Hydrogen bonding preference of equatorial versus axial hydroxyl groups in pyran and cyclohexane rings in organic crystals

Abstract

In a study of the hydrogen bonding counts in crystalline pyranose monosaccharides, we noticed that equatorial hydroxyls formed more hydrogen bonds, on average, than axial groups. A survey of the Cambridge Structural Database was extended to all pyran and cyclohexane rings which have at least one hydroxyl substituent. There is a definite tendency for equatorial hydroxyls to be involved in two hydrogen bonds, whereas axial are more likely to form only one. The sterically accessible surface of the hydroxyl oxygens was calculated, and shows a positive correlation with the number of hydrogen bonds. The axial groups in general have a lower accessible surface than equatorial groups, and are closer to the molecular centre of mass, giving less opportunity for forming hydrogen bonds in the closely packed crystal structures.