Thermochemical properties from ab initio calculations: π‐ and σ‐Free radicals of importance in soot formation: ^•C₃H₃ (propargyl), ^•C₄H₃, ^•C₁₃H₉ (phenalenyl), ^•C₆H₅ (phenyl), ^•C₁₀H₇ (naphthyl), ^•C₁₄H₉ (anthryl), ^•C₁₄H₉ (phenanthryl), ^•C₁₆H₉ (pyrenyl), ^•C₁₂H₇ (acenaphthyl), and ^•C₁₂H₉ (biphenylyl)

Abstract

Abstract The calculated difference in the standard heat of formation Δ Δ f H°(298.15) of n ‐ and i ‐C 4 H 3 • free radicals is 37.9 kJ mol −1 for G3MP2B3 and 45.0 kJ mol −1 for CCSD(T)‐CBS (W1U) calculations, which seems to preclude the direct even‐carbon radical pathway to benzene and higher PAH (polycyclic aromatic hydrocarbon) formation including soot in a hydrocarbon flame. For the phenyl‐type σ‐radicals listed in the title, absolute values of Δ f H°(298.15) have been calculated using G3MP2B3‐computed values of bond dissociation energies D°(298.15) and combined with experimental values of Δ f H° (298.15) for the parent hydrocarbon because of a slight systematic overprediction of the thermodynamic stability of large PAHs by the applied computational G3MP2B3 method. Standard enthalpies of formation Δ f H°(298.15) as well as absolute entropies S° and heat capacities C° p are given for a series of π‐ and σ‐free radicals important to combustion as a function of temperature. A spread of roughly 40 kJ mol −1 in the average CH bond strength of PAH leading to σ‐radicals has been calculated, the lowest leading to 4‐phenanthryl (463.6 kJ mol −1 ), the highest leading to 2‐biphenylyl radical (502.5 kJ mol −1 ). © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 395–415, 2008