Chemistry Reference
In-Depth Information
can only be achieved via translational motion with fixed charges. Ion clusters
form ''cages'' that are long lived and slow the dynamics. When charge fluctua-
tions are allowed as with a polarizable model, however, an additional charge
screening mechanism is operative that allows the system's dynamics to
increase somewhat.
Force Fields and Properties of Other Ionic Liquids
There exist several other cation types that can be made into ionic liquids,
although, as mentioned previously, most experimental and simulation studies
have been limited to the imidazolium cation. A small number of computa-
tional studies have been conducted on systems containing other cations,
including pyridinium, phosphonium, guanidinium, and triazolium, and, there
has also been work on developing force fields and computing properties for
ionic liquids containing anions beyond the standard set of halides, PF
6
,
BF
4
,andNO
3
. Here we review some of those studies.
Cadena and co-workers
64
developed a force field for the alkyl-pyridinium
cation, and simulated 1-
n
-hexyl-3-methylpyridinium bis(trifluoromethylsulfo-
nyl)imide ([C
6
mpy][Tf
2
N]), 1-
n
-octyl-3-methylpyridinium bis(trifluoromethyl-
sulfonyl)imide ([C
8
mpy][Tf
2
N]), and 1-
n
-hexyl-3,5-dimethylpyridinium bis
(trifluoromethylsulfonyl)imide ([C
6
dmpy][Tf
2
N]). They used a mix of quantum
calculations and parameters available in the CHARMM database
42
to parame-
terize the pyridinium force field, which has the functional form of Eq. [5]. For
the [Tf
2
N] anion the parameters developed by Canongia Lopes and Padua were
used.
65
Molecular dynamics simulations were conducted at a series of tempera-
tures for 5 ns, each using NAMD.
66
Liquid densities, compressibilities, and
expansivities were computed between 298 and 348 K and compared to experi-
mental data. They observed that there exists a significant history dependence
of the liquid densities. That is, computed densities depended on the equilibra-
tion history, even though 5-ns production runs were carried out. As Del
Popolo and Voth
62
observed for [C
2
mim][NO
3
] at 400 K, significant non-
Gaussian dynamics are in effect, suggesting that, at least for these ionic liquids,
calculating even static properties such as densities can be problematic due to
the sluggish dynamics of the system and that great care must be taken to
ensure proper equilibration. The authors also conducted experimental nuclear
magnetic resonance (NMR) investigations to measure self-diffusion coefficients
of the cations and anions. They showed that the MD simulations had not yet
achieved diffusive behavior, even after 5 ns, so direct comparison of experimen-
tal and calculated self-diffusivities was not possible. Their computed rotational
relaxation times for the cations and anions showed that the cations with the
longest alkyl groups had rotational relaxation times greater than 10 ns at
298K, and greater than 2 ns even at 348K. The [Tf
2
N] anion had faster rota-
tional relaxation times, but they still ranged from 4 ns at 298K to 0.8 ns at
