NMR spin-rotation relaxation and diffusion of methane

The translational diffusion-coefficient and the spin-rotation contribution to the H-1 NMR relaxation rate for methane (CH4) are investigated using MD (molecular dynamics) simulations, over a wide range of densities and temperatures, spanning the liquid, supercritical, and gas phases. The simulated diffusion- coefficients agree well with measurements, without any adjustable parameters in the interpretation of the simulations. A minimization technique is developed to compute the angular velocity for non-rigid spherical molecules, which is used to simulate the autocorrelation function for spin-rotation interactions. With increasing diffusivity, the autocorrelation function shows increasing deviations from the single-exponential decay predicted by the Langevin theory for rigid spheres, and the deviations are quantified using inverse Laplace transforms. The H-1 spin-rotation relaxation rate derived from the autocorrelation function using the kinetic model agrees well with measurements in the supercritical/gas phase, while the relaxation rate derived using the diffusion model agrees well with measurements in the liquid phase. H-1 spin-rotation relaxation is shown to dominate over the MD-simulated H-1-H-1 dipole-dipole relaxation at high diffusivity, while the opposite is found at low diffusivity. At high diffusivity, the simulated spin-rotation correlation time agrees with the kinetic collision time for gases, which is used to derive a new expression for H-1 spin-rotation relaxation, without any adjustable parameters. Published by AIP Publishing.