Inclusion complexes of 2-phenoxyethanol and alkoxyethanols in cyclodextrins: an 1H NMR study

The association in aqueous solutions of small amphiphilic molecules 2-phenoxyethanol, PhE₁, and some α-n-alkyl-ω-hydroxyoligo(oxiethylenes], C₄E₁, C₄E₂ and C₆E₂) with β-cyclodextrin (βCD), heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) was investigated by ¹H NMR spectroscopy. The upfield shifts observed for the H3 and H5 NMR signals due to anisotropic shielding confirm that the host–guest associations are of inclusion type. The stoichiometries and the apparent inclusion constants, K _app, were determined by ¹H NMR spectroscopy using the H5 and H3 signals. The relative differences in the K _app values for βCD inclusion complexes seem to reflect the hydrophobic/hydrophilic balance of the guests. The K _app values for the PhE₁ inclusion complexes can be related to the degree of methylation and hydrophobicity variation within the considered hosts. In addition, a comparative study between βCD and TRIMEB inclusion complexes using 2D ROESY (Rotating-frame Overhauser Enhancement SpectroscopY) NMR spectra provides structural features for these complexes which are inaccessible by other experimental methods.