Direct Kinetic Studies of the Reactions Br + CH₃OH and CH₂OH + HBr: The Heat of Formation of CH₂OH

Abstract

The chemical equilibrium Br + CH3OH ⇌ HBr + CH2OH (1, −1) has been studied by investigating the kinetics of the forward and reverse reactions. Excimer laser photolysis coupled with Br atom resonance fluorescence detection was used over the temperature range 439−713 K to obtain k1 = (3.41 ± 0.89) × 109T1.5 exp[−(29.93 ± 1.47) kJ mol-1/RT] cm3 mol-1 s-1. The reverse reaction was studied with the fast flow technique, in the temperature range 220−473 K, using laser magnetic resonance for monitoring the CH2OH radicals. Thus, k-1 = (1.20 ± 0.25) × 1012 exp[(3.24 ± 0.44) kJ mol-1/RT] was obtained. The kinetic results were compared with available literature data and possible causes of the deviations were discussed. Kinetic information on the foward and back reactions was combined to obtain the heat of formation for CH2OH. Both second-law and third-law procedures were used in the derivations, giving a recommended value of ΔfH°298(CH2OH) = −16.6 ± 1.3 kJ mol-1, which corresponds to the C−H bond dissociation energy of DH°298(H−CH2OH) = 402.3 ± 1.3 kJ mol-1. These thermochemical data obtained from kinetic equilibrium studies agree within the error limits with current photoionization mass spectrometric and ab initio theoretical results.