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Molecular dynamics (MD) simulations are used to investigate $^1$H nuclearmagnetic 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 usingsimulation configurations are well-correlated with measured diffusion data atambient conditions. The agreement between the predicted and experimentallymeasured NMR relaxation times and diffusion coefficient also validate theforcefields used in the simulation. The molecular simulations naturallyseparate intramolecular from intermolecular dipole-dipole interactions helpingbring new insight into the two NMR relaxation mechanisms as a function ofmolecular chain-length (i.e. carbon number). Comparison of the MD simulationresults of the two relaxation mechanisms with traditional hard-sphere modelsused in interpreting NMR data reveals important limitations in the latter. Withincreasing chain length, there is substantial deviation in the molecular sizeinferred on the basis of the radius of gyration from simulation and the fittedhard-sphere radii required to rationalize the relaxation times. This deviationis characteristic of the local nature of the NMR measurement, one that iswell-captured by molecular simulations.

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