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incidence angle in comparison to the scattered F energies. Also, there is very little
difference in the final translational energy of the HF product with regard to input
translational energy. Furthermore, for the 0.5 eV incident energy it is even common
that the HF product has more final translational energy than F had initially. This
brings clarity to the notion that the molecular energy transferred to the product
states is mainly from the chemical reaction that takes place and not from the
collision energy.
4.2
[emim][NO
3
]
We have also applied our dynamic QM/MMmodel to study the surface reactivity of
room temperature ionic liquids (RTIL). These RTIL are quite unique liquids in that
they are comprised solely of cation and anion pairs that can be interchanged with
other types of cations or anions; therefore, their physical and chemical properties
are a bit tunable. Generally these liquids are nonvolatile and nonflammable, chemi-
cally and thermally stable, and possess high ionic conductivity. Our interest in
RTIL is in their use as environmentally conscious hypergolic bipropellants.
Although these liquids are becoming more readily utilized as solvents, little is
known about the surface reactivity of these liquids. Since we have had some
success in analyzing the reactivity of squalane via gaseous atom/surface scattering,
we extended our work to include a simple RTIL, 1-ethyl-3-methyl-imidazolium
nitrate, referred to as [emim][NO
3
]. This liquid can be viewed as three distinctive
components - the NO
3
anion, the imidazolium ring cation, and the nonpolar
hydrocarbon tail (ethyl-chain) - all with their own unique local chemistry.
The [emim][NO
3
] RTIL was chosen because an OPLS-AA force field had
already been developed for it [
50
], and it limits the chemical interactions to only
C, H, O, and N, atoms for which we have confidence in using MSINDO to predict
reasonably accurate thermochemistry of nitrogen-containing ring systems [
51
,
52
].
In order to grasp the difference in the reactivity of the different surface compo-
nents with each other. As opposed to the incident atom, we chose to compare the
nonreactive scattering of Ar with the surface to the chemistry that results in reactive
scattering with O(
3
P), both with an initial 5.0 eV of translational energy.
The gas-liquid scattering setup is very similar to that described with the squa-
lane experiments in the previous section. However, as mentioned in Sect.
2.3
, the
QM region in these simulations was fixed to a localized region near the surface. Our
surface analysis of [emim][NO
3
] indicates that the ethyl-C sticks up out of the
surface, followed by an even distribution of cations and anions. This means that
there are readily accessible H atoms sticking up out of the surface for H abstraction
or H elimination reactions to occur before the incident O crosses the surface
threshold (the point where the density is equal to half the bulk density). Our
study was done in tandem with Minton and coworker's similar experimental
study [
36
] that used the RTILs [emim][NTf
2
] and [C
12
mim][NTf
2
], which they
are able to obtain with high purity, unlike [emim][NO
3
] at the time of their study.
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