Assessing thermochemical data

Abstract

This paper describes, with examples, a critical assessment of thermochemical data for some small molecules and free radicals. The available heats of formation, Δ f H° (all 298 K values). for simple alkyl hydroperoxides and di-alkyl peroxides were compared and new data are provided. The Δ f H° values, all ±5 kJ/mol, are: CH 3OOH, −135; CH 3CH 2OOH, −168; n-C 3H 7OOH, −189; s-C 3H 7OOH, −205; t-C 4H 9OOH, −240; CH 3OOCH 3, −132; CH 3CH 2OOCH 3, −165; C 2H 5OOC 2H 5, −198; n-C 3H 7OO n-C 3H 7, −240; s-C 3H 7OO s-C 3H 7, −272; t-C 4H 9OO t-C 4H 9, −342. These are consistent with established O–O bond dissociation energies and with additivity considerations. Δ f H° values for the corresponding alkoxy radicals are also addressed. A similar survey was applied to the homologous n-alkyl aldehydes, C2 to C8, for which recommended Δ f H° values, all ±1.5 kJ/mol, are: −166.5, −189, −207.5, −227, −248, −268 and −289, respectively. Particular attention was given to Δ f H°(CH 3CO ) = −10.3 ± 1.8 kJ/mol. The current NIST WebBook datum, Δ f H°(CS) = 280.3 kJ/mol, is arguably the best value, being consistent with related thermochemical data. Finally the Δ f H° values for the allylic free radicals CH 2CHCH 2, 174 ± 3 kJ/mol, CH 2CHCH (OH), 4.5 ± 4 kJ/mol, and (CH 2CH) 2 C(OH), 37 ± 4 kJ/mol, derived from experimental data and results of computational chemistry are described, together with some related homolytic bond strengths.