Symmetry, disorder, and dynamics in solid crown ether complexes: 2H NMR studies of 15-crown-5·NaI, 15-crown-5·NaClO4, and 21-crown-7·KI

Abstract

2H NMR line shapes have been obtained as a function of temperature for partially deuteriated 15-crown-5·NaI, 15-crown-5·NaClO4 and 21-crown-7·KI. Sudden changes in the line shapes above 330 K correspond to phase transitions: DSC shows transitions at 338, 347 K for 15-crown-5·NaI, and 356, 374, 383 K for 15-crown-5·NaClO4. The 2H NMR line shapes for the room temperature (RT) phases show the onset of a motion of the macrocycles above 200K, which is rapid a little above room temperature. Through detailed analysis of the complicated line shapes it has been shown that the most consistent interpretation of the dynamics in these RT phases is a "merry-go-round" type of motion similar to that found for 18-crown-6 and 12-crown-4 complexes in which O-CH2-CH2 units exchange sites around the ring, simultaneously adjusting their conformation to fit the site. In the high-temperature phases of all three complexes all the ring atoms are dynamically equivalent. This can only happen through a combination of increased symmetry and disorder for which possible models have been devised.Key words: 2H NMR, crown ether complexes, dynamics, disorder.