Chemistry Reference
In-Depth Information
Force Fields and Properties of Ionic Liquids Having
Dialkylimidazolium Cations
Given that almost all of the initial experimental work focused on ionic
liquids having a dialkylimidazolium cation, it is perhaps not surprising that
most simulation studies have likewise focused on this cation. After all, if
this was what experimental researchers thought to be the most useful system,
modelers certainly wanted to make sure that they studied the most relevant
systems. In hindsight, following that trajectory was unfortunate because there
are
many
other cations that are both useful from a technological perspective
and of interest to scientists from a fundamental perspective. Nonetheless, for
several years almost all of those other cations were ignored by modelers in
favor of ionic liquids containing the imidazolium cation. As discussed earlier,
already five different imidazolium force fields had been proposed and used to
model ionic liquids. Over the next few years, several more force fields, all hav-
ing the same basic functional form as Eq. [5], were proposed for the imidazo-
lium cation. Each new force field was derived in a more rigorous manner,
using higher accuracy quantum methods and relying more on first-principles
calculations than on literature parameters from similar organic compounds.
Urahata and Riberio
56
proposed an imidazolium force field in which the
methyl and methylene units of the alkyl chain were treated as united-atom seg-
ments. They varied the length of the chain from methyl to octyl, and also
studied F
,Cl
,Br
, and PF
6
anions. Importantly, their calculations
matched experimental structure factors. Their work was interesting because
they were the first to show that the detailed structure of the liquid changes
depending on the length of the alkyl chain. Later studies would demonstrate
that long alkyl chains lead to the formation of nanoscale polar and nonpolar
domains in these systems, but the work by Urahata and Riberio gave the first
inkling of this concept.
Canongia Lopes, Deschamps and Padua
57
introduced what they termed
a ''systematic'' explicit-atom force field for imidazolium-based ionic liquids.
Their force field is based on the OPLS-AA/AMBER functional form, and the
authors sought to make it as transferable as possible. That is, components of
a molecule already contained in the OPLS-AA database should be compatible
with this force field, and, in fact, many of the terms from the OPLS force field
were used for the ionic liquid. To perform the parameterization, they first
minimized the energy of a gas-phase ion at the HF/6-31G(d) level. Single-
point energy calculations were then conducted at the more accurate MP2/
cc-pVTZ(f) level. Bond and angle terms were reparameterized if the mini-
mized structure value deviated significantly from the OPLS value. Otherwise,
the OPLS value was used. Key dihedral angle energy profiles were computed
from quantum calculations, and customized dihedral potential parameters
were fit to this energy profile. This was the first time dihedral angle para-
meters were specifically fit to a quantum calculation of an ionic liquid. As
