Ionization Energy of Methylene Revisited: Improved Values for the Enthalpy of Formation of CH₂ and the Bond Dissociation Energy of CH₃ via Simultaneous Solution of the Local Thermochemical Network

Abstract

The appearance energy of the CH2+ fragment from CH2CO has been carefully remeasured and fitted by a model curve, producing EA0(CH2+/CH2CO) = 13.743 ± 0.005 eV. This value can be sequentially propagated through selected thermochemical cycles to yield individual values for EI(CH2), D0(H−CH2), ΔH°f 0(CH2), and ΔH°f 0(CH2CO). A set of values with a statistically larger weight is produced by analyzing a local thermochemical network, which combines the present measurement with thirteen other experimental determinations from the literature and encompasses the enthalpies of formation of CH3, CH3+, CH2, CH2+, and CH2CO. The recommended simultaneously adjusted thermochemical values are: ΔH°f 0(CH3) = 35.86 ± 0.07 kcal/mol (35.05 ± 0.07 kcal/mol at 298 K), ΔH°f 0(CH3+) = 262.73 ± 0.06 kcal/mol (261.83 ± 0.06 kcal/mol at 298 K), ΔH°f 0(CH2) = 93.18 ± 0.20 kcal/mol (93.31 ± 0.20 kcal/mol at 298 K), ΔH°f 0(CH2+) = 332.92 ± 0.19 kcal/mol (333.04 ± 0.19 kcal/mol at 298 K), ΔH°f 0(CH2CO) = −11.10 ± 0.21 kcal/mol (−11.85 ± 0.21 kcal/mol at 298 K), as well as D0(H3C−H) = 103.42 ± 0.03 kcal/mol (104.99 ± 0.03 kcal/mol at 298 K), D0(H2C−H) = 108.95 ± 0.20 kcal/mol (110.35 ± 0.20 kcal/mol at 298 K), D0(H2CCO) = 77.08 ± 0.02 kcal/mol (78.73 ± 0.02 kcal/mol at 298 K), EI(CH3) = 9.3830 ± 0.0005 eV, and EI(CH2) = 10.3962 ± 0.0036 eV. These values are in excellent agreement with current and several previous experimental measurements. The recommended enthalpy of formation of CH2 implies that the reaction of singlet methylene with water is essentially thermoneutral (to within ±0.2 kcal/mol) at 0 and 298 K, and slightly endothermic (0.5 ± 0.2 kcal/mol) at 1000 K.