K. Patil et R. Pawar, Near-infrared spectral studies for investigating the hydration of 18-crown-6 in aqueous solutions, J PHYS CH B, 103(12), 1999, pp. 2256-2261
The near-infrared difference spectra of 18-crown-6 in aqueous solutions hav
e been obtained in the 1350-1650 nm region. Resolution of the difference sp
ectra 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-c
rown-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 concentrati
ons, four water molecules remain hydrogen-bonded to the crown ether ring ha
ving D-3d conformation. These complexed water molecules have been further d
ifferentiated into two types: (i) two water molecules doubly hydrogen-bonde
d (bridging) to the crown ether oxygen atoms and (ii) the other two singly
hydrogen-bonded to the remaining oxygen atoms and rapidly exchangeable.