Neutron diffraction and molecular dynamics investigations of the temperature dependence of the local ordering in liquid cyclopropane

Abstract

The structure of the liquid cyclopropane and the evolution of the local ordering with temperature has been investigated along the liquid-vapour coexistence curve between the melting point (T m ≈ 146 K) and room temperature by neutron diffraction and molecular dynamics simulation. Experimentally, the intramolecular structure factor is well described using the molecular parameters close to those reported from spectroscopic calculations and is found to be almost temperature independent. In contrast, the intermolecular structure factor shows a strong variation in the translational and orientational ordering by cooling the liquid from room temperature to a temperature close to the melting point. Molecular dynamics simulations were performed using an atom-atom Lennard-Jones potential to model the intermolecular interaction between cyclopropane molecules. Excellent reproduction of thermodynamical and dynamical data reported for cyclopropane has allowed an assessment of the validity of the potential parameters used. Moreover, good agreement between the experimental and the simulated pair correlation function has been obtained, allowing the use of the simulated functions to discuss the evolution of the liquid structure with temperature. It is found that g CC(r) is the correlation function best able to describe the variation in the orientational order in the first shell of neighbours, from the relative evolution against temperature of the first two peaks observed at about 4·0 and 5·1 Å, respectively. A discussion of this evolution using angular pair correlation function and geometrical models based upon solid-phase data shows that, at low temperatures, the favoured configurations of cyclopropane molecules are those where, on average, their symmetry axes are perpendicular or parallel. When the temperature increases, configurations where the molecular axes are more randomly distributed tend to increase, thus leading to a more orientationally disordered liquid. It is emphasized that this trend is also consistent with the interpretation of light scattering experiments.