A molecular merry-go-round: Motion of the large macrocyclic molecule 18-crown-6 in its solid complexes studied by2H NMR

2H NMR line shape measurements were used to confirm and refine a model to account for large-amplitude motions in solid 18-crown-6 complexes previously identified from 13C and 1H NMR results. Reminiscent of the motion of a merry-go-round, the motion is a combined rotation and conformational adjustment of the macrocycle, in which individual -OCH2CH2- units jump to adjacent sites in the crystal. Specifically, for the malononitrile complex, 18-crown-6·2CH2(CN)2, jump rates were obtained by modeling the intermediate-rate line shapes, yielding an activation energy of 47.6 ±0.8 kJ/mol. Because the motion does not introduce disorder, it is diffraction invisible