Formation of resonantly stabilised free radicals via the reactions of atomic carbon, dicarbon, and tricarbon with unsaturated hydrocarbons: theory and crossed molecular beams experiments

Resonance stabilised free radicals (RSFRs) play an important role in the growth of polycyclic aromatic hydrocarbons and ultimately in the production of soot and carbonaceous particles in combustion flames, in the interstellar medium, and in planetary atmospheres. This article reviews extensive experimental crossed molecular beams and theoretical ab initio/Rice–Ramsperger–Kassel–Marcus studies in the last two decades of the reactions of atomic carbon, C(3P), dicarbon, C2(X1Σg+/a3Πu), and tricarbon, C3(X1Σg+), with unsaturated hydrocarbons, from acetylene to benzene, showing that the reactions form various types of RSFR via Cn(n = 1–3)-for-H, Cn-for-CH3, and Cn-for-CxHy exchange mechanisms. The RSFRs produced in these reactions include CxH (x = 1–8), propargyl (C3H3) and its substituted analogues, 2,4-pentadiynyl-1 (i-C5H3) and 1,4-pentadiynyl-3 (n-C5H3) together with their methyl substituted counterparts, butatrienyl (i-C4H3) and its substituted analogues, and hexenediynyl, i-C6H3, as well as cyclic five-, six-, and seven-member ring radicals including aromatic phenyl, benzyl, and tolyls. The reactions of atomic carbon and dicarbon proceed by barrierless additions to double, triple, or ‘aromatic’ bonds of the unsaturated hydrocarbons, form highly exothermic products, and are fast even at very low temperatures, whereas the reactions of singlet tricarbon require high barriers to be overcome, often leading to endothermic products, and can occur only at high temperatures. The paper summarises typical reaction mechanisms for small carbon species (C, C2, and C3) with unsaturated hydrocarbons and describes implications of the considered reactions in combustion chemistry and astrochemistry.