Computational Study of Room Temperature Molten Salts Composed by 1-Alkyl-3-methylimidazolium CationsForce-Field Proposal and Validation

Abstract

We present a complete force field for liquid-state simulations on ionic liquids containing 1-ethyl-3methylimidazolium and 1-n-butyl-3-methylimidazolium cations and the tetrachloroaluminate and tetrafluoroborate anions. The force field is compatible with the AMBER methodology and is easily extendable to other dialkylimidazolium salts. On the basis of the general AMBER procedures to develop lacking intramolecular parameters and the RESP approach to calculate the atomic point charges, we obtained an all-atom force field which was validated against the experimental density, diffusion coefficient, vibrational frequencies, as well as X-ray (crystal state) and neutron (liquid state) diffraction structural data. Moreover, molecular mechanics calculations for the developed force field produce the cation's structures and dipole moments in very good agreement with quantum mechanical ab initio calculations. In addition, a basic study concerning the simulated liquid structure in terms of the radial distribution functions has been undertaken using molecular dynamics simulation. In summary, we achieved a very consistent picture in the computed data for the four room-temperature molten salts.

The simulation of imidazolium-based ionic liquids†

“…Diffusion rates appear to be influenced by the size of the alkyl groups for low n (C 1 C 1 im > C 2 C 1 im > C 4 C 1 im) [15], however on increasing the alkyl chain length (n > 6) diffusion coefficients do not appear to vary systematically. [65] On changing only one of the cation or anion, both self diffusion constants tend to change in the same direction, [15] , [32,65] moreover there is evidence that mobile ions are not randomly distributed but form well defined clusters, [73] and evidence from time correlation functions that cation anion motion is collective. [23,91] All of these factors indicate that ions may diffuse in association (rather than independently).…”

“…[71,80] Changing the anion tends to have a larger effect on the radial distribution functions than exchanging the cations. [15,17] However, longer alkyl chains on the and 11.0Å in a separate simulation [17]). Long range structuring of the liquid also appears to be persistent beyond 20Å [13,30,32], however limitations on cell dimensions mean that length scales beyond this have not been explored.…”

“…The lack of good experimental data has not only limited the development of force fields, but also their validation. [11,15] Further complications arrise because the atom types for the imidazolium cation are not necessarily straight forward to assign. For example, the imidazolium ring has variously been described using imidazole ring (figure 2) parameters from OPLS-AA/AMBER, [30] or protonated histidine parameters from CHARMM.…”

“…The imidazolium ring vibrational modes are particularly poorly reproduced by the AMBER force field, and one recent improvement has been to adjust bond and angle force constants so as to improve this discrepancy. [15,17] In addition, problems have arisen because the torsional parameters for rotation of the alkyl chain with respect to the imidazolium ring are not defined in standard force fields. These parameters are important because they affect the position of the anions (by excluding them from the volume encompased by the rotation) and therefor packing in the liquid phase.…”

“…The partial charges used in the simulation of imidazolium based ionic liquids has varied particularly widely. [12,33] Specifically the charges associated with the nitrogen atoms of the imidazolium ring vary from postive [11,21,30, 34] through neutral [15,17] to negative [12,13,33] depending on the parametrisation. Urahata and Ribeiro have commented upon this inconsistency, but they also note, however, that the dipole moments obtained from the calculations.…”