Guest orientation within carceplexes and capsules was determined qualitativ
ely from NMR data, and the molecular mobility of guests was determined via
coalescence of H-1 NMR signals. Both are highly dependent on guest size and
shape, as is interconversion of twistomers. Incarceration of 1,4-thioxane
results in a large (1.8 kcal/mol) constraint on thioxane's conformational m
obility (chair-to-chair interconversion). Similar conformational constraint
s (1.6 kcal/mol) were determined for 1,4-dioxane both when incarcerated in
carceplex 1b and when encapsulated reversibly in capsule 3b. Encapsulation-
induced conformational constraints of this magnitude are unprecedented, and
are particularly striking for the noncovalently linked capsules.