HIGH-PRESSURE X-RAY-DIFFRACTION STUDY OF PENTAERYTHRITOL

Pentaerythritol [2,2-bis(hydroxymethyl)-1,3-propanediol], C(CH2OH)(4), tetragonal, <I(4)over bar>, Z = 2, a = 6.0752 (8), c = 8.733(2)Angstrom at T = 291K and 0.1MPa, has been studied at high pressures by X-ray diffraction. The pressure dependence of the unit-cell dimensions has been measured to 1.15 GPa, and the crystal structure has been determined at 1.15 GPa: a = 6.017(1), c = 8.267 (3) Angstrom; final R = 0.043 for 185 observed reflections; the C and O atoms were refined with anisotropic thermal parameters, and H atoms located in a Delta F map and refined with isotropic thermal parameters. The compressibility of van der Waals contacts between the sheets of the hydrogen-bonded molecules is several times larger than along the sheets, and above 280 MPa the distances between the centres of near molecules of neighbouring sheets become shorter than the distances between the centres of the hydrogen-bonded molecules; at ambient pressure these distances become equal when the structure transforms to the plastic phase at 453 K. Small changes in molecular dimensions are consistent with transformations of the crystal structure. The OH ... O hydrogen bond is compressed to 98% of its ambient pressure length.