STRUCTURE OF CYCLODEXTRINS AND THEIR COMPLEXES .4. CHROMATOGRAPHIC AND NMR-STUDY OF 1,2,3-TRI-T-BUTYLNAPHTHALENE AND ITS COMPLEX WITH GAMMA-CYCLODEXTRIN

The process of complex formation of 1,2,3-tri-t-butylnaphthalene with gamma-cyclodextrin was studied by means of gas-liquid chromatography, reversed-phase high performance liquid chromatography and H-1 and C-13 NMR spectra. In spite of the large volume of the guest molecule, using chromatographic methods, the complex was found to be unusually stable. The stability constant K-G,K-gamma-CD was estimated to be ca. 10(4) M(-1) in 75 vol% of ethanol in water at 20 degrees C and is greater than 4.4 m(-1) in glycerin at 90 degrees C. H-1 and C-13 spectra at room temperature exhibit only minor shifts of the signals upon complexation and the complex formation manifests itself in carbon spectra at low temperatures by significant broadening of the signals. Both reversed-phase high performance liquid chromatography and room temperature H-1 NMR spectra suggest a shallow insertion of the guest into the gamma-cyclodextrin cavity. There is a surprising inconsistency between the high stability of the complex and the shallow insertion of the guest into the host cavity. Internal rotation of t-butyl groups in the positions 1 and 2 is frozen at 193 K while the remaining group in position 3 rotates almost freely at this temperature. Molecular mechanics calculations using PCMODEL do not reproduce this trend yielding the smallest barrier hindering the rotation for t-butyl group in position 2.