The structure and dynamics of crystalline 2,4-hexadiyne

Abstract

This paper reports the crystal structures and heat capacity of solid 2,4-hexadiyne between 3 K and 300 K. A phase transition observed at 117·9 K has a transition enthalpy ΔH _trans = 996·6 ± 1 J mol^-1 and entropy ΔS _trans = 8·42 ± 0·01 J K^-1 mol^-1. Neutron diffraction from powdered samples revealed that the phase change is a first order transition from a rhombohedral structure below 117·9 K to a monoclinic form above this temperature. An empirical interatomic potential based on atom-atom parameters derived from aromatic hydrocarbon data is used to model the crystal structures and thermodynamic properties of the two phases and to calculate the associated phonon dispersion curves. Only qualitative agreement is found indicating the non-transferability of parameters.