Computer simulations and the interpretation of incoherent neutron scattering experiments on the solid rotator phases of long-chain alkanes

Abstract

Molecular dynamics calculations have been carried out for the solid rotator phases of the long-chain alkanes n-nonadecane, C₁₉H₄₀, and n-tricosane, C₂₃H₄₈. The work represents an extension of that reported previously (Ryckaert, J. P., McDonald, I. R., and Klein, M. L., 1989, Molec. Phys., 67, 957), but the new calculations are made on much larger systems (up to 1620 molecules) and the simulations are run for much longer times (up to 1 ns). Correlations in the lateral motion of the molecular centres of mass are observed and found to be strongly system-size dependent. Detailed comparison is made with the results of quasielastic neutron scattering experiments, from which information on the longitudinal and rotational motions of the molecules can be extracted. The origins of certain discrepancies between simulation and experiment are discussed and strategies for computing the relevant physical quantities within a simulation are compared. Assessment is also made of the adequacy of rigid-body models of the chain dynamics on which the interpretation of the experimental data is based.