Towards quantitative prediction of proton chemical shifts in imidazolium chloride ionic liquids by computational NMR

We present the results of a computational investigation based on molecular dynamics (MD) and density functional theory (DFT) aimed at testing the performance of different classical Force Fields (FFs) to accurately model the microscopic structure of the ionic liquids butylmethylimidazolium tetrafluoroborate and chloride and predict their NMR properties. The FFs used are a full-charge FF, a scaled-charge FF, a polarizable FF, and a recently presented virtual-site FF (Doherty, B.; Zhong, X.; Acevedo, O. J. Phys. Chem. B 2018, 122, 2962-2974). We use the NMR parameters (namely ¹H chemical shifts of the [C₄C₁im] cation) computed by DFT methods over a MD trajectory as a probe to judge the performance of the various FFs. While the agreement between experimental and calculated NMR resonances is almost quantitative for the tetrafluoroborate salt, using all four types of FFs, for the chloride salt we observe a very different performance: The full-charge and scaled-charge FFs exhibit a rather significant disagreement, while an improvement is obtained with the polarizable FF and a rather good agreement is achieved by the virtual-site FF. The high sensitivity of NMR parameters to geometrical factors allows us to pinpoint specific deficiencies of different FFs in correctly representing the hydrogen-bond between the positively charged imidazolium ring protons and the chloride anion. In turn, such analysis enables us to propose future directions for improving the performance of classical FFs for ionic liquids.