Dynamic QM/MM: A Hybrid Approach to Simulating Gas–Liquid Interactions - Multiscale Molecular Methods in Applied Chemistry - page 60

cation/anion pair. Once neutralization occurs, it is inevitable that the neutral species
will eventually desorb from the charged liquid. Instead of proton transfer, the
surface collision can also supply enough energy to enable the anion to react with
the cation. This bond usually occurs between an O in [NO 3 ] and one of the C-H
carbons on the imidazolium rings. It is likely that these recombinations with the
cation and anion are only short lived in the actual system. However, within our
simulation time, we found that the probability of these events was 0.12. When the
proton transfer event occurs to a molecular species that is located at the surface
edge, there is sometimes enough momentum to “push” the neutral species off of the
surface, which is listed as “total” in Table 1 . The probability for this event to happen
to [emim] is 0.10 and for [NO 3 ] is 0.06. There is only a very slight probability of
0.01 that more than one molecular species departs the surface together. Overall, for
nonreactive scattering of Ar from the surface, little change occurs chemically to the
ionic liquid molecules. This adds to knowledge concerning the chemical and
thermal stability of RTIL.
The collision of a reactive atom, O( 3 P), with [emim][NO 3 ] causes a multitude of
chemical reactions to ensue at the surface. This is in contrast to the scenario that we
just presented with the bombardment with Ar, a nonreactive atom, although there
are some common features as well. The types of reactions that occur have been
grouped into reaction types in Table 3 . One of the most common reactions, with a
probability of 0.43, is the elimination of NO 3 , which occurs when the incident O
collides with [NO 3 ] forming NO 2 or HNO 2 and O 2 or occasionally OOH. The
second most prevalent reaction type, occurring with a probability of 0.21, is ring
scission - O addition; this occurs when O addition causes a bond scission in the
imidazolium ring, opening it up into an intact chain. About a quarter of the time
there is enough vibrational energy in the chain that it splits apart creating the
channel labeled ring fragmentation - O addition. Commonly, in both the ring
scission reaction types, an aldehyde is formed. As with the O

þ squalane reactions,

H abstraction also occurs in the O [emim][NO 3 ] reaction yielding OH with a

probability of 0.09. Note that this is much lower than the 0.41 that is found with

þ

Table 3 Reaction probability
from O( 3 P) + [emim][NO 3 ]
by reaction type

Distribution of reactions by type
O scattering 0.13 0.01

O addition to NO 3 0.02 0.004

Elimination of NO 3 0.43 0.02

O addition to HC a 0.01 0.002

Elimination of HC from ring 0.04 0.01

Ring scission - O addition b 0.21 0.01

Ring fragmentation - O addition c 0.05 0.01

H abstraction 0.09 0.01

NO 3 substitution 0.003

HC substitution - H elimination 0.02 0.004
a HC refers to the ethyl or methyl hydrocarbon on the imidazo-
lium ring
b After ring scission the [emim] becomes an intact chain
c After becoming a chain, fragmentation occurs

0.01