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(H2CCO) = 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.

The protonation of carbenes: structural effects on α-proton acidity of carbocations

“…For CH 3 + in the gas phase ΔHIA(g) = 76.7 kcal mol -1 can be derived from the heat of formation 38 and entropy of the ion reported by Aboud et al 37 This yields ΔHIA(aq) = An indication of the uncertainty of this value is obtained from an alternative extrapolation. Inspection of the points for the alkyl cations in Figure 1 suggests that they are consistent with a mild saturation of substituent effects in the gas phase relative to solution.…”

Ketene Thermochemistry

“…[11] The lower values clustered around À54 of Aubry et al [12] and Traeger [13] are not supported and neither are the much lower results due to Orlov and colleagues [14] and Golovin and others [15] of À87.2 and À96.2 kJ mol À1 , respectively. In very recent work Bui et al [16] summarise previous experimental and computational studies and by calculating the reaction enthalpy for reaction (5):…”

Inhibition of hydrogen oxidation by HBr and Br2

“…The thermochemistry of the bromine-containing species of interest, given in Table 1, was obtained using the Active Thermochemical Tables (ATcT) approach [41,42], which, in contrast to the traditional "sequential" approach, derives accurate, reliable, and internally consistent thermochemical values by analyzing and simultaneously solving [43][44][45] all the available thermochemical interdependencies defined in the underlying Thermochemical Network (TN) [46,47]. The most recent ATcT TN (ver.…”