N.J. Trappeniers

The density dependence of the self-diffusion coefficient of liquid methane

Self-diffusion coefficients for liquid methane obtained by the NMR spin-echo technique are reported at 110, 140 and 160 K. The density range extends to the freezing line at 110 and 140 K, and to 30 mol/dm3 at 160 K. Using the hard sphere diameter as a single temperature dependent parameter it was found that the reduced diffusivity isotherms fall on a common curve when plotted against reduced density [n∗#620.26, 0.58⩽Tr⩽1.70]. Literature viscosities could also be correlated in this way using diameters obtained from the diffusion data. The experimental results are in good agreement with molecular dynamics calculations for the hard sphere fluid, except at densities above n∗ = 0.87. This difference at high densities is similar to deviations from the rough hard sphere model observed for more complex molecular liquids (CCl4, C6H6). The diffusion data are also compared with the results of molecular dynamics and other calculations for more sophisticated potential models.

High resolution nuclear magnetic resonance spectroscopy in liquids and gases at pressures up to 2500 bar

This article is the first in a series of four papers, which deal with the effect of hydrostatic pressure on the high resolution proton magnetic resonance spectra of methane, ethylene, methanol and ethanol. The present paper describes the spectrometer in which high resolution spectra can be recorded at pressures up to 2500 bar. The most essential part of the spectrometer is a beryllium copper high pressure vessel in which the sample can be rotated under pressure. The positions of the spectral lines are referred to an external reference of benzene, contained in a sealed glass capillary. Frequency modulation of the transmitter is used in the measurement of the line positions. The paper describes how the apparatus can also be used for the measurement of the density dependence of the diamagnetic volume susceptibility.

The self-diffusion coefficient in the gas phase at moderate densities, obtained by computer simulations

The self-diffusion coefficient for Lennard-Jones molecules has been determined by molecular dynamics for densities up to the critical one and for temperatures ranging from T = 1.3∈/k to T = 5.56∈/k. At low density, the results are in a good agreement with the theoretical predictions; at elevated densities agreement with the available experimental results is found, although the scarcity of experimental data prevents a general comparison between real systems and the molecular dynamical calculations.

The orientational relaxation of methane molecules in the solid phase II at low temperatures

A model for the dynamics of the coupling between the orientations of the ordered CH4 molecules in phase II of solid methane at low temperatures is proposed. The model is equivalent to the dynamics of disordered solid hydrogen. The effective interaction strength is determined by the overlap of the librational ground states in the molecular field potential and vanishes in the classical limit. An approximate expression for the effective interaction strength is derived, showing an exponential dependence on the uncertainty of orientation in the librational ground states. This parameter is estimated from the experimental values of the tunnel energies. The second moments of the spectral densities of several anisotropic operators are evaluated in the infinite temperature limit. The resulting gaussian approximations for the spectra are applied in a derivation of the spin lattice relaxation time. The calculated values of the spin lattice relaxation time are compared to experiment.

The thermodynamics of discontinuous orientational phase transitions in quantum crystals

Generalized Clausius-Clapeyron relations are derived for discontinuous orientational phase transitions in the quantum-mechanical regime. Some qualitative features of the isotope effect on the transition temperature are explained. The quantum correction to the density dependence of the h.c.p.-f.c.c. transition temperature in solid hydrogen, the stability of the partially ordered phase of solid CH4 and the anomalous isotope effect in ammonium bromide are treated in thermodynamic terms.

High resolution nuclear magnetic resonance spectroscopy in liquids and gases at pressures up to 2500 bar: II. Density dependence of the proton magnetic shielding constants in the nonpolar gases methane and ethylene

Abstract The density dependence of the proton magnetic shielding constants in liquid methanol and ethanol has been accurately measured at a temperature of 29.2°C and at external pressures between 1 and 2500 bar. For comparison, a measurement in the vapor coexisting with the liquid at 29.2°C has also been made. For the carbonyl protons the results appear to be in qualitative agreement with previous measurements of the density dependence of the shielding constants in methane and ethylene. The results are discussed theoretically on basis of the Buckingham electric field model.

High resolution nuclear magnetic resonance spectroscopy in liquids and gases at pressures up to 2500 bar: IV. Density dependence of the 13C-H indirect spin-spin coupling constant in gaseous methane

Abstract The density dependence of J 13C−H in 13 CH 4 has been accurately measured in a methane sample containing the natural amount of 13 CH 4 . A linear increase of J 13C−H with increasing density has been found. The total variation over a density interval from 0−575 amagat is 0.24 Hz.

The rotational kinetic energy in the orientationally disordered phase of solid methane. A quasi-classical approach

It is shown that, as a consequence of the orientational correlations, the quantum rotational kinetic energy of interacting molecules in the orientationally disordered phase of solid methane exceeds the free rotor value. The expectation value of kinetic energy is obtained from the initial terms of a quasiclassical high temperature expansion of the free energy. The calculation is based on an analytic function approximating the configuration representation of the statistical density operator of the free spherical top. It is demonstrated that in the classical limit the function tends asymptotically to the true density operator, the error being exponentially dependent on temperature. As an application, the quantum correction to the second moment of an infrared absorption band is estimated.

The ordering phase transitions in solid methane and the cerium monochalcogenides compared. A test of universality

Using data from literature, it is shown that the antiferrorotational transition in solid methane and antiferromagnetic transition in the Ce monochalcogenides are described by the same n = 4 Ginsburg-Landau-Wilson Hamiltonian. Arguments are given that this agreement still applies in the case of a compressible lattice. The conflicting experimental observations in the two systems seem to violate the universality hypothesis.

Low temperature PVT data for ethylene by an NMR method

Densities of ethylene are reported as a function of pressure in the temperature interval of 123.15 K to 223.15 K and a pressure range of 20 to 2800 bar. The experimental method based on NMR is discussed and a comparison is made with the results of other authors.