Abstract
Isotopic separation factors of lithium ion complexed by crown-ethers are obtained from statistical mechanics. The effect of the organic arm which links the crown-ether to a silica matrix is considered. Ab initio HF/6-31+G* calculations, PM3 semiempirical calculations and hybrid quantum mechanics/molecular mechanics PM3/MM Molecular Dynamics simulations have been performed.
In vacuum, it is found that 7Li is better complexed than 6Li by the crown-ether with or without its organic arm, in contrast to what is experimentally observed by liquid chromatography with ethanol solvent and crown-ethers grafted on silica.
To model the solvent effect, some ethanol molecules are first included in the PM3 calculations to solvate the free lithium isotopes and the lithium isotopes complexed by the crown-ether. The method reproduces the observation that 6Li isotope is more complexed than 7Li, but the hierarchy of the experimental separation factor in the crown-ether series is not correctly recovered.
The PM3/MM Molecular Dynamics simulations of the crown-ether 12C4 system show that when taking into account the effect of the organic arm, it is possible to find a separation factor in agreement with the experimental result.