Structure and intermolecular interactions in fluid ammonia: An investigation by neutron diffraction at high pressure, statistical-mechanical calculations and computer simulations

Abstract

Neutron scattering experiments on deuterated ammonia were performed in the overcritical region at high pressure. The structure factors and the radial distribution functions were determined at a temperature of 449 K at four densities extending from 0.318 to 0.700 g cm−3. With increasing density the intramolecular structure changes as the covalent bond becomes slightly longer and the angle between D–N–D is getting larger. At the two highest densities a shoulder at about 2.5 Å appears in the radial distribution function which cannot be assigned to a specific atom pair. The experimental radial distribution functions were compared with theoretical results obtained from the site–site-Ornstein–Zernike-(SSOZ) equation in combination with the HNC closure (XRISM). A series of potentials were tested and a site–site Lennard-Jones potential was fitted to the experimental results. The reverse Monte Carlo technique was used to calculate the atom pair correlation and angular distribution functions by minimizing the deviation of the radial distribution function of a configuration from the experimentally determined function. The agreement between SSOZ calculations and simulations are quite good in the overcritical region. The angular distribution functions show no significant features, but with increasing density a parallel orientation of the molecules become more unlikely.