UIC

Molecular Dynamics Simulations of Xe Chemical Shifts and Solubility in n-Alkanes

By Huajun Yuan

Using the MD method, we demonstrate that intermolecular NMR (Nuclear Magnetic Resonance) chemical shifts can be used to evaluate and develop intermolecular potentials for cross-interactions. We examine the average Xe chemical shifts in n-alkanes over a range of temperatures using the OPLS- all atom force field for the solvent molecules. Comparing the present results with earlier results using non-realistic rigid model solvent molecules, we obtain chemical shift contributions arising from flexibility of the solvent molecules. Modification of parameters of the exponential-6 potential model for solute-solvent interaction which leads to Xe chemical shifts in better agreement with experimental values likewise leads to improved estimates of Xe solubility. Since the average chemical shift converges in a fraction of the steps necessary to obtain converged solubility, testing of solute-solvent potentials against average chemical shift values, prior to time-intensive calculations of solubility, leads to more efficient development of potentials for mixtures. In the present work the MD simulations reproduce the signs and relative magnitudes of the Xe chemical shifts in n-alkanes, as well as the signs and relative magnitudes of their temperature coefficients. A rational comparison of Xe chemical shifts in different solvents can be made when the solvents are in the same thermodynamic state. In an atomistic MD simulation the additive chemical shift contributions arising from the CH₃ and CH₂ groups are obtained separately. We determine these constitutive contributions to the Xe chemical shift for each temperature in each solvent. We find the per-CH₃ contributions are greater than the per-CH₂ contributions for each case. We investigate the transferability of these contributions.