Fig. 8 shows how k3[NO] varies with [NO] at room tempera-
ture The derived rate constant k3(298 K) ¼ (1.33 ꢁ 0.08) ꢂ
10ꢀ11 cm3 moleculeꢀ1 sꢀ1 compares well with the recom-
mended value of 1.3 ꢂ 10ꢀ11 cm3 moleculeꢀ1 sꢀ1, which carries
an uncertainty factor of two.12
We present an Arrhenius plot of the rate constants for the
CF3CFHO2 + NO reaction in Fig. 5(c). As with reaction (3)
between CF3CF2 and NO, temperature-dependence of the rate
constant is, at most, very slight, the gradient of the Arrhenius
plot yielding:
RO2 radicals. The rate constants that we have measured and
their temperature-dependence are summarised in Table 1,
where they are compared with previously recommended values
of these rate constants.
The large values of the rate constants for the reactions
between NO and these peroxy radicals, together with their
small, but significant, negative temperature-dependence, are
consistent with reactions that proceed across potential energy
surfaces with no barrier and long-range attraction between
the reagents and which exhibit minima associated with short-
lived and energised RO2NO complexes.
k3ðTÞ ¼ ð1:7 ꢁ 0:5Þ ꢂ 10ꢀ11
ꢂ expððꢀ71 ꢁ 69Þ K=TÞ cm3 moleculeꢀ1 sꢀ1
Alternatively the data can be fitted to the expression:
Acknowledgements
We are grateful to the NERC (the Natural Environment
Research Council) for a grant in aid of this research and to
Dr Ian Sims for useful comments on a draft of this paper.
We thank Hannelore Keller-Rudek and Geert Moortgart with
advice over the absorption cross-sections of the ethyl and
substituted ethyl iodides.
k3ðTÞ ¼ ð1:35 ꢁ 0:02ÞðT=298 KÞð0:29ꢁ0:09Þ
ꢂ 10ꢀ11 cm3 moleculeꢀ1 sꢀ1
There has been no previous report on the temperature-
dependence of the rate constant for this reaction.
In this case, the value of k3 at 198 K is significantly below
that obtained by extrapolating the data from higher tempera-
tures. As well as experimental error, this discrepancy may
reflect uncertainties in other rate constants used in the model-
ling. Also it should be remembered that the total variation in
the experimental values of k3 for this reaction over the tem-
perature range 298 to 198 K amounts to no more than a factor
of ca. 1.5.
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Fig. 8 The values of k3[NO], for the reaction of CF3CFH and NO, at
room temperature plotted against [NO].
3442
Phys. Chem. Chem. Phys., 2003, 5, 3436–3442