Cyanoalkynes and Cyanopolyynes: From Crossed Beam Experiments to Astrochemistry - Polyynes: Synthesis, Properties, and Applications - page 294

a loose transition state with no entrance barrier and produces a strongly
bound (242 kJ/mol) intermediate, the cis/trans-1-cyanovinyl-2 radical inter-
mediate, HCCHCN, with a C s symmetry on a 2 A 0
Both the cis and trans forms can isomerize easily via TS1(1) located only
14 kJ/mol above the initial collision complexes; since the isomerization
barrier lies well below the total available energy, the cis and trans forms are
expected to be present in equal amounts. The CN radical can attack the
acetylene molecule also with its N-side without an entrance barrier. This
approach, however, was not found to be efficient at leading to the isocyano-
acetylene product because of significant barriers (

PES, cf. Figure 14.4 .

30 kJ/mol) along that
reaction pathway. The cis/trans 1-cyanovinyl-2 radical intermediates can
undergo either an H atom elimination to form cyanoacetylene through the
transition state TS3(1) (

72 kJ/mol) or a 1,2-H atom shift through the

transition state TS2(1) ( 65 kJ/mol) leading to the 1-cyanovinyl radical

which is the absolute minimum of the potential energy surface. This inter-
mediate can also decompose to cyanoacetylene þ H through the transi-

tion state TS4(1) ( 87 kJ/mol). Two high-energy singlet cyanovinylidene

( 227 kJ/mol) isomers also

exist (not shown here). Since our maximum collision energy is limited to
27 kJ/mol, none of the vinylidenes is relevant to our experiments.
We have already commented on the fact that our experimental data are
consistent with the formation of the most stable isomer cyanoacetylene, but
by using the experimental results only we cannot exclude a small contri-
bution from the HCCNC formation channel; we have employed RRKM
calculations to tackle this problem. This procedure shows that HCCCN is
the only reaction product and that only about 15% of it is formed after
the rearrangement of cis/trans 1-cyanovinyl-2 radical

þ 115 kJ/mol) and singlet isocyanovinylidene ( þ

intermediates into

1-cyanovinyl radical.
A last comment on the H atom abstraction channels leading to HCN (or
HNC) and C 2 H is due. These reaction channels could not be experimentally
investigated because of the presence of a strong signal at the m/e of the
reactant CN and the very unfavorable kinematics associated with the pro-
duct masses. However, according to the ab initio calculations, the two
H-abstraction channels are endothermic by 23 kJ/mol (HCN þ C 2 H) and

83 kJ/mol (HNC C 2 H), respectively, and involve transition states located

at 41 kJ/mol and 96 kJ/mol above the reactants. In other words, they can
occur neither under the present experimental conditions nor under the con-
ditions of the ISM and Titan's atmosphere. Very recently, a room tempera-
ture kinetic study in which the H atoms could be monitored by VUV-LIF
on the Lyman a transition confirmed that the H-displacement channels are
the only active pathways [5].
Interestingly, in this specific case the reaction dynamics study confirms
that the HCCCN

þ

H channel is the main one, as could be simplistically
predicted by mere thermochemistry. This conclusion should not be gener-
alized; in many well-known cases the main reaction channel is not the one

þ