| Abstract
| - The appearance energy of the CH2+ fragment from CH2CO has been carefully remeasured and fitted by amodel curve, producing EA0(CH2+/CH2CO) = 13.743 ± 0.005 eV. This value can be sequentially propagatedthrough selected thermochemical cycles to yield individual values for EI(CH2), D0(H−CH2), ΔH°f0(CH2),and ΔH°f0(CH2CO). A set of values with a statistically larger weight is produced by analyzing a localthermochemical network, which combines the present measurement with thirteen other experimentaldeterminations from the literature and encompasses the enthalpies of formation of CH3, CH3+, CH2, CH2+,and CH2CO. The recommended simultaneously adjusted thermochemical values are: ΔH°f0(CH3) = 35.86± 0.07 kcal/mol (35.05 ± 0.07 kcal/mol at 298 K), ΔH°f0(CH3+) = 262.73 ± 0.06 kcal/mol (261.83 ± 0.06kcal/mol at 298 K), ΔH°f0(CH2) = 93.18 ± 0.20 kcal/mol (93.31 ± 0.20 kcal/mol at 298 K), ΔH°f0(CH2+)= 332.92 ± 0.19 kcal/mol (333.04 ± 0.19 kcal/mol at 298 K), ΔH°f0(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 ± 0.03kcal/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, andEI(CH2) = 10.3962 ± 0.0036 eV. These values are in excellent agreement with current and several previousexperimental measurements. The recommended enthalpy of formation of CH2 implies that the reaction ofsinglet methylene with water is essentially thermoneutral (to within ±0.2 kcal/mol) at 0 and 298 K, andslightly endothermic (0.5 ± 0.2 kcal/mol) at 1000 K.
|
