Chemistry Reference
In-Depth Information
This group then used their force field to carry out MD simulations with
the program MDynaMix.
61
They simulated five different pure ionic liquids:
[C
1
mim][Cl] (the same system examined by the Lynden-Bell group),
[C
1
mim][PF
6
], [C
2
mim][BF
4
], [C
4
mim][BF
4
], and [C
4
mim][PF
6
]. For each
liquid, a total of 192 ion pairs were simulated for 100 ps. Liquid densities gen-
erally agreed with experiment and with previous simulations. Although this
force field appears to be the most advanced of all the imidazolium force fields,
the liquid densities computed with it are not significantly different from those
obtained with the earlier, more ad hoc force fields. This is a recurring theme
for many systems that have been studied to date. While liquid densities are an
attractive experimental property with which to compare simulation results, it
is a property that is remarkably insensitive to the force field and consequently
is not a particularly useful measure for discriminating between ''good'' and
''bad'' force fields. Liu, Huang, and Wang
60
point this out, stating that liquid
densities are fairly insensitive to charge distributions. This force field did
achieve slightly better agreement with experimental densities than previous
studies, however, and the authors attribute this to the fitting they did on the
C2 hydrogen van der Waals parameters. They also computed enthalpies of
vaporization and the various components of the intermolecular energy show-
ing that, as the alkyl chain length increases, the van der Waals interactions
become more dominant and more negative in energy. This points to the fact
that the ''nonpolar'' regions of the ionic liquid are interacting in a favorable,
stabilizing manner, a finding similar to that of Urahata and Riberio.
56
It was noted that the anions tend to localize near the C2 carbon on the
imidazlium ring in many of the previous simulation studies.
37,39,49
The corre-
sponding ab initio calculations show this region of the ring to have the greatest
concentration of positive charge, and that the hydrogen attached to the C2
carbon has a high Bronsted acidity, validating the tendency for the negatively
charged anion to localize near this region. Liu, Huang and Wang
60
showed
this in a graphical way, plotting the relative probability distribution of differ-
ent anions about the cations, an example of which is shown in Figure 6. The
different colors in the original publication correspond to different probabilities
of observing the center of a PF
6
anion about a [C
4
mim] cation. The anion
resides in several different places during the course of the simulation, but
the most populated regions are near C2. Interestingly, the region near the butyl
chain is devoid of anion density; apparently, the nonpolar alkyl group ''sweeps
out'' a region around the cation, such that the anion resides, on average, at
other locations that are more polar and less sterically congested.
We finish the discussion of force fields developed for imidazolium-based
systems by describing the work of Voth and co-workers who simulated
[C
2
mim][NO
3
] first with traditional fixed-charge models
62
and then with a
model that included electronic polarizability.
63
For the fixed-charge system,
Del Popolo and Voth
62
used a force field having the AMBER function form,
with parameters for the cation and anion taken from existing sources. They
