Molecular Dynamics Simulations of NMR Relaxation and Diffusion of Bulk Hydrocarbons and Water

TL;DR

This study uses molecular dynamics simulations to accurately predict NMR relaxation times and diffusion coefficients of hydrocarbons and water, providing insights into relaxation mechanisms and highlighting limitations of traditional models.

Contribution

The paper demonstrates that MD simulations can reliably reproduce NMR relaxation and diffusion data without adjustable parameters, offering detailed mechanistic insights beyond classical models.

Findings

MD simulations agree with experimental NMR relaxation times and diffusion coefficients.

Simulations distinguish intra- and intermolecular dipole interactions, revealing relaxation mechanisms.

Traditional hard-sphere models show limitations in interpreting NMR data for longer hydrocarbons.

Abstract

Molecular dynamics (MD) simulations are used to investigate $^1$H nuclear magnetic resonance (NMR) relaxation and diffusion of bulk $n$-C$_5$H$_{12}$ to $n$-C$_{17}$H$_{36}$ hydrocarbons and bulk water. The MD simulations of the $^1$H NMR relaxation times $T_{1,2}$ in the fast motion regime where $T_1 = T_2$ agree with measured (de-oxygenated) $T_2$ data at ambient conditions, without any adjustable parameters in the interpretation of the simulation data. Likewise, the translational diffusion $D_T$ coefficients calculated using simulation configurations are well-correlated with measured diffusion data at ambient conditions. The agreement between the predicted and experimentally measured NMR relaxation times and diffusion coefficient also validate the forcefields used in the simulation. The molecular simulations naturally separate intramolecular from intermolecular dipole-dipole interactions helping bring new insight into the two NMR relaxation mechanisms as a function of molecular chain-length (i.e. carbon number). Comparison of the MD simulation results of the two relaxation mechanisms with traditional hard-sphere models used in interpreting NMR data reveals important limitations in the latter. With increasing chain length, there is substantial deviation in the molecular size inferred on the basis of the radius of gyration from simulation and the fitted hard-sphere radii required to rationalize the relaxation times. This deviation is characteristic of the local nature of the NMR measurement, one that is well-captured by molecular simulations.