Cyanoalkynes and Cyanopolyynes: From Crossed Beam Experiments to Astrochemistry - Polyynes: Synthesis, Properties, and Applications - page 301

( 25 kJ/mol). The CN radical can also abstract one of the hydrogen

atoms of the methyl group, since this pathway is attractive at the B3LYP
level of theory. The formation of HCN

þ

þ CH 2 CCH is strongly exothermic,

170 kJ/mol, and also the formation of HNC þ CH 2 CCH is exothermic by

110 kJ/mol. However, the related reaction CN CH 3

is known to be characterized by an entrance barrier and a room temperature
rate constant of

þ CH 4 ! HCN/HNC þ

10 13 [2,3], so we do not expect this channel to signi- 6

ficantly contribute.
In conclusion, the crossed molecular beam study of the CN

CH 3 CCH
reaction clearly demonstrated the occurrence of a CN versus H atom
exchange channel. The investigation of isotopic variant CN

þ

CD 3 CCH
suggested the existence of two distinct reaction products, cyanomethylace-
tylene and cyanoallene. The yield of the two products is about the same at
the collision energies investigated. The addition to the b position at the
expense of the a C atom, based on the enhanced spin density of the a C and
a simultaneous steric hindrance of the methyl group to attack the b position,
appears to favor the H-loss over the CH 3 -loss channel.
Interestingly, the presence of the methyl group of methylacetylene
strongly increases the complexity of the reaction if compared to that of the
unsubstituted acetylene. In particular, amongst the possible products only
cyanomethylacetylene and cyanoacetylene (routes 14.9a and 14.9c) preserve
the triple acetylenic bond in the nitrile products, while the formation of
cyanoallene (routes 14.9b) or allene (route 14.9d and 14.9e) can also occur.
This observation implies two consequences relevant to observations and ast-
rochemical models: on one side, it should be possible to observe cyanoallene
in the same extraterrestrial environments where cyanomethylacetylene was
observed, if reaction 14.9a is the real formation route of cyanomethylace-
tylene; on the other side, the models should take into account that the
reactions of CN radical with short methyl-substituted polyynes do not
necessarily form cyanopolyynes.

þ

14.4.3 T HE R EACTION CN þ CH 3 CCCH 3

In the previous section, we have seen how the presence of a -CH 3 group has
a strong influence on the chemistry of CN addition to a triple acetylenic
bond. In this section we further explore such an effect by moving to the
fully CH 3 -substituted species, dimethylacetylene. Also, in this case, we
expect a similar reactive approach with the cyano radical attacking the

-orbital of the dimethylacetylene molecule. The possible reaction routes

are ( Figure 14.10 )

X 2 þ Þþ CN ð CH 3 C C CH 3 ! CH 3 þ CH 3 C C CN ð 14 : 10a Þ

! CH 2 ¼ C ¼ C ð CH 3 Þ CN þ H ð 14 : 10b Þ

! CH 3 þ CH 3 C C NC ð 14 : 10c Þ