Kinetics of the R + HBr → RH + Br (R = CH2I or CH3) reaction. An ab initio study of the enthalpy of formation of the CH2I, CHI2 and CI3 radicals

Abstract

The kinetics of the reaction of the CH₂I and CH₃ radicals, R, with HBr have been investigated separately in a heatable tubular reactor coupled to a photoionization mass spectrometer. The CH₂I (or CH₃) radical was produced homogeneously in the reactor by a pulsed 248 or 351 nm exciplex laser photolysis of CH₂I₂ (or CH₃I). 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 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(CH₂I + HBr) = (3.8 ± 0.7) × 10⁻¹³exp[ + (1.4 ± 0.6) kJ mol⁻¹/RT] and k(CH₃ + HBr) = (2.3 ± 0.5) × 10⁻¹² exp[ + (0.60 ± 0.17) kJ mol⁻¹/RT]. The threshold energies of the reverse reactions, Br + R′H → R′ + HBr (R′ = CH₂I, CHI₂ or CI₃), were calculated by ab initio methods at the MP2(fc)/6-311G(df)//MP2(fc)/6-311G(df) level of theory. These were combined with the experimentally determined activation energies of the forward reactions in a second-law method to determine the enthalpies of the reactions. The enthalpy of formation values at 298 K are (in kJ mol⁻¹): 228.0 ± 2.8 (CH₂I), 314.4 ± 3.3 (CHI₂) and 424.9 ± 2.8 (CI₃). The C–H bond strengths of analogous iodomethanes are (in kJ mol⁻¹): 431.6 ± 2.8 (CH₃I), 412.9 ± 3.3 (CH₂I₂) and 391.9 ± 3.1 (CHI₃). The Arrhenius expression of the reverse reactions as determined by the thermodynamic transition state theory. The entropies of activation of the reactions were obtained by ab initio calculations.