Recombination versus Disproportionation Reactions of Hydrogen Atoms with ClCF₂CHF, ClC₂F₄, BrC₂H₄, BrC₂F₄, and BrCF₂CFBr Radicals and Unimolecular Reactions of the Haloethane Molecules from Recombination

Abstract

The reactions of hydrogen atoms with the ClCF2CHF, ClC2F4, BrC2H4, BrC2F4, and BrCF2CFBr radicals have been studied at room temperature and 1 Torr pressure of Ar by an infrared chemiluminescence technique in a flow reactor. The H + CF3CH2 recombination reaction was also examined to provide a reference point to earlier experiments from this laboratory. The recombination step generates vibrationally excited molecules that undergo HX(X = Br, Cl, F) elimination at 1 Torr of pressure. The characteristic low vibrational excitation, 〈f V(HX)〉 ≈ 0.15, with a monotonically declining distribution from unimolecular 1,2-HX elimination reactions versus 〈f V(HX)〉 ≈ 0.35 with an inverted distribution from disproportionation, or direct halogen atom abstraction, reactions is used as a diagnostic test for recombination versus disproportionation mechanisms. Upon the basis of the observed HBr vibrational distributions, the H + BrC2F4 reaction has a small Br atom abstraction component that is superimposed upon the HBr vibrational distribution from unimolecular HBr elimination. The other reactions proceed only by a recombination−elimination mechanism. The multiple reaction channels, including C−Br rupture and 1,1-HX elimination as well as 1,2-HX elimination, of the haloethanes are discussed. The 1,1-HX elimination channel is important for the CF2XCF2H and CF2XCFXH molecules. The H + PBr3 reaction is discussed in an appendix.

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

“…[7] In addition, CF 2 ClCH 2 F has been studied by the observation of the infrared emission of HCl and of HF from molecules with 435 kJ mol À1 of energy formed by recombination of H atoms and CF 2 ClCHF radicals. [26] The experimental ratio of HCl to HF products was 3.4 AE 0.3, which is twofold smaller than the ratio of HF and HCl elimination rate constants, which is approximately 6. The experimental ratio can only be explained by isomerisation to CF 3 CH 2 Cl followed by HF elimination.…”

“…The latter were obtained from analysis of previously published chemical activation data, [24,25] as well as consideration of thermal activation studies. [26][27][28][29] In anticipation of the results, the substitution of fluorine or chlorine atoms atom for a hydrogen atom on the carbon atom, C H , from which the H atom departs, always seems to raise the threshold energy for HCl and HF elimination.…”

Mixed Quantum-Classical Reaction Path Dynamics of C2H5F → C2H4 + HF

“…[14][15][16][17][18] More recently, chemical activation has once again been used to investigate product energy distributions and determine the effects of various substituents. [19][20][21][22]24,25 Some of these later experimental studies have also included computational modeling results. 23,25 Nonstatistical vibrational energy distributions of the HF product for the unimolecular decomposition of fluoroethane and related molecules have been obtained by monitoring HF infrared emission following infrared multiphoton dissociation 17 and chemically activated decomposition.…”

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct

“…The lowest vibrational frequencies of the transition state also are sensitive to the C−Br distance. The vibrational energy released 28 to HBr from CH 3 CH 2 Br* activated by H + CH 2 CH 2 Br recombination resembles the energy disposal from C 2 H 5 Cl* and C 2 H 5 F*, and the dynamics for four-centered elimination 29−31 seems to be similar for the three reactions. The 3 kcal mol −1 higher energy of C 2 D 5 CHFBr* partially compensates for the H/D kinetic isotope effects, and the twofold lower rate constant corresponds to E 0 (2,1-DBr) = 50 kcal mol −1 , which would be 49 kcal mol −1 for C 2 H 5 CHFBr, and the F atom had little effect on the threshold energy for 2,1-HBr elimination.…”

“…The lowest vibrational frequencies of the transition state also are sensitive to the C–Br distance. The vibrational energy released to HBr from CH 3 CH 2 Br* activated by H + CH 2 CH 2 Br recombination resembles the energy disposal from C 2 H 5 Cl* and C 2 H 5 F*, and the dynamics for four-centered elimination − seems to be similar for the three reactions.…”