NMR analyses of cyclodextrin complexes with substituted benzoic acids and benzoate anions

Abstract

Inclusion modes of α-cyclodextrin with para-substituted benzoic acids (X = H, OH, CH₃, CN, NO₂) and the corresponding anions are analyzed with NOEs from ROESY experiments and with ¹H NMR shielding variations. The data indicate a predominant deep inclusion of the carboxy group in the complexes with all substituted benzoic acids, irrespective of the nature of the substituents. Protons in ortho- and meta-positions to the carboxylic group show complexation shifts larger than medium-induced shifts observed for any solvent. Complexation-induced ¹³C NMR shift (CIS) changes of the phenyl derivatives are also measured, with partial correction of literature signal assignments. Measurements of the same derivatives in several aprotic solvents reveal no correlation with complexation induced shifts, showing that polarity effects resulting from the medium cannot be the major source of the observed CIS values.