Guest dynamics in carceplexes: a 2H NMR study

Abstract

Solid-state ²H NMR powder spectra and T₁ 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 C₆ axis with an activation energy of 25.8 ± 0.4 kJ mol^–1. In addition, it reorients about the C₄ axis of the carceplex cavity with a rate (ca. 10⁶–10⁷ s^–1) that is in the intermediate regime on the ²H 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 C₄ symmetry of the carceplex geometry. Acetonitrile reorients rapidly ( 10⁵ s^–1) about a four-fold symmetric axis for T 181 K.