GUEST DYNAMICS IN CARCEPLEXES - A H-2 NMR-STUDY

Solid-state H-2 NMR powder spectra and T-1 longitudinal relaxation data have been collected for carceplexes containing benzene and acetonitrile guest molecules. Analysis of the data suggests that benzene rotates rapidly about its C-6 axis with an activation energy of 25.8 +/- 0.4 kJ mol(-1). In addition, it reorients about the C-4 axis of the carceplex cavity with a rate (ca. 10(6)-10(7) s(-1)) that is in the intermediate regime on the H-2 NMR spectral timescale in the temperature range T = 310-400 K. Lineshape simulations suggest that this reorientation has less than four-fold symmetry owing to unequal site populations, probably a reflection of the deviation from C-4 symmetry of the carceplex geometry. Acetonitrile reorients rapidly (much greater than 10(5) s(-1)) about a four-fold symmetric axis for T greater than or equal to 181 K.