The accurate computational prediction of hydrocarbon radical thermochemistry is vital for the development of reliable combustion and soot-formation models. Ordinarily, accurately predicting enthalpies of formation, for example, requires the application of robust coupled cluster methods paired with large basis sets and basis set extrapolations, appended with corrections for harmonic and anharmonic zero-point vibrational energies, relativistic effects, and sundry other factors. Unfortunately, such robust approaches are only applicable to small systems, with perhaps 4-5 non-hydrogen atoms. Balanced reaction classes, as typified by the isodesmic bond separation reactions of Pople and co-workers, can provide a route to accurate thermochemical predictions from inherently inaccurate electronic structure methods through systematic cancellation of errors.  This enables accurate thermochemical predictions at a drastically reduced computational cost. Although there have long existed myriad error-balanced reaction types (homodesmotic, hyperhomodesmotic, etc), we recently introduced a refined hierarchy of homodesmotic reactions that ameliorates inconsistencies regarding the definition of these reaction types that pervade the literature. When paired with a generalization of the concept of bond separation reactions, ascending this hierarchy provides increasingly accurate computational predictions of reaction enthalpies at a given level of theory. The homodesmotic hierarchy has been explicitly defined for hydrocarbons and hydrocarbon radicals, and we demonstrate the error cancellation afforded by this hierarchy for several test sets of molecules as well as key intermediates in combustion and soot-formation.

Hierarchy of homodesmotic reactions for computational thermochemistry of hydrocarbon radicals

245th National Meeting (2013)

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