Title: NMR spin-rotation relaxation and diffusion of methane

Abstract

The translational diffusion-coefficient and the spin-rotation contribution to the ¹H NMR relaxation rate for methane (CH ₄) are investigated using MD (molecular dynamics) simulations, over a wide range of densities and temperatures, spanning the liquid, supercritical, and gas phases. Here, 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 ¹H 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. ¹H spin-rotation relaxation is shown to dominate over the MD-simulated ¹H- ¹H 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 expressionmore » for ¹H spin-rotation relaxation, without any adjustable parameters.« less

Authors:

Singer, P. M.  ^[1];

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• Search DOE PAGES for ORCID "0000-0003-0061-112X"
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Asthagiri, D.  ^[1];

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• Search DOE PAGES for ORCID "0000-0001-5869-0807"
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Chapman, W. G.  ^[1];

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• Search DOE PAGES for ORCID "0000-0002-8789-9041"
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Hirasaki, G. J.  ^[1]

• Search DOE PAGES for author "Hirasaki, G. J."
• Search DOE PAGES for ORCID "0000-0002-3405-9428"
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+ Show Author Affiliations

1. Rice Univ., Houston, TX (United States)

Publication Date:

2018-05-22

Research Org.:

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Univ. of California, Oakland, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC)

OSTI Identifier:

1543861

Grant/Contract Number:  

AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Chemical Physics

Additional Journal Information:

Journal Volume: 148; Journal Issue: 20; Journal ID: ISSN 0021-9606

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Chemistry; Physics