1H NMR Study of Intramolecular Hydrogen-Bonding Interaction in Cyclodextrins and Their Di- O -methylated Derivatives

Intramolecular hydrogen bonding of α-, β-, and γ-cyclodextrins, and α- and β-dimethyl cyclodextrins has been investigated in dimethyl sulfoxide-d6 by 1H NMR spectroscopy. The temperature dependence of 1H chemical shifts of secondary hydroxyl protons of cyclodextrins and dimethyl cyclodextrins reveals the existence of the intramolecular hydrogen bond. The strength of hydrogen bond becomes stronger as the ring size becomes smaller and by methylation. The intramolecular hydrogen bond in which the OH(3) is proton donor is preferred in α-cyclodextrin as well as in α- and β-dimethyl cyclodextrins. The orientation of secondary hydroxyl groups was estimated with the Karplus-type equation and the orientation is found to be suitable for making intramolecular hydrogen bond.