The reactivity of ground-state carbon atoms with unsaturated hydrocarbons in combustion flames and in the interstellar medium

This article reviews recent crossed-beams and ab-initio studies of reactions of ground-state carbon atoms C(P-3(j)) with unsaturated hydrocarbons and their radicals. All reactions have no entrance barrier and are initiated via an addition of the carbon atom to the pi system. With the exception of the carbon atom reaction with acetylene, which also shows a significant fraction of molecular hydrogen loss, these bimolecular collisions are dominated by an atomic carbon versus hydrogen atom exchange mechanism. In some systems, homolytic cleavages of carbon-carbon bonds present additional decomposition routes of typically a few per cent at the most. The impact-parameter-dependent chemical dynamics are interpreted in terms of statistical and non-statistical decomposition of reaction intermediates involved. The polyatomic reaction products are highly hydrogen-deficient resonance-stabilized free radicals. The latter are strongly suggested as suitable precursors to form the first (substituted) aromatic ring molecule, polycyclic aromatic hydrocarbon-like species and carbonaceous nanoparticles in combustion processes, chemical vapour deposition and the outflows of carbon stars.