Reactions of Acetonitrile with Cl and F
J. Phys. Chem., Vol. 100, No. 2, 1996 667
matrix and measured infrared and ultraviolet spectra of the
products.39 She observed the bands below 300 nm seen here
but attributed them to CH2CNH. It is unlikely that this molecule
could be formed in the pulse radiolysis experiments, and we
tentatively assign those bands to CH2CN. The bands below
359 nm could be associated with a fluorine atom adduct,
although the good correspondence between the rate coefficients
measured by fluorine atom loss and by the relative rate method
suggest that the ultimate fate of the adduct is reaction.
It is interesting that the reaction of fluorine atoms with
acetonitrile involves an addition mechanism, while that of
chlorine atoms appears to proceed entirely by abstraction. The
reaction of chlorine atoms with alkenes and alkynes is thought
to proceed by electrophilic addition. The strong carbon-
nitrogen triple bond and high ionization potential of acetonitrile
probably make this mechanism unfavorable. However, it is not
clear why the addition of fluorine atoms is so rapid.
Figure 14. Arrhenius plot showing all measurements of k1: filled
triangles, this work; filled circles, Poulet et al.;11 open triangles, Olbregts
et al.;9 open square, upper limit of Kurylo and Knable;10 line, fit to
data described in text.
Although the reaction of F atoms with CH3CN is very fast,
with a rate coefficient of (5.5 ( 1.5) × 10-11 cm3 molecule-1
s-1 at 296 K and ambient pressure, the concentration of F atoms
in the stratosphere is very small, of the order of 1 molecule
production of CH2ClCN versus CCl4 in the competitive pho-
tochlorination of mixtures of CH3CN and CHCl3.
cm-3 40
, and so reaction 2 will not impact the calculated profiles
of CH3CN.
Cl + CHCl3 f HCl + CCl3
(15)
(16)
Acknowledgment. NCAR is sponsored by the National
Science Foundation. Part of the work carried out at NCAR
was funded through NASA’s Upper Atmosphere Research
Program, Grant W-18067. The authors thank Chris Cantrell
and Mark Smith of NCAR and Steve Japar of Ford for their
comments on the manuscript.
CCl3 + Cl2 f CCl4 + Cl
Olbregts et al. evaluated the available data to reach their
reference expression, based largely on the direct data of Clyne
and Walker30 and indirect data of Knox31 and others (discussed
in ref 31). The data produced by Clyne and Walker are usually
regarded as being too high, and current evaluations12,13 are based
on the data of Knox. However, recent relative rate studies by
Wallington32 and Beichert et al.33 at room temperature gave a
rate coefficient k15 ) 1.13 × 10-13 cm3 molecule-1 s-1, which
is roughly 50% higher than the room temperature value derived
from the data of Knox. In order to reevaluate the data of
Olbregts et al., we have taken the room temperature value of
k15 measured by Wallington, with the activation energy recom-
mended in refs 12 and 13 (the mean of two determinations by
Knox). The revised values for k1 measured by Olbregts then
become 4.6 × 10-14 and 9.4 × 10-14 cm3 molecule-1 s-1 at
370 and 413 K, respectively. The revised data of Olbregts et
al. are included in the Arrhenius plot (Figure 14) and are found
to be in excellent agreement with the current data set.
In the stratosphere, the strong temperature dependence of k1
means that it will not compete with the OH reaction, since OH/
Cl is typically about 20, and kOH/kCl is calculated to be around
3-4 for all temperatures encountered in the atmosphere.12
Nevertheless, the reaction will have a larger effect than
previously thought, especially at higher altitudes, where CH3-
CN profiles may be more sensitive to reaction 1.
We have reported the first measurements of the pressure
dependence of the rate coefficient for the reaction of fluorine
atoms with acetonitrile. There is evidence from the FTIR
product study that abstraction occurs at least part of the time,
and the time-resolved UV experiments show the production of
two absorbing species. The species OCN and NHCN, which
are both isoelectronic with CH2CN, are known to have banded
spectra between 300 and 420 nm,34-36 and it would be expected
that CH2CN would also absorb in this region. Although CH2-
CN has been observed in interstellar space through its millimeter
wave spectrum,37,38 its ultraviolet spectrum has not been
positively identified. The spacings of the vibrational progres-
sions in Figure 8 are 400 ( 50 cm-1 for the bands below 350
nm and 980 ( 20 cm-1 for the bands below 300 nm. Jacox
codeposited CH3CN and excited argon atoms in a cryogenic
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