Our results demonstrate that the equatorial C-H groups of 1,3-dioxane form hydrogen-bond-like C-H... 0 interactions more readily than do the axial C-H groups. The peak frequency of the strong axial C-H stretch band of 1,3-dioxane in a dilute D2O solution possesses an unusual non-monotonic pressure dependence, which indicates enhanced C-(HO)-O-... hydrogen bond formation at high pressure. We performed density functional theory calculations to predict the relative energies and total interaction energies of 1,3-dioxane/(water)(n) clusters and found that the equatorial C-H groups are more favorable sites for hydrogen bonding than are the axial C-H groups. (c) 2005 Elsevier B.V. All rights reserved.