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)

The calculated difference in the standard heat of formation Delta Delta H-f degrees(298.15) of n- and i-C4H3 center dot free radicals is 37.9 kJ mol(-1) for G3MP2B3 and 45.0 kJ mol(-1) for CCSD(T)-CBS (W 1 U) 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 sigma-radicals listed in the title, absolute values of Delta(f) H degrees(298.15) have been calculated using G3MP2B3-computed values of bond dissociation energies D degrees(298.15) and combined with experimental values of Delta(f) H degrees (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 Delta(f) H degrees(298.15) as well as absolute entropies S degrees and heat capacities C-p degrees are given for a series of pi- and sigma-free radicals important to combustion as a function of temperature. A spread of roughly 40 k mol(-1) in the average C-H bond strength of PAH leading to a-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)). (C) 2008 Wiley Periodicals, Inc.