Anisotropy of Intermolecular Interactions from the Study of the Thermal-Expansion Tensor

The anisotropy of the intermolecular interactions in the low-temperature ordered phases of three chemically and structurally related compounds [neopentylglycol, (CH₃)₂C(CH₂OH)₂, pivalic acid, (CH₃)₃C(COOH), and neopentylalcohol, (CH₃)₃C(CH₂OH)], all of which display an orientationally disordered high-temperature phase, has been shown by means of the isobaric thermal-expansion tensor. The variation of the directions of the principal components of the thermal-expansion tensor as a function of temperature, as well as the variation of its principal coefficients, is evidence of the large differences in the intermolecular interactions for each compound; or, more precisely, between the strong intermolecular hydrogen bonds and the weak van der Waals interactions. In addition, the differences in the hydrogen-bonding schemes expected a priori from the molecular structures of the studied compounds have been enhanced. Finally, the volume expansivity as well as the packing coefficient have been analysed in the orientationally disordered high-temperature phase of each of the three compounds.