Kinetics and Thermochemistry of the Cl(²P_J) + C₂Cl₄ Association Reaction

Abstract

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the Cl( 2 P J ) + C 2 Cl 4 association reaction as a function of temperature (231-390 K) and pressure (3-700 Torr) in nitrogen buffer gas. The reaction is found to be in the falloff regime between third and second order over the range of conditions investigated, although the second-order limit is approached at the highest pressures and lowest temperatures. At temperatures below 300 K, the association reaction is found to be irreversible on the experimental time scale of ∼20 ms. The kinetic data at T < 300 K have been employed to obtain falloff parameters in a convenient format for atmospheric modeling. At temperatures above 330 K, reversible addition is observed, thus allowing equilibrium constants for C 2 Cl 5 formation and dissociation to be determined. Second-and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction: ∆H°2 98 ) -18.1 ( 1.3 kcal mol -1 , ∆H°0 ) -17.6 ( 1.3 kcal mol -1 , and ∆S°2 98 ) -27.7 ( 3.0 cal mol -1 K -1 . In conjunction with the well-known heats of formation of Cl( 2 P J ) and C 2 Cl 4 , the above ∆H values lead to the following heats of formation for C 2 Cl 5 at 298 and 0 K: ∆H°f ,298 ) 8.0 ( 1.3 kcal mol -1 and ∆H°f ,0 ) 8.1 ( 1.5 kcal mol -1 . The kinetic and thermochemical parameters reported above are compared with other reported values, and the significance of reported association rate coefficients for understanding tropospheric chlorine chemistry is discussed. † Present address: Cl 2 + hν (355 nm) f nCl( 2 P 3/2 ) + (2n)Cl( 2 P 1/2 ) (2) ln{[Cl( 2 P J )] 0 /[Cl( 2 P J )] t } ) (k 1 [C 2 Cl 4 ] + k 3 )t ) k′t (I)

Gas phase oxidation of n-decane and PCE by photocatalysis using an annular photoreactor packed with a monolithic catalytic bed coated with P25 and PC500

“…However such reduction was not observed which may indicate further mechanisms in the photodegradation of PCE besides the classical attack of hydroxyl radicals. Chlorine radicals chain propagation reactions have been a matter of discussion by several research groups in an attempt to explain the PCO mechanism of chlorinated compounds [45][46][47].…”

“…Several authors have followed another meaningful approach to clarify the mechanism of PCE degradation by PCO [45][46][47][48][60][61][62]. However, agreement is yet to be found.…”

“…However, agreement is yet to be found. On one hand, different authors [45][46][47] claimed that Cl • radical addition to PCE occurs several times faster than HO • radical addition, neglecting therefore the role of HO • radical. Such conclusion was further substantiated by Lu et al [61] and Fan and Yates [62] after finding that the surface hydroxyl groups were inactive in the oxidation of methyl chloride and trichloroethylene.…”

Temperature‐dependent kinetics study of the gas‐phase reactions of atomic chlorine with acetone, 2‐butanone, and 3‐pentanone

“…Chlorine atom kinetics as a function of temperature and pressure of N 2 bath gas were studied using the laser flash photolysis (LFP)–resonance fluorescence (RF) technique. The LFP–RF apparatus was similar to those we have employed in a number of previous studies of chlorine atom reactions with atmospheric gases 24–32. A schematic diagram of the apparatus is published elsewhere 33.…”

Theoretical study of the relative reactivity of chloroethenes with atmospheric oxidants (OH, NO3, O(3P), Cl(2P) and Br(2P))

“…In the troposphere the reaction with OH radicals is the dominant primary oxidation step 2 and also the reaction with chlorine atoms should be considered since the rate constant for this process is larger than for the reaction with OH. 1,3,4 The possibility of Br-atom reaction occurring in the troposphere has also been suggested. 5 The reaction with O( 3 P) atoms is generally not competitive with the other paths because of the low concentration of O( 3 P) atoms although it is a prototype system for the addition reactions to the C=C bond.…”