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secondary reactions was assessed; CH3 is generated on a longer
time scale than C2H3. The rate coefficient for the combination
of C2H3 was determined to be (1.41 ( 0.60) × 10-10 cm3
molecule-1 s-1 in the 1 Torr region. This value is close to the
gas kinetic collision rate. Earlier studies at higher pressures
(>100 Torr) report a similar rate coefficient, but both 1,3-
butadiene and C2H2 were observed.
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Various mechanisms involving unimolecular isomerizations
and/or decompositions have been tested using steady state
RRKM calculations. The observed pressure independence for
k1 () k1a + k1b) and the pressure dependence for [C2H2]/[1,3-
butadiene] suggests a mechanism where the chemically activated
1,3-butadiene unimolecularly reversibly isomerizes to cy-
clobutene. At low pressures the chemically activated cy-
clobutene can undergo a symmetry forbidden retro 2 + 2
addition to produce C2H2 and C2H4. Thus, each C2H3 combina-
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The cyclobutene decomposition is not expected to be observed
in thermal systems, since the barrier is approximately 30 kcal/
mol higher than for the isomerization to 1,3-butadiene. This
contrasts with chemical activation systems in which energies
are in excess of the critical energy for reaction; in thermal
(collisional activation) systems the observed rate coefficient is
dramatically reduced by the small Boltzmann factor for large
critical energies. Future studies in which the pressure depen-
dence in the transition region, 5-100 Torr, is determined will
provide information on the validity of the proposed mechanism.
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cyclobutene will not be observed. Thus, monitoring the pressure
dependence of both 1,3-butadiene and cyclobutene will provide
information on the proposed mechanism. The possibility of a
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Acknowledgment. We thank Paul Romani for many helpful
discussions. This work was supported by the NASA Planetary
Atmospheres Research Program. R. Peyton Thorn thanks the
National Academy of Sciences for the award of a research
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