Local order of liquid propionic acid as studied by original neutron scattering, DFT calculations and molecular dynamics simulations

Abstract

Original neutron scattering data on completely deuterated liquid propionic acid (PA) at large momentum transfer are presented. Experimental data, at atmospheric pressure and 298 K, Density Functional Theory (DFT) calculations, Natural Bond Orbital (NBO) and Atoms-In Molecules (AIM) analyses and Molecular Dynamics (MD) simulations were performed to describe the local order of PA. The structure factor SM(q), the molecular form factorF1(q) and the intermolecular pair correlation function gL(r) of the liquid were determined by analyzing the neutron scattering data. Some possible molecular arrangements including linear and cyclic dimers and a cyclic trimer, were optimized using DFT calculations to examine their ability of characterizing the liquid's local order. The experimental data are found to be well reproduced by cyclic clusters. NBO and AIM calculations have been then conducted to determine the strength and the stabilization energy of hydrogen bonds inside the likeliest associations. Molecular dynamics (MD) simulations involving a large number of particles were performed to analyses the experimental structure factor at medium and large distances. Using the OPLS-AA flexible force field, simulation results corroborate well our neutron scattering data and clearly highlight the H-bond interactions in the liquid PA.