Retarding Effects of Cyclodextrins on the Decomposition of Organic Isothiocyanates in an Aqueous Solution

Abstract

One- and two-dimensional ¹H nuclear magnetic resonance (¹H NMR) spectra were measured in order to estimate the dissociation constants (K_diss) and molecular geometries of cyclodextrin (CD) with three organic isothiocyanates (ITC), allyl-ITC, 3-butenyl-ITC, and 4-pentenyl-ITC, in an aqueous solution (pH 5.0, I_c=0.75 M). In every ITC, the K_diss values decreased in the order of α-CD>β-CD>γ-CD, and the three rod-like linear molecules of ITCs were accommodated with the best fit into the smallest cavity of α-CD. By rotating-frame nuclear Overhauser enhancement spectroscopy, cross peaks were found between the H-3 of α-CD and the H-1 of AITC. From the result, the molecular geometry of α-CD–AITC complex presume that the isothiocyanate group of AITC is located some where around the wideing rim and a hydrophobic allyl group is included within the hydrophobic cavity of α-CD.