Coarse-Grained Modeling for Macromolecular Chemistry - Multiscale Molecular Methods in Applied Chemistry - page 301

structure much closer than the mapping scheme of Hermandaris et al. [ 22 ] for
different RDFs. The difference between RDFs of the two mapping schemes could
also be due to the different ways of developing the CG force fields, which we will
discuss in Sect. 3 . However, it is clear that the different mappings for PS implicitly
provide a bias of the resulting model towards the description of certain properties: if
the superatom location is on the main chain, at the center of mass of the repeat unit,
or at the center of the phenyl ring it will lead to a better reproduction of the main-
chain structure or the phenyl ring arrangements, for example.
Two mapping schemes have also been used for [C 4 mim][PF 6 ] in Fig. 1d
(IL-MS1 and IL-MS2). In IL-MS1, every three carbon atoms in the alkyl tail is
represented by one bead (bead CT), whereas the imidazolium ring is split into three
beads (R1, R2, and R3). This model has been proposed for study of the orientation
of the imidazolium ring in the CG model. In IL-MS2, three different superatoms
are used to describe the methyl moieties (identified by labels M1 and M2) and the
imidazolium ring (bead R). In the aliphatic chain, three methylene units are
collected into one single site called C. We have found that the two schemes,
which retain different features of the original molecule, perform differently depend-
ing on the properties. Both mapping schemes are able to reproduce with sufficient
accuracy the experimental X-ray and neutron scattering results (Fig. 3 ). However,
the difference between the models becomes more apparent in the details of the
position of the two major peaks ( 1A 1 ) and of the specific

interactions that contribute to them. The inset in Fig. 3 compares the position of
the two peaks as obtained from the CG simulations and as measured experimen-
tally. The figure shows that IL-MS1 better fits the data corresponding to the lower q ,

1.5A 1

and

Fig. 3 Neutron and X-ray scattering structure factors of [C 4 mim][PF 6 ] obtained from CG
simulation (IL-MS1) and experiment at 200 K. The inset shows the positions of the two major
peaks of the X-ray scattering structure factors of [C 4 mim][PF 6 ] for the two mapping schemes
(IL-MS1 and IL-MS2) at different temperatures. Neutron and X-ray scattering data are from [ 23 ]