RAMAN-SPECTROSCOPIC STUDY OF THE HYDRATES OF 18-CROWN-6

The hydrates of 18-crown-6 have been studied by Raman spectroscopy. The analysis of the Raman spectra, in combination with the examination of the liquidus curve, of the 18-crown-6-water system shows the formation of at least four distinct hydrates at liquid nitrogen temperature. The ratios of 18-crown-6 to water for the two hydrates are 1:6 and 1:4.5, and their congruent melting points are 23.7 and 24.0 degrees C, respectively. The conformation of the crown ring in these hydrates is essentially D-3d The 1:6 hydrate agrees with the hexahydrate determined by a recent crystal structure analysis. The 1:4.5 hydrate is suggested to have a hydration structure similar to that predicted by Monte Carlo simulations. The present experimental findings are well correlated to the results of previous computer simulations.