Manifestation of chiral recognition of camphor enantiomers by alpha-cyclodextrin in longitudinal and transverse relaxation rates of the corresponding 1:2 complexes and determination of the orientation of the guest inside the host capsule

The 1:2 complexes of camphor enantiomers with alpha-cyclodextrin in (2)H(2)O manifest differences in longitudinal and transverse relaxation rates of camphor methyl protons owing to chiral recognition. The relaxation data obtained at two magnetic fields were quantitatively analyzed using the model of anisotropic overall tumbling with internal motion. In experimental conditions (guest-to-host ratio = 1:20, T = 300.6K), all camphor molecules are complexed. The complexes are not rigid but the rotational diffusion of camphor enantiomers embedded inside the capsules formed by two alpha-cyclodextrin hosts is well outside the extreme narrowing region. Both differences in the anisotropic overall tumbling and internal rotation of all methyl groups participate in enantiomeric differentiation of the relaxation rates. Anisotropic tumbling of camphor molecules provides information on the orientation of the guest in the host capsule that for the complex under study could not be obtained by other methods.