title: Molecular dynamics simulations of NMR relaxation and diffusion of bulk hydrocarbons and water.
abstract: Molecular dynamics (MD) simulations are used to investigate1H nuclear magnetic resonance (NMR) relaxation and diffusion of bulk n-C5H12to n-C17H36hydrocarbons and bulk water. The MD simulations of the1H NMR relaxation times T1,2in the fast motion regime where T1=T2agree with measured (de-oxygenated) T2data at ambient conditions, without any adjustable parameters in the interpretation of the simulation data. Likewise, the translational diffusion DTcoefficients calculated using simulation configurations agree 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.
authors.affiliation: Rice University, Department of Chemical and Biomolecular Engineering, 6100 Main St., Houston, TX 77005, USA. Electronic address: ps41@rice.edu.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Singer
authors.affiliation: Rice University, Department of Chemical and Biomolecular Engineering, 6100 Main St., Houston, TX 77005, USA.
authors.forename: Dilip
authors.initials: D
authors.lastname: Asthagiri
authors.affiliation: Rice University, Department of Chemical and Biomolecular Engineering, 6100 Main St., Houston, TX 77005, USA.
authors.forename: Walter G
authors.initials: WG
authors.lastname: Chapman
authors.affiliation: Rice University, Department of Chemical and Biomolecular Engineering, 6100 Main St., Houston, TX 77005, USA.
authors.forename: George J
authors.initials: GJ
authors.lastname: Hirasaki
keywords: Autocorrelation function; Hard-sphere model; Intermolecular relaxation; Intramolecular relaxation