Experiments and Theory on the Thermal Decomposition of CHCl₃ and the Reactions of CCl₂

Abstract

Rate constants for the thermal decomposition of CHCl3 in Kr diuent have been measured by the laser schlieren density gradient method. The only decomposition process indicated is molecular elimination giving the singlet carbene, CCl2, and HCl. Rate constants are determined under different conditions of density over the temperature range 1282−1878 K, giving k(±15%) = 4.26 × 1016 exp(−22 516 K/T) cm3 mol-1 s-1. Electronic structure calculations have provided models for both the transition state and molecule. With these models, both semiempirical Troe and Rice−Ramsperger−Kassel−Marcus unimolecular theoretical calculations are carried out. The experimental results agree with theory provided E 0 = 56.0 kcal mol-1 and 〈ΔE〉down = (820 ± 30) cm-1, suggesting that the barrier for back reaction is 3.8 kcal mol-1. Cl-atom atomic resonance absorption spectrometric (ARAS) experiments, also in Kr diluent, are then carried out, confirming that atom formation is entirely due to the thermal reactivity of CCl2. On the basis of Cl-atom yield measurements, a mechanism for Cl-atom formation is devised. Chemical simulations of the absolute Cl-atom profile data then provide estimates of the temperature dependences for the rate constants used in the mechanism. These results are discussed in terms of unimolecular reaction rate theory suggesting that the heat of formation for CCl radicals is 100 ± 4 kcal mol-1 at 0 K. Our calculated results (R-CCSD(T)) extrapolated to the complete basis set limit give values of Δf = 53.0 and Δf = 102.5 kcal mol-1 and are consistent with the experimental results reported herein. Additionally, the results suggest that CCl2 undergoes dissociative recombination with a substantial activation energy.

Kinetics and mechanism for the thermal chlorination of chloroform in the gas phase: Inclusion of HCl elimination from CHCl3

“…We use the thermochemical properties available in Huybrechts mechanism in this study, since they are close to our own data, and this avoids differences in model predictions because of differences in thermochemical parameters. The values for species not considered in Huybrechts mechanism are calculated in this study or taken from literature as noted in Table III. The high-pressure limit rate constant of CHCl 3 → 1 CCl 2 + HCl in three-parameter Arrhenius expression is then determined from the above thermochemical parameters using canonical transition state analysis in a "THERMKIN" [15] [30] and Kohn et al [29]), which results in their insertion barrier (reverse reaction) of 3.8 kcal/mol at 0 K. Our calculated high-pressure limit rate constants for CHCl 3 → 1 CCl 2 + HCl at three levels are compared with that of Kumaran et al [11] in Table IV, which shows that the values are in good agreement. The above high-pressure limit rate constant for CHCl 3 → 1 CCl 2 + HCl at G3//B3LYP/6-311G(d,p) level is used as input to a kinetic analysis [31,32] for decomposition of CHCl 3 as functions of temperature and pressure (Table V).…”

“…The high-pressure limit rate constant for primary dissociation of CHCl 3 → CCl 2 + HCl was estimated as 1.6 × 10 14 exp(−56 kcal/mol/RT ) s −1 . Kumaran et al [11] report the thermal decomposition of CHCl 3 based on the laser schlieren density gradient study in 101-365 torr Kr bath at a higher temperature range (1282-1878 K). They also applied two unimolecular rate theories (Troe and RRKM) using ab initio determinations, CCSD(T)/cc-pvDZ//MP2/6-31G(d), for both the transition states and molecules.…”

CCl4 Decomposition in RF Thermal Plasma in Inert and Oxidative Environments

“…Since the 1980s several papers have been published on this topic using incineration technology like burning, catalytic oxidation or even plasma technology. Most of these papers are experimental work [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. In our previous papers, the decomposition of CCl 4 in thermal RF plasma was investigated in inert (CCl 4 -Ar) [20] and in oxidative (CCl 4 -O 2 -Ar) [21] environments.…”

The Molecular Structures and Energetics of Cl2CO, ClCO, Br2CO, and BrCO

“…(All bond energies calculated at 0 K. [2][3][4] Our laboratory has recently been developing a composite theoretical approach that is intended to reliably predict a variety of thermodynamic quantities, including heats of formation, without recourse to empirical parameters. 1,[5][6][7][8][9][10][11][12][13] As described below, our approach starts with existing, reliable thermodynamic values (obtained from either experiment or theory). Missing pieces of information are then computed by using high-level ab initio electronic structure methods, such as coupled cluster methods including single, double, and connected triple excitations, with the latter being handled perturbatively.…”