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.