Abstract
The intramolecular 1H NMR dipole-dipole relaxation of molecular fluids has traditionally been interpreted within the Bloembergen-Purcell-Pound (BPP) theory of NMR intramolecular relaxation. The BPP theory draws upon Debye's theory for describing the rotational diffusion of the 1H-1H pair and predicts a monoexponential decay of the 1H-1H dipole-dipole autocorrelation function between distinct spin pairs. Using molecular dynamics (MD) simulations, we show that for both n-heptane and water this is not the case. In particular, the autocorrelation function of individual 1H-1H intramolecular pairs itself evinces a rich stretched-exponential behavior, implying a distribution in rotational correlation times. However, for the high-symmetry molecule neopentane, the individual 1H-1H intramolecular pairs do conform to the BPP description, suggesting an important role of molecular symmetry in aiding agreement with the BPP model. The intermolecular autocorrelation functions for n-heptane, water, and neopentane also do not admit a monoexponential behavior of individual 1H-1H intermolecular pairs at distinct initial separations. We suggest expanding the autocorrelation function in terms of modes, provisionally termed molecular modes, that do have an exponential relaxation behavior. With care, the resulting Fredholm integral equation of the first kind can be inverted to recover the probability distribution of the molecular modes. The advantages and limitations of this approach are noted.
| Original language | English |
|---|---|
| Pages (from-to) | 10802-10810 |
| Number of pages | 9 |
| Journal | Journal of Physical Chemistry B |
| Volume | 124 |
| Issue number | 47 |
| DOIs | |
| State | Published - Nov 25 2020 |
| Externally published | Yes |
Funding
We thank Edmund J. Fordham for discussions on using MaxEnt in NMR and for helpful comments on Laplace transforms. D.A. thanks Lawrence Pratt for many rewarding discussions on MaxEnt methods in general. We thank Arjun Valiya Parambathu for helpful comments on the manuscript. We thank Chevron Energy Technology Company, the Rice University Consortium on Processes in Porous Media, and the American Chemical Society Petroleum Research Fund (No. ACS PRF 58859-ND6) for financial support. We gratefully acknowledge the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy (No. DE-AC02-05CH11231) and the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for high-performance computer time and support. We also gratefully acknowledge the reviewers for their insightful comments and suggestions which improved the manuscript.