of the NMR line shape and 2D exchange experiments shows that the ring inversion in the Cr form occurs via the BC form.Interconversion rate constants and equilibrium constants between the two forms over a wide temperature range were determined in several normal and liquid crystalline solvents.In the solid state tetroxocane crystallizes in a (slightly distorted) crown form.Deuterium NMR in a powder sample of solid tetroxocane-d8 exhibits dynamic spectra consistent with molecular 4-fold jumps about their pseudo-C4 axes.Quantitative analysis of the dynamic line shape is consistent with a small C2 deformation of the molecule, as found previously by X-ray analysis, and yields the following kinetic equation for the jump rate kJ(s-1)=5.9E16exp(-15.9/RT), where R is in kcal mol-1 K-1.
- Calucci, L.,Zimmermann, H.,Poupko, R.,Luz, Z.
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