Near-infrared spectral studies for investigating the hydration of 18-crown-6 in aqueous solutions

The near-infrared difference spectra of 18-crown-6 in aqueous solutions have been obtained in the 1350-1650 nm region. Resolution of the difference spectra yielded the spectra of hydrated 18-crown-6 molecule and with the help of density data has allowed the determination of hydration numbers of 18-crown-6 at different concentrations (0.5-4 m). The hydration number obtained at 1 m concentration of 18-crown-6 is about similar to 20, while at higher concentrations the value is around 4. These results have been interpreted in terms of hydrophobic hydration of 18-crown-6 in the dilute concentration region, resulting in clathratelike equilibria, while at higher concentrations, four water molecules remain hydrogen-bonded to the crown ether ring having D-3d conformation. These complexed water molecules have been further differentiated into two types: (i) two water molecules doubly hydrogen-bonded (bridging) to the crown ether oxygen atoms and (ii) the other two singly hydrogen-bonded to the remaining oxygen atoms and rapidly exchangeable.