Conformational behaviour of cyclic and open-chain poly(oxyethylene) compounds in water studied by infrared spectroscopy

The conformational behaviour of cyclic and open-chain poly(oxyethylene) compounds in water was studied by infrared spectroscopy. The compounds studied are 15-crown-5 [cyclo-(OCH2CH2)(5)], 18-crown-6 [cyclo-(OCH2CH2)(6)] and CH3(OCH2CH2)(m)OCH3 with m = 5 and 6. The conformational changes of the C-C and C-O bonds with varying concentration were analysed by examining the intensity ratio of the relevant conformational key bands. The analysis has shown that the gauche conformation around the C-C bond for the cyclic compounds is much more stabilized in water than that for the corresponding open-chain compounds, although the conformational behaviour of the C-O bond is similar for these compounds. The stronger gauche preference of the C-C bond in the cyclic compounds is associated with their ring structure. The ring geometry constrains the ether oxygens in the molecule to be located in a certain spatial region, where the water molecules can favourably form concerted hydrogen bonds with these oxygens. The hydration structure is suggested to be similar to that in the crystalline hydrates of 18-crown-6. (C) 1998 Elsevier Science B.V. All rights reserved.