Flip-flop hydrogen bonding in a partially disordered system

Abstract

Cyclodextrins have proved useful as model systems for the study of hydrogen bonding¹, α-Cyclodextrin (α-CD) crystal-lizes from water as a hexahydrate having well defined hydrogen bonds (O—H…O) in linear and circular arrangements favouring ‘endless’ …O—H…O—H…O—H… chains^1–3. The larger β-cyclodextrin (β-CD) forms a crystalline dodecahydrate, β-CD·12H²O in which X-ray localization of the hydroxyl hydrogen atoms is made difficult because 25 of the 45 hydroxyl groups are statistically disordered and water molecules enclosed within the β-CD cavity are distributed over several sites that are not fully occupied⁴. We have therefore carried out a neutron diffraction study of β-CD at the Oak Ridge high flux isotope reactor to determine whether there is any well defined hydrogen bonding in disordered systems. In β-CD there are 19 hydrogen bonds of the type O—H…H—O. In these bonds, oxygen atoms are in the normal O—H…O distance range⁵, but two statistically half-occupied H atoms are arranged between them. The fact that the H…H separation of ∼1 Å is so short that the two H atoms positions are mutually exclusive suggests an equilibrium between two states: O—H…O⇌O…H—O. Of the two H atoms only one is in hydrogen-bonding contact at a given time; the other one is flipped out to form a hydrogen bond with an adjacent acceptor group and vice versa. Because long hydrogen-bonding chains are involved in a cooperative, concerted motion (domino effect), we have coined the term ‘flip-flop hydrogen bond’. This study demonstrates that hydrogen bonds are opera-tive in disordered systems and display dynamics even in solid state.