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

The appearance energy of the CH 2 + fragment from CH 2 CO has been carefully remeasured and fitted by a model curve, producing EA 0 (CH 2 + /CH 2 CO) = 13.743 ± 0.005 eV. This value can be sequentially propagated through selected thermochemical cycles to yield individual values for EI(CH 2 ), D 0 (H−CH 2 ), Δ H ° f 0 (CH 2 ), and Δ H ° f 0 (CH 2 CO). 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 CH 3, CH 3 +, CH 2, CH 2 +, and CH 2 CO. The recommended simultaneously adjusted thermochemical values are: Δ H ° f 0 (CH 3 ) = 35.86 ± 0.07 kcal/mol (35.05 ± 0.07 kcal/mol at 298 K), Δ H ° f 0 (CH 3 + ) = 262.73 ± 0.06 kcal/mol (261.83 ± 0.06 kcal/mol at 298 K), Δ H ° f 0 (CH 2 ) = 93.18 ± 0.20 kcal/mol (93.31 ± 0.20 kcal/mol at 298 K), Δ H ° f 0 (CH 2 + ) = 332.92 ± 0.19 kcal/mol (333.04 ± 0.19 kcal/mol at 298 K), Δ H ° f 0 (CH 2 CO) = −11.10 ± 0.21 kcal/mol (−11.85 ± 0.21 kcal/mol at 298 K), as well as D 0 (H 3 C−H) = 103.42 ± 0.03 kcal/mol (104.99 ± 0.03 kcal/mol at 298 K), D 0 (H 2 C−H) = 108.95 ± 0.20 kcal/mol (110.35 ± 0.20 kcal/mol at 298 K), D 0 (H 2 C CO) = 77.08 ± 0.02 kcal/mol (78.73 ± 0.02 kcal/mol at 298 K), EI(CH 3 ) = 9.3830 ± 0.0005 eV, and EI(CH 2 ) = 10.3962 ± 0.0036 eV. These values are in excellent agreement with current and several previous experimental measurements. The recommended enthalpy of formation of CH 2 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.