Journal of Physical Chemistry p. 7300 - 7307 (1991)
Update date:2022-08-18
Topics:
Danis, F.
Caralp, F.
Rayez, T.
Lesclaux, R.
The kinetics of reaction 1, CCl3 + O2 + M -> CCl3O2 + M, has been investigated in detail as a function of temperature and over a large pressure range.At low pressure, 0.8-12 Torr, the reaction was investigated by laser photolysis and time-resolved mass spectrometry, while at high pressure (760 Torr), flash photolysis with UV absorption spectrometry was employed.At the low pressure limit, the rate expression, k1(0) = (1.6 +/- 0.3) x 10-30(T/298)-(6.3+/-0.5) cm6 molecule-2 s-1 (M = N2), exhibits a quite strong negative temperature coefficient.The obtained strong collision rate expression, 7.0 x 10-30(T/298)-4.3 cm6 molecule-2 s-1, using either RRKM calculations or Troe's factorized expression, is unable to reproduce the experimental temperature dependence, unless an unreasonably strong temperature dependence is assigned to the collisional efficiency factor: βc = 0.23(T/298)-2.0 (M = N2).Similar results are obtained for other chlorofluoromethyl radicals.The falloff curves were constructed by using RRKM calculations obtained by adjusting βc and the transition-state model, in order to reproduce the experimental data.The rate expression at the high-pressure limit was derived from these calculations k1(infinite) = (3.2 +/- 0.7) x 10-12(T/298)-(1.2+/-0.4) cm3 molecule-1 s-1.All the parameters to be used in Troe's analytical expression for calculating the bimolecular rate constant at any pressure and temperature are given.The rate constant at the low-pressure limit k1(0) is more than an order of magnitude lower than for the CF3 radical.The RRKM calculations show that this arises from a large difference in the C-C bond dissociation energies in the corresponding peroxy radicals: 81.9 kJ mol-1 for CCl3O2 instead of ca. 145 kJ mol-1 for CF3O2.
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