Kinetic studies of the reaction of atomic sulfur with acetylene

The rate constant for reaction of sulfur atoms with acetylene was measured. Laser flash photolysis of CS2 precursor was employed to generate ground-state S(3P) atoms, which were monitored with time-resolved resonance fluorescence as they reacted with C2H2 in a large excess of Ar bath gas. Temperatures from 295 to 1015 and pressures from 10 to 500 mbar were investigated. A pressure-dependence was observed at all temperatures, revealing that adduct formation is the dominant reaction channel. The necessary stability suggests H2CCS or possibly HCCSH are the products at high temperatures, so that the reaction is spin-forbidden. The fall-off curves may be represented with a broadening factor Fcent = 0.6, and low and high-pressure limiting rate constants of k0 = 1.0 × 10−18 (T/K)−3.55 exp(−1990 K/T) cm6 molecule−2 s−1 and k∞ = 2.1 × 10−11 exp(−11.2 kJ mol−1/RT) cm3 molecule−1 s−1, respectively. An entrance barrier to recombination of about 10 kJ mol−1 is proposed to arise where the singlet and triplet potential energy curves cross.