Chemistry Reference
In-Depth Information
Since the average kinetic energy is typically confined to 0.8 kJ/mol in diffuse
clouds and 0.08 kJ/mol in dark molecular clouds, the formation of
cyanopolyynes, as well as that of other molecular species, via gas phase
reactions must involve reactions with no barriers and only binary collisions.
Ternary encounters occur only once in a few 10
9
years and can be neglected
considering mean interstellar cloud lifetimes of 10
6
years. That is why ion-
molecule reactions are postulated to play a central role in interstellar
reaction networks [47,48]. In particular, early models of interstellar cloud
chemistry suggest that the HCCCNH
þ
ion is the key intermediate in the
formation of HCCCN and its isomer HCCNC, via the dissociative recom-
bination of HCCCNH
þ
with an electron of the cosmic radiation field. The
ion HCCCNH
þ
is thought to be formed via ion-molecule reactions. How-
ever, if this sequence of reactions is the real route of formation, the relative
abundances of HCCCN and HCCNC should be about the same — in strong
disagreement with astronomical observations which report a ratio of 160:1
[49]. In addition, the HCCCN fractional abundance predicted by the model
are orders of magnitude lower than that observed [49]. A second short-
coming is the observation of
13
C isotopic enrichments in HCCCN isotopo-
mers towards TMC-1 [50]. A detailed mapping indicates that H
13
CCCN and
HC
13
CCN have similar intensity, but that H
13
CCCN versus HCCC
13
CN
shows a ratio of 1.4. If ion-molecule reactions are responsible for HCCCN
formation, all three isotopomers should be formed with the same yield since
the carbon atoms of c-C
3
H
3
þ
are indistinguishable. Finally, a recent
theoretical investigation of the reaction N(
4
S)
C
3
H
3
þ
(the most important
step in this ion-molecule reaction network) showed that this reaction is not
feasible in molecular clouds [51]. In conclusion, ion-molecule reactions are
not able to explain the formation of HCCCN in the ISM. New formation
mechanisms, based on the generalization of reaction 14.1a,
þ
X
2
þ
Þþ
2
S
1
=
2
Þ
CN
ð
H
ð
C
C
Þ
n
H
!
H
ð
C
C
Þ
n
CN
þ
H
ð
,
ð
14
:
2
Þ
are being considered [33,52-54].
To verify if reactions 14.1 and 14.2 are responsible for the formation of
cyanoacetylene/cyanopolyynes in the low temperature environments of Titan
and ISM, a confirmation from laboratory experiments is required. Provided
that the elementary reactions of interest are thermodynamically feasible, it is
necessary to reach the knowledge of at least two other factors: the relevant
rate constants and the yield of the possible reaction products. Particularly
this last piece of information will allow us to draw the sequence of elemen-
tary steps which account for the global reaction.
Certainly the perspective from which neutral-neutral reactions were con-
sidered changed significantly after a series of sophisticated experiments
on the rate constants for CN reactions with acetylene, methylacetylene, and
ethylene [55,56]. From these studies experimental evidence was given that
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