H. Kang et al.
FULL PAPERS
[
38,39]
tiates the passage to the reaction transition state.
We
face, the solvated ethyl cationic species may be able to pro-
vide an effective route to the transition state.
observe that the temperature range of the p-complex forma-
tion (<93 K) overlaps only partially with the temperature at
We may discuss how an ice surface can isolate the ethyl
cationic intermediate, even though it is unstable in the gas
phase and aqueous solutions. An ice surface has an intrinsi-
cally heterogeneous structure at the atomic level owing to
+
which C H5 (80–100 K) appears. Also, even though the
2
[34]
p complex can be formed in a nonhydrating environment,
a C H5 signal cannot be observed in the LES inspection of
+
2
[40]
Sample C. This indicates that the unhydrated p complex is
the Bernal–Fowler ice structure, which statistically distrib-
utes dangling hydrogen atoms, dangling oxygen orbitals, and
four-coordinated molecules on the surface. Reactant mole-
cules can find various adsorption sites and solvating environ-
+
not the source of the C H signal. The hydrated p complex
2
5
+
on Sample A cannot explain the C H emission from this
2
5
surface at high temperatures (>93 K). We can imagine that
another species with a charged character and an intermedi-
ary structure between the p complex and ethyl cation may
be formed on the hydrating surface of Sample A, and this
[41]
ments on such a heterogeneous ice surface, which is quite
a different situation from a liquid-phase environment with
time-averaged homogeneity. As a result, the interaction of
HCl and ethene on an ice surface can occur in diverse ways,
and some reaction routes may pose a significantly lower
energy barrier than other pathways. For example, local ad-
sorption sites that can effectively solvate and stabilize an
ethyl cationic structure may be available on the surface. The
importance of a local solvation structure for proton transfer
has been pointed out in studies of the acid-catalyzed addi-
+
species is detected as C H . This possibility is supported by
2
5
the desorption of ethene in the temperature region of 100–
20 K for Sample A, which is observed as an additional tail
1
on the TPD curve (Figure 3). This TPD signature correlates
better with an ethyl cationic species than with the p com-
plex, as mentioned in Section 2.1. We cannot, however, de-
termine whether the ethyl cationic species is an isolated
ethyl cation or another species with a charged character and
intermediary structure, owing to the lack of sufficient spec-
troscopic information at present.
[42]
tion reaction of water to ethene. Once an ethyl cationic
species is produced on these special sites, this species can be
protected from being converted into other low-energy struc-
tures by a kinetic barrier. The kinetic barrier is provided by
the restricted mobility of water molecules on a low-tempera-
Other possible sources may also be considered, including
the collision-induced proton transfer from adjacent HCl to
+
[31]
ethene molecules, or from H O to ethene. However, the
ture ice surface. In this respect, the ethyl cationic species
3
+
absence of a C H5 signal in Samples B and C rules out
formed on the ice surface is a metastable product that is sta-
bilized by the kinetic constraints of the surface, rather than
a thermodynamically stabilized product. Indeed, such kinetic
isolation of metastable species is an important characteristic
of ice surface reactions at low temperatures, as has been ob-
2
these possibilities. Further, the collisional fragmentation of
+
C H Cl or C H OH into C H is obviously not a possible
2
5
2
5
2
5
explanation, because these species are absent from the sur-
faces.
According to our results, the ethyl cationic species is
formed by the proton transfer from molecular HCl to
ethene, rather than from hydronium ions to ethene after the
ionization of HCl. Water solvation also plays a crucial role
[8,15–23]
served in several examples.
This poses an interesting
question of whether a similar kinetic route to an ethyl cat-
ionic species is available in aqueous solutions. As a reaction
medium, an aqueous solution has properties that are quite
different than those of an ice surface. While an ice surface
has a heterogeneous molecular structure with a solid-like
character, an aqueous solution consists of fluctuating, ran-
domly oriented solvent molecules that provide a homogene-
ous environment on a time average. However, this solvent
fluctuation may be able to briefly generate solvation struc-
tures analogous to those found on the ice surface. If this
happens, it may be possible to form ethyl cationic species in
the liquid for a transient period.
in this proton transfer. The evidence for this includes the
+
fact that a C H5 signal appears only when ethene and HCl
2
interact in the presence of water molecules (Sample A) and
also at a low temperature (<100 K) at which some HCl
+
molecules remain un-ionized on the surface. A C H signal
2
5
is not observed in the reaction of hydronium ions and
ethene (Sample B); nor is it observed in the reaction of HCl
and ethene in a nonhydrating environment (Sample C). Fur-
+
thermore, the absence of a C H D signal (Figure 2a) indi-
2
4
+
cates that the proton transfer occurs directly between HCl
and ethene, rather than through a bridging water molecule.
The C H signal from Sample A disappears at tempera-
2
5
tures above 100 K (Figure 2b), which indicates that the
ethyl cationic species is destroyed at higher temperatures.
This can be explained by the increased mobility of surface
+
C H D would be produced if a proton (deuteron) were
2
4
transferred through a D O bridge between HCl and ethene.
2
[31]
This suggests that water molecules assist the proton transfer
by solvating the HCl-ethene complex structure and that
they stabilize the ethyl cationic intermediate with an ionic
character. The solvating water molecules, however, do not
molecules at high temperatures, which removes the kinet-
ic barrier that has protected the ethyl cationic structure on
the surface. The ethyl cationic species dissociates into
ethene and hydronium and chloride ions, according to the
LES and TPD spectra obtained at these temperatures. Nev-
ertheless, the reaction does not cross over the final transi-
tion state to form ethyl chloride, though this step is thermo-
dynamically downhill and occurs in liquids. The formation
of ethyl chloride requires a bimolecular encounter between
[38,39]
act as a proton transfer bridge. Theoretical calculations
show that the transition state of the reaction in the gas
phase has an ion-pair structure consisting of an ethyl cation
and a chloride anion. If a similar transition state is involved
in the reaction in the water-solvating environment of ice sur-
942
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Chem. Asian J. 2011, 6, 938 – 944