Reaction kinetics to low temperatures. Dicarbon + acetylene, methylacetylene, allene and propene from 77 ≤ T ≤ 296 K

Abstract

The temperature dependence of the reactions of the dicarbon molecule in its ground singlet (X ¹Σ⁺_g) and first excited triplet (a ³Π_u) states with acetylene, methylacetylene, allene and propene has been studied using a recently constructed continuous supersonic flow reactor. Four Laval nozzles have been designed to access specified temperatures over the range of 77 ≤ T ≤ 220 K and measurements have been performed at 296 K under subsonic flow conditions. C₂ was produced in its two lowest electronic states via the in situ multiphoton dissociation of C₂Br₄ at 266 nm. The time dependent losses of C₂ in these two states in the presence of an excess of co-reagent species were simultaneously followed by laser-induced fluorescence in the Mulliken and Swan bands for the detection of singlet and triplet state C₂, respectively. The rate coefficients were measured to be very fast, with values larger than 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ and up to 5 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The reactions of ¹C₂ are seen to be essentially temperature independent from 77 ≤ T ≤ 296 K whereas the rate coefficients for the ³C₂ reactions are seen to increase until they are equivalent to the ¹C₂ values at 77 K.