Ab InitioCalculations and Three Different Applications of Unimolecular Rate Theory for the Dissociations of CCl₄, CFCl₃, CF₂Cl₂, and CF₃Cl

Abstract

Previously measured Cl dissociation rate constants for CCl 4 and CFCl 3 were analyzed with three different kinetics modeling calculations. The three models differ in detail but primarily are distinguished by the manner in which the high-pressure limiting rate constant is determined: model 1 involves a calibration to transport properties of the dissociated fragments, model 2 uses a Gorin model with a hindrance parameter, and model 3 requires variational transition state theory on an ab initio reaction path where all low-frequency motion off the path is presumed to be a free rotation. All three models have two adjustable parameters: the dissociation energy E 0 and the average energy transferred to the buffer gas 〈∆E〉 down . All three models are found to give comparable fits to the experiment and produce quite similar values for the adjustable parameters. For CCl 4 , the values are E 0 ) (68.2 ( 1.2) kcal mol -1 and 〈∆E〉 down ) (750 ( 125) cm -1 . For CFCl 3 , the values are E 0 ) (76.5 ( 0.5) kcal mol -1 with 〈∆E〉 down ) (800 ( 215) cm -1 . These values are compared to those obtained in similar studies for CF 2 Cl 2 and CF 3 Cl. The results indicate a substantial and consistent decrease in the C-Cl bond energy with each additional chlorine substitution in the chlorofluoromethanes. Isodesmic electronic structure calculations at the MP2 level confirm this effect but find it to be a little smaller than the experimental results indicate. Extended electronic structure calculations provide heats of formation for all nine CH x F y Cl z methyl radicals.

Thermal decomposition of CF3Br using Br-atom absorption

“…Similar equations have also been derived by Benson [29]. Unfortunately, when these equations are applied to the homologous cases, CF 3 Cl [1], CF 3 Br, and CF 3 I [27], all in Kr bath gas, the predicted vibrational energy transfer always shows a monotonic decrease from lower to higher mass. The experimental ͗⌬E͘ down values for these three cases are cm Ϫ1 , respec-(1100 Ϯ 200), 550, and (850 Ϯ 150) tively, showing a minimum for CF 3 Br.…”

“…identifies the bond energy, E 0 , as the reaction endothermicity, ⌬H for reaction (1). Lennard -Jones pa-0 , 0 rameters for both collisional deactivation and the reverse reaction, (Ϫ1) (i. e., recombination at the high-pressure-limit), are estimated from polarizabilities by the methods of Cambi et al [23].…”

“…Dymov et al [10] in a static system measured not only equilibrium constants as summarized in eq. (5) but also estimated rate constants for reaction (1). The experiments were performed in pure CF 3 Br between 773 -924 K at a density of (2.05 Ϯ molecules cm…”

“…Hence, the present theory and experiments are also completely consistent with this earlier work. Lastly, we have recently commented on how halogen substitution affects the energy transfer parameter in halomethanes [1,4,5] and have specifically compared the dissociations of CH 3 I and CF 3 I [26] because the respective threshold energies are almost identical. Though the fraction of molecules with energy exceeding the threshold was greater in CF 3 I, the ␤ c values necessary to explain the both sets only differed by 1.8.…”

“…Dissociation rate measurements are essential for describing the pyrolysis kinetics in high-temperature combustion and, also, for the estimation of threshold energies (i.e., bond energies) with statistical rate theories [1]. We have already completed studies on several molecules using the Cl-and I-atom atomic resonance absorption spectroscopic (ARAS) techniques and also using the [2 -5].…”