Kinetics of the R + HBr ⇄ RH + Br (R = CH2Br, CHBrCl or CCl3) equilibrium. Thermochemistry of the CH2Br and CHBrCl radicals

Abstract

The kinetics of the reaction of the CH₂Br, CHBrCl or CCl₃ radicals, R, with HBr have been investigated separately in a heatable tubular reactor coupled to a photoionization mass spectrometer. The CH₂Br (or CHBrCl or CCl₃) radical was produced homogeneously in the reactor by a pulsed 248 nm exciplex laser photolysis of CH₂Br₂ (or CHBr₂Cl or CBrCl₃). The decay of R was monitored as a function of HBr concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature. The reactions were studied separately over a wide ranges of temperatures and in these temperature ranges the rate constants determined were fitted to an Arrhenius expression (error limits stated are 1σ + Student’s t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₂Br + HBr) = (7.5 ± 0.9) × 10⁻¹³ exp[−(2.53 ± 0.13) kJ mol⁻¹/RT], k(CHBrCl + HBr) = (4.9 ± 1.1) × 10⁻¹³ exp[−(8.2 ± 0.3) kJ mol⁻¹/RT] and k(CCl₃ + HBr) < (6 ± 1) × 10⁻¹⁵ at 787 K. The kinetics of the reverse reactions, Br + R′H → HBr + R′ (R′ = CH₂Br or CHBrCl), were taken from the literature and also calculated by ab initio methods at the MP2(fc)/6-31G(d,p)//MP2(fc)/6-31G(d,p) level of theory in conjunction with the thermodynamic transition state theory to calculate the entropy and the enthalpy of formation values of the radicals studied. The thermodynamic values were obtained at 298 K using a second-law method. The results for entropy values are as follows (units in J K⁻¹ mol⁻¹): 263 ± 7 (CH₂Br) and 294 ± 6 (CHBrCl). The results for enthalpy of formation values at 298 K are (in kJ mol⁻¹): 171.1 ± 2.7 (CH₂Br) and 143 ± 6 (CHBrCl). The C–H bond strength of analogous halomethanes are (in kJ mol⁻¹): 427.2 ± 2.4 (CH₃Br) and 406.0 ± 2.4 (CH₂BrCl). Thermodynamic properties of the CH₂Br radical were calculated by statistical thermodynamic methods over the temperature range 100–1500 K.