Thermodynamic, structural and transport properties of tetramethyl ammonium fluoride: First principles molecular dynamics simulations of an unusual ionic liquid

First principle molecular dynamics (FPMD) simulations in the isobaric-isothermal ensemble were carried out to investigate the liquid phase of pure tetramethyl ammonium fluoride (TMAF), its equimolar mixture with water, and its dilute solution in water. These simulations were performed using Born-Oppenheimer molecular dynamics with the quantum many-body potentials and forces obtained directly from on-the-fly electronic structure calculations at the Kohn-Sham density functional theory level. For the equimolar mixture, simulations were carried out using two different generalized gradient approximation functionals and two values of the charge density cutoff, and it is found that thermodynamic properties are rather sensitive to these details of the electronic structure calculations. The FPMD simulation for neat TMAF at a temperature of 600 K did not yield any indication for the decomposition of TMAF over the 50 ps production period, but trimethylamine and fluoromethane decomposition products were found during a short trajectory at 1200 K. At 400 K and 1 atm, the simulations indicate that significant structural rearrangement and changes in cohesive energy density and compressibility occur upon addition of water to TMAF. The hydration water is found to preferentially solvate the fluoride ions, and the distribution of F^--H₂O hydrogen bond lifetimes is broad. The simulations do not reveal any significant differences in the transport behavior of TMA⁺, F^-, and H₂O over the production periods.