Enthalpy of formation of methyl and methylene radicals of photoionization studies of methane and ketene

Photoion yield curves for CH₃⁺ and CH₂⁺ from methane have been measured near threshold at 295 and 115 °K, and the curves for CH₂⁺ from ketene have been obtained at 295 and 130 °K. Although the detection efficiences for positive and negative ions were nearly equal, a search for the ion‐pair process yielding CH₃⁺+H⁻ gave negative results. The methane data are successfully fitted on the assumption that the full rotational energy is available for formation of CH₃⁺, but that only two rotational degrees of freedom contribute to the available energy for the process yielding CH₂⁺. Neglecting excess energy at threshold, the values ΔHf°₀(CH₃) =149.4±0.5 kJ/mole (35.70±0.12 kcal/mole and ΔHf°₀(CH₂) =392.5±2.1 kJ/mole(93.8±0.5 kcal/mole) from methane. Correction of the threshold for CH₂⁺ from ketene for rotational energy results in the concordant value ΔHf°₀(CH₂) =390.8±1.7 kJ/mole (93.4±0.4 kcal/mole) on the assumption that excess energy can be neglected at threshold. The mean of the two determinations is selected as the preferred value ΔHf°₀(CH₂) =391.6±1.7 kJ/mole (93.6±0.4 kcal/mole).