16 Structure simulation of (bmim)(PF6)



STRUCTURES


Quantum-mechanical calculations, motional parameters, and results from molecular dynamics (MD) simulations support the existence of hydrogen bonding and the formation of ion pairs in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate.

Conclusions

Forcefield development

To begin simulating ionic liquids, the hypothesis was that a fixed-charge atomistic forcefield, with partial charges and intramolecular terms derived from gas phase quantum calculations and non-bond dispersion potentials taken from the literature, would enable to accurately model thermophysical properties of ionic liquids.
In the figure below is shown the charge distribution from a single calculation of 1-n-butyl-3-methylimidazolium hexafluorophosphate, using density functional theory (a standard functional form had been used for the forcefield).

: Charge distribution for (bmim)(PF6). Red corresponds to regions of high relative negative charge, and blue to high relative positive charge.
Charge distribution for (bmim)(PF6).
Red corresponds to regions of high relative negative charge, and blue to high relative positive charge.

The structure of the anion about the cation was also computed. The figure below shows site-site radial distribution functions for this liquid. Note that the anion shows strong preferential ordering about the C2 carbon.

Site-site g(r) for PF6 anion about different carbon atoms of the cation. There is stronger localization near the acidic C2 carbon of the imidazolium ring.
Site-site g(r) for PF6 anion about different carbon atoms of the cation. There is stronger localization near the acidic C2 carbon of the imidazolium ring.

This organization of the cation and anion can also be seen in the minimized gas phase structure obtained from a density functional theory calculations, shown below:


: Minimum energy conformation of 1-n-butyl-3-methylimidazolium PF6 obtained from density functional theory calculations. As with the liquid phase, the anion prefers to associate with the hydrogen at the C2 position.
Minimum energy conformation of 1-n-butyl-3-methylimidazolium PF6 obtained from density functional theory calculations.
As with the liquid phase, the anion prefers to associate with the hydrogen at the C2 position.

When the H at the C2 position is blocked with a CH3 group, however, the organization of the anion about the cation changes. Now, the anion associates to an equal extent with all three carbon sites. A typical conformation for this ionic liquid is shown in the figure below:


Minimum energy conformation of a cation-anion pair from DFT calculations. Now the anion can associate with the hydrogen atoms off the 4- and 5-positions on the ring.
Minimum energy conformation of a cation-anion pair from DFT calculations. Now the anion can associate with the hydrogen atoms of the 4- and 5-positions on the ring.
As with the liquid phase, the anion prefers to associate with the hydrogen at the C2 position.

Conclusions

It is well known that hydrogen bonding intensifies the formation of ion pairs in electrolyte solutions significantly when compared to systems without specific interactions. Thus it is assumed for ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate that the hydrogen bonding leads to the formation of ion pairs or even higher aggregates. It can be speculated that in ionic liquids not only ion pairs are formed, but also higher aggregates with a kind of layer structure, in which the anions are located mainly above and below the aromatic ring near C2. The occurrence of the hydrogen bonding in addition to the Coulombic interactions between the ions might explain the high viscosiy and some of the other specific macroscopic properties of the ionic liquid.20

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