Studies of Inclusion Complexes of Dichloromethane in Cryptophanes by Exchange Kinetics and 13C NMR in Solution and the Solid State

Cryptophanes are molecular hosts having high binding affinities for small, neutral molecules in weakly polar solvents. We have previously studied dichloromethane as a guest inside the cavity of cryptophane-E (cryptophane-333), in solution as well as in the solid state. Here, we present a solution 13C relaxation study of dichloromethane as the guest molecule complexed in the cavity of two smaller hosts: cryptophane-A (cryptophane-222) and cryptophane-223. 13C relaxation measurements were performed for both guest and host molecules. Exchange kinetics have to be explicitly taken into consideration in order to derive 13C relaxation properties of the bound guest. Rate coefficients were measured using proton 1D-exchange experiments. We have also investigated dichloromethane complexed in the cavity of cryptophane-233 in the solid state, by estimating the motionally averaged 13C−1H dipolar interactions from recoupling experiments under magic angle spinning (MAS) conditions. The measurements for the three cryptophanes provide a consistent set of results for the extent of rotational freedom of the guest inside the host cavities.