Accurate thermochemistry of key soot formation intermediates
Wheeler, Steven Edge
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Precise enthalpies of formation of key soot formation intermediates C3H3, C4H3, and C4H5 have been determined through systematic extrapolations of ab initio energies within the focal point method of Allen and co-workers. Electron correlation has been accounted for using second-order Z-averaged perturbation theory (ZAPT2) and coupled cluster theory through full triple excitations [CCSD, CCSD(T), and CCSDT]. We have also explicitly examined the convergence of ZAPTn energies and predicted spectroscopic constants for a series of systems to compare it with more oft-applied restricted open-shell perturbation theories. The results offer justification for the inclusion of ZAPT2 energies as the leading correlation contribution in the focal point method over alternative perturbation theories in cases exhibiting spin-contamination and suggest ZAPT2 as the best available open-shell perturbation theory for routine applications. Based on the extrapolated results, recommended enthalpies of formation are as follows: °°° H(propargyl) = 84.7, H(1-propynyl) = 126.6, H(cycloprop-1-enyl) = 125.7, f0f0f0°°°° H(cycloprop-2-enyl) = 117.3, H(i-C4H3) = 119.0, H(E-n-C4H3) = 130.8, H(i-f0f0f0f0–1°C4H5) = 78.4, and f0H(E-n-C4H5) = 89.1 kcal mol. These constitute the most reliable enthalpies presently available, and their incorporation into detailed kinetic models of soot formation will allow for the development of more definitive models of PAH and soot formation during combustion.