Size makes a difference: Chiral recognition in complexes of fenchone with cyclodextrins studied by means of NMR titration.

Gibbs energies of complex formation between enantiomers of bicyclic terpenoid, fenchone, and naturally occurring cyclodextrins, βCD and γCD, were determined by means of C and H nuclear magnetic resonance (NMR) titration data. These results were compared with the corresponding data obtained previously for the diastereomeric fenchone/αCD complexes. The size of the inner cavity of host molecules significantly influences stoichiometry, association constants, and enantiomeric differentiation of the studied complexes. These complementary data allow us to discuss qualitatively the influence of the host size on the guest-host interactions. A method of the simultaneous use of titration data collected for several resonances of different isotopes in the determination of association constants was worked out and thoroughly analyzed. Comparison of the results of global data analyses with weighted means of individual ones revealed that both these approaches are equally trustworthy.