Abstract
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-(OCH 2CH 2) 5], 18-crown-6 [cyclo-(OCH 2CH 2) 6] and CH 3(OCH 2CH 2) m OCH 3 with m = 5 and 6. The conformational changes of the CC and CO 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 CC 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 CO bond is similar for these compounds. The stronger gauche preference of the CC 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.
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