Real-World Predictions from Ab Initio Molecular Dynamics Simulations - Multiscale Molecular Methods in Applied Chemistry - page 139

have appeared in the last few years in the literature. AIMD simulations were
performed as well to investigate their extraordinary behavior [ 88 , 110 , 113 - 118 ].
In 2005, AIMD simulations on dimethyimidazolium chloride [Mmim][Cl] car-
ried out by Del P ´ polo et al. showed significant differences compared to both the
classical simulations and the neutron diffraction results [ 113 ]. In particular, and
unlike the gas-phase ion pair, chloride ions tended to be located near a ring C-H
proton in a position suggesting hydrogen bonding. It should be noted, that these
results might be due to the choice of the applied functional. No dispersion correc-
tion was used and later it was shown that GGA functionals favor the hydrogen
bonded states [ 119 , 120 ].
Bhargava and Balasubramanian found that the apparently good agreement
between the pair correlation functions from classical MD and AIMD conceal subtle
but crucial, differences [ 114 ]. The radial pair distribution functions between the
most acidic proton of the [Mmim] cation and the chloride anion were extremely
different regarding location and width of the peaks. Furthermore, differences
between AIMD and MD in the spatial distribution of chloride ions around the
cation were found. The data were explained in terms of the formation of a hydrogen
bond between the most acidic hydrogen of the imidazolium ring and the chloride
ion. Size effects were excluded by simulations of 32 ion pairs with traditional MD
simulations. The cation-anion hydrogen bond present in the melt was observed as a
red-shift in the C-H stretching frequency.
The structural and dynamical hydrogen bonding in the IL [Emim][SCN] was
investigated by Thar et al. in 2009 [ 117 ]. The geometric picture indicated a superior
role for the most acidic hydrogen bond as compared to the two other hydrogen
atoms at the rear. Despite the structural picture, the hydrogen bond dynamics at the
most acidic hydrogen atomwas found to decay faster than the corresponding dynamics
at the other ring hydrogen atoms. Neglecting the directionality in the hydrogen
bond analysis provided dynamics which reflected the geometrical analysis. Two
movements were identified. First, a fast ( 0.3 ps) hopping of the anion above and

below the imidazolium ring and, second, a translational motion of the anion away
from the cation in-plane of the imidazolium ring (5-10 ps).
The first AIMD simulation of an IL applying dispersion-corrected functionals
[ 52 ] was carried out on the protic ionic liquid monomethylammonium nitrate [ 118 ].
On average, 1.8 of 3 possible hydrogen bond contacts formed, leaving one free
acceptor and donor site similar to water. Furthermore, like water, monomethy-
lammonium nitrate exhibited a fast fluctuating hydrogen bond network. However,
the hydrogen bond network of monomethylammonium nitrate and water also
showed some important structural differences. While the hydrogen bonds in
water arrange in parallel fashion, the hydrogen bonds of monomethylammonium
nitrate prefer angles of 0 ,90 , and 180 . The ion dynamics of monomethy-
lammonium nitrate was described by a model of ions rattling in long living cages
[ 118 ].
Other liquids , like liquid ammonia NH 3 [ 121 ], formamide HCONH 2 [ 122 ], and
liquid hydrogen fluoride HF [ 123 ], as well as more exotic liquids, like liquid

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