´
225
D. Sarzynski et al. / Chemical Physics Letters 514 (2011) 220–225
experimental results, values of the rate constants, kH at 298 K of
(3.2–4.2) ꢀ 10ꢁ13 cm3 moleculeꢁ1 sꢁ1 for CH3F + Cl [29–31], (3.2–
European Social Fund, Human Capital, National Cohesion Strategy
(contract No. UDA-POKL.04.01.01-00-010/08-01).
6.8) ꢀ 10ꢁ14 cm3 moleculeꢁ1 sꢁ1
(CH2F2 + Cl)
[31,32]
and
4.8 ꢀ 10ꢁ16 cm3 moleculeꢁ1 sꢁ1 (CHF3 + Cl) [33] are in line with
the general trend in kH-values for reactions of the H-abstraction
from the other halomethanes by chlorine atoms. The kinetic data
for the reactions of iodomethanes with chlorine atoms are still
incomplete. However, recent kinetic studies [34,35] show a com-
plex reaction mechanism. The hydrogen abstraction from iodome-
thanes could either proceed directly in a bimolecular reaction or
indirectly via the formation of a weakly bound adduct, which then
decomposes via HCl elimination.
The abstraction of deuterium is related with the higher energy
barrier comparing to the hydrogen abstraction reaction. The acti-
vation energy of 10.7 kJ molꢁ1derived for CD2Cl2 + Cl is
2.8 kJ molꢁ1 higher than that for the reaction CH2Cl2 + Cl. Conse-
quently, values of the rate constant kD are lower than kH, and fur-
thermore, kD depends on a temperature slightly higher than kH.
Our value of kD of (9.7 1.0) ꢀ 10ꢁ14 cm3 moleculeꢁ1 sꢁ1 at room
temperature is almost four time less than that of kH at 298 K. The
experimental measurements of kH/k3 and kD/k3 were done at the
same temperatures, which allows us to calculate values of the
kinetic isotope effect (KIE), described by the ratio kH/kD, directly
from the results of the relative rate measurements. The room
temperature value of KIE is 3.8 0.2. However, the significance of
KIE is distinctly reduced with rising temperature (see Figure 5),
and at the highest temperature of this letter (527 K), the derived
value of KIE is 2.3 0.2.
It is interesting to compare the available values of KIE for reac-
tions of atomic chlorine with chloromethanes. The values of KIE of
5.4 0.3 [27] for CH3Cl/CD3Cl + Cl, 3.8 0.2 for CH2Cl2/CD2Cl2 + Cl
derived in this letter, and 4.7 0.6 [28] for CHCl3/CDCl3 + Cl are
considerably lower than the 17.5 2.5 [5,6] estimated for CH4/
CD4 + Cl at room temperature. An explanation of the reported val-
ues of KIE needs theoretical calculations that enable insight into
the molecular structure and properties of the reactants during
the reaction. The results presented in this letter complete the ki-
netic data for the reactions of chloromethanes with chlorine
atoms: CH3Cl/CD3Cl + Cl, CH2Cl2/CD2Cl2 + Cl and CHCl3/CDCl3 + Cl
obtained previously in our laboratory using the same experimental
technique, the same reference reaction and performed in the same
temperature range. These results are valuable reference data, use-
ful in the evaluation of the rate constants using the computational
methods of the reaction rate theory.
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This study was supported by research fellowship within ‘Devel-
opment program of Wroclaw Medical University’ funded from