Reaction of HCCN with NO and O2
J. Phys. Chem. A, Vol. 101, No. 5, 1997 869
CNO could abstract H from the precursor to make HCNO. Both
H atom abstraction channels to make HCNO and to make
HONC are exothermic.13 There appear to be no measurements
of any CNO H-atom abstraction rates or of the reaction rate
between CNO and NO.
While there are a multitude of plausible exothermic channels
associated with the reaction of HCCN with molecular oxygen,
most can be eliminated because at least one of the resulting
products is not observed. Only HNC, HCN, and CO2 were
observed as products. Since the ground vibrational states of
these species were formed more slowly than HCCN was
destroyed, we cannot tell whether they were formed by the
highly exothermic direct reactions (3a and 3c) into vibrationally
excited states, some of which were indeed observed or were
created via secondary processes.
No CHNO isomers were detected, although a lack of
knowledge of the strengths of their infrared absorptions prevents
us from being sure that none were present. We can, however,
be certain that virtually no C2H or HCO was formed. Previously
we have been able to detect C2H at a concentration estimated25
to be less than 1012 molecule cm-3. Therefore, although this
species is quite reactive, it should have been detectable at short
reaction times if it had been formed in significant amounts. Also
we should have been able to detect HO2, which is formed from
HCO, since it has been observed in the past26 with good S/N.
An ab initio investigation of the potential energy surfaces
involved in the reaction of HCCN with O2 seems highly
desirable. Without guidance from such a calculation, we feel
it is pointless to speculate about the pathway of this reaction,
especially in view of our doubts that any of the observed
products are produced directly by the reaction.
Figure 7. Possible geometries for HCCN + NO reaction channels.
See text for discussion of these three channels.
However, Maclagan’s initial calculations indicate that cy-
clization of the cis adduct for the cyclic structure shown in
Figure 7a is feasible and that the energy of the transition state
for this rearrangement is approximately 15 kcal/mol below the
energy of the reactants. A crude RRKM calculation indicates
that the rate of such a cyclization is at least 2 orders of
magnitude faster than redissociation to reactants. Therefore, if
it proves to be energetically possible for the cyclic structure to
break up as shown in Figure 7a, this pathway would seem a
plausible route to HCN. The completion of Maclagan’s
calculations should provide a clearer picture of what can be
taking place.
Guided by theoretical work23 on the HCCO + NO system,
which in many ways is similar to that investigated here, an
isomerization/fragmentation pathway leading to the exothermic
reaction product, HCN, is shown in Figure 7a. This pathway
mirrors one of the energetically favorable rearrangements
identified by Nguyen et al.23 as having accessible entrance
channels. In the present study HCN was observed as a major
product, formed at a rate equal to that at which HCCN was
destroyed. Therefore, it is reasonable to assume that it is formed
directly as indicated in Figure 7a, although formation from a
secondary reaction with NO or the photolyte cannot be entirely
ruled out.
Conclusions
The overall rate constants for reaction of HCCN with O2 and
NO have been determined using pseudo-first-order methods.
These rates have been found to be (3.5 ( 0.6) × 10-11 cm3
molecule-1 s-1 for reaction with NO and (1.8 ( 0.4) × 10-12
cm3 molecule-1 s-1 for reaction with O2. Several products for
these reactions were identified. From reaction with NO, HCN,
and HCNO were found, and HCN could be a primary product.
From reaction with O2, HCN, HNC, and CO2 were observed.
None of these species are believed to be produced directly by
the reaction.
If the mechanism is that of Figure 7a, which is analogous to
the pathway proposed by Nguyen et al.,23 our failure to observe
HNCO formed by H atom abstraction by NCO from dibro-
moacetonitrile is easily explained by the rapid reaction17 between
NCO and NO which has24 two roughly equal channels:
Acknowledgment. This work was supported by the Depart-
ment of Energy and the Robert A. Welch Foundation. We wish
to thank Dr. Robert Maclagan for sharing his preliminary
ab initio results on the HCCN + NO reaction and Dr. Carl
Melius for the MP-4 energies of the species involved in this
reaction.
NCO + NO f N2O + CO
f CO2 + N2
44%
56%
(R5a)
(R5b)
References and Notes
and a rate constant of 3.3 × 10-11 cm3 molecule-1 s-1 at 296
K. However, no CO2 could be observed as a product in the
NO system.
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The observation of HCNO as a reaction product in the HCCN
+ NO reaction cannot be explained by the endoergic channel
2i. A second reaction must be occurring. The observation of
HCNO could be interpreted as evidence that at least some
HCCN‚NO collision complexes are stable enough to survive
to a collision with a second NO when they could react to form
HCNO + ONCN as illustrated in Figure 7b. Alternatively,
CNO could be produced by channel 2f through a five-membered
ring intermediate as depicted in Figure 7c (such a five-membered
ring intermediate was not proposed by Nguyen et al.). This
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