Thermochemistry of Key Soot Formation Intermediates:  C₃H₃ Isomers^†

Accurate standard enthalpies of formation for allene, propyne, and four C₃H₃ isomers involved in soot formation mechanisms have been determined through systematic focal point extrapolations of ab initio energies. Auxiliary corrections have been applied for anharmonic zero-point vibrational energy, core electron correlation, the diagonal Born−Oppenheimer correction (DBOC), and scalar relativistic effects. Electron correlation has been accounted for via second-order Z-averaged perturbation theory (ZAPT2) and primarily through coupled-cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [ROCCSD, ROCCSD(T), ROCCSDT, and UCCSDT(Q)]. The correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, 6), was employed. The CCSDT(Q) corrections do not exceed 0.12 kcal mol^-1 for the relative energies of the systems considered here, indicating a high degree of electron correlation convergence in the present results. Our recommended values for the enthalpies of formation are as follows:  (propargyl) = 84.76, (1-propynyl) = 126.60, (cycloprop-1-enyl) = 126.28, (cycloprop-2-enyl) = 117.36, (allene) = 47.41, and (propyne) = 46.33 kcal mol^-1, with estimated errors no larger than 0.3 kcal mol^-1. The corresponding C₃H₃ isomerization energies are about 1 kcal mol^-1 larger than previous coupled-cluster results and several kcal mol^-1 below those previously obtained using density functional theory.