The kinetics of the reactions CH3CCl2 + O2⇌CH3CCl2O2 → products (1) and (CH3)2CCl + O2 ⇌ (CH3)2-CClO2 → products (2) have been studied using laser photolysis/photoionization mass spectrometry. Decay constants of the radicals were determined in time-resolved experiments as a function of temperature (299-1000 K (reaction 1) and 299-700 K (reaction 2)) and bath gas density ([He] = (3-48) × 1016 molecules cm-3 (reaction 1) and (3-24) × 1016 molecules cm-3 (reaction 2)). At room temperature the rate constants are in the falloff region under the conditions of the experiments. Relaxation to equilibrium in the addition step of the reaction was monitored within the temperature ranges 430-500 K (reaction 1) and 490-550 K (reaction 2). Equilibrium constants were determined as functions of temperature and used to obtain the enthalpies of the addition step of the reactions 1 and 2. At high temperatures (600-700 K) the rate constant of reaction 2 is independent of both pressure and temperature within the uncertainty of the experimental data and equal to (1.72 ± 0.24) × 10-14 cm3 molecule-1 s-1. The rate constant of reaction 1 is independent of pressure within the experimental range and increases with temperature in the high-temperature region: k1-(791 K ≤ T ≤ 1000 K) = (1.74 ± 0.36) × 10-12 exp(-6110 ± 179 K/T) cm3 molecule-1 s-1. Structures, vibrational frequencies, and energies of several conformations of CH3CCl2O2, (CH3)2CCl, and (CH3)2CClO2 were calculated using ab initio UHF/6-31G** and MP2/6-31G** methods. The results were used to calculate the entropy changes of the addition reactions: ΔS°298 = -159.6 ± 4.0 J mol-1 K-1 (reaction 1) and ΔS°298 = -165.5 ± 6.0 J mol-1 K-1 (reaction 2). These entropy changes combined with the experimentally determined equilibrium constants resulted in the R-O2 bond energies: ΔH°298 = 112.2 ± 2.2 kJ mol-1 (reaction 1) and ΔH°298 = 136.0 ± 3.8 kJ mol-1 (reaction 2).
|Number of pages||10|
|Journal||Journal of Physical Chemistry A|
|Publication status||Published - Mar 5 1998|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry