24
Can. J. Chem. Vol. 78, 2000
ity to both reactions increases, with the positive sensitivity
to k17 being significantly compensated by the negative sensi-
tivity to k–17. At the lower temperatures, the sensitivity to re-
action [–23] dominates, therefore adjustment of k–23
provides a sensitive and selective means of fitting the calcu-
lated yield of C4H6 to the experimental yield. However, the
sensitivity of this adjustment decreases with increasing tem-
perature and is dependent to some degree on the estimates of
kinetic parameters for reactions [17] and [–17]. For these
reasons, the rate constants used for reaction [–23] in fitting
the calculated yields of C4H6 to the experimental data are
perhaps best regarded as estimates which may depend sig-
nificantly on other parameters related to the reactions of
C2H3 and C4H7.
The values of k–23 which provided agreement with the
measured yields of C4H6 are presented in Table 5 and Fig. 5.
Also shown in the figure is a line calculated from the Arrhenius
parameters for reaction [–23] estimated by Weissman and
Benson (40). Our results are approximately a factor of seven
lower than their estimate with about the same activation en-
ergy. Our scatter is fairly large at the highest temperatures,
possibly reflecting a problem resulting from the increased
complexity of the dependence of the C4H6 yield on other
reactions in the model. The uncertainty in the values of the
kinetic parameters for some of the other reactions of C2H3
and C4H7 will almost certainly have a bearing on this behav-
iour. However, with these reservations in mind, our rate con-
stants lead to an activation energy of 160 kJ mol–1 and a
preexponential factor of 2.2 × 1013 s–1, both of which are
consistent with results for other hydrogen atom elimination
reactions of hydrocarbon free radicals.
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Acknowledgements
The experimental work upon which the model is based
was supported by funding from the Department of Energy,
Mines and Resources Canada. General support for the mod-
elling was provided by an operating grant from the Natural
Sciences and Engineering Research Council Canada.
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© 2000 NRC Canada