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

Solid-state H-2 NMR powder spectra and T-1 longitudinal relaxation dat a have been collected for carceplexes containing benzene and acetonitr ile guest molecules. Analysis of the data suggests that benzene rotate s 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 carce plex cavity with a rate (ca. 10(6)-10(7) s(-1)) that is in the interme diate regime on the H-2 NMR spectral timescale in the temperature rang e T = 310-400 K. Lineshape simulations suggest that this reorientation has less than four-fold symmetry owing to unequal site populations, p robably a reflection of the deviation from C-4 symmetry of the carcepl ex 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.