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.