High-pressure neutron diffraction on fluid methane

Orientational and translational correlations of liquid methane over the nanometer-picosecond scales by molecular dynamics simulation and inelastic neutron scattering

Five models for the site-site intermolecular pair interactions of methane are compared in some detail and used to investigate both structural and dynamical properties of the dense liquid deuteromethane by means of molecular dynamics (MD) simulations. The orientational distribution probabilities of molecular pairs are carefully analyzed for each anisotropic potential model. We propose a revision of existing classification methods used to group the innumerable relative orientations of methane-methane pairs into six basic geometries. With this new approach, our results for the probability of the six basic categories as a function of the intermolecular distance are different from the ones present in the literature, where the role of the angular spread on the anisotropic interaction energy is not taken in full consideration and certain configurations with no significant change in the pair-potential are assigned to different categories. The analysis of the static orientational correlations in liquid methane and the prevalence of certain configurations in different ranges guide the subsequent discussion of the MD model-dependent results for the dynamic structure factor. Comparison with our inelastic neutron scattering results for liquid CD(4) at the nanometer and picosecond space and time scales allows us to confirm the full adequacy of the Tsuzuki, Uchimaru and Tanabe model of 1998 with respect to more recent potentials.

Investigations on the Structure of Fluid Methane by Neutron Diffraction Experiments

High pressure neutron diffraction experiments on a mixture of CH4 and CD4 as well as on pure deuterated methane CD4 were performed at three supercritical states. The structure factor was determined at three different densities at a temperature of T = 370K. The intra- and intermolecular structure was determined from the structure factor. The results from pure CD4 are in very good agreement with former neutron diffraction experiments. The experiments on the isotopic mixture allowed the direct determination of the pair correlation function of the molecular centres. The results of the diffraction experiments are presented in this paper.

Inelastic neutron scattering and molecular dynamics determination of the interaction potential in liquid CD4

Anisotropic interactions of liquid CD4 are studied in detail by comparison of inelastic neutron Brillouin scattering data with molecular dynamics simulations using up to four different models of the methane site-site potential. We demonstrate that the experimental dynamic structure factor S(Q,omega) acts as a highly discriminating quantity for possible interaction schemes. In particular, the Q evolution of the spectra enables a selective probing of the short- and medium-range features of the anisotropic potentials. We show that the preferential configuration of methane dimers at liquid densities can thus be discerned by analyzing the orientation-dependent model potential curves, in light of the experimental and simulation results.