Crossed-beam and theoretical study of the S(1D) + C2H2 reaction

The reaction dynamics of excited sulfur atoms, S(1D), with acetylene has been investigated by the crossedbeam scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis at the collision energy of 35.6 kJ mol-1. These studies have been made possible by the development of intense continuous supersonic beams of S(3P,1D) atoms. From product angular and TOF distributions, center-of-mass product angular and translational energy distributions are derived. The S(1D) + C2H2 reaction is found to lead to formation of HCCS (thioketenyl) + H, while the only other energetically allowed channels, those leading to CCS(3Σ-, 1Δ) + H2, are not observed to occur to an appreciable extent. The dynamics of the H-elimination channel is discussed and elucidated. The interpretation of the scattering results is assisted by synergic highlevel ab initio electronic structure calculations of stationary points and product energetics for the C2H2S groundstate singlet potential energy surface. In addition, by exploiting the novel capability of performing product detection by means of a tunable electron-impact ionizer, we have obtained the first experimental information on the ionization energy of thioketenyl radical, HCCS, as synthesized in the reactive scattering experiment. This has been complemented by ab initio calculations of the adiabatic and vertical ionization energies for the ground-state radical. The theoretically derived value of 9.1 eV confirms very recent, accurate calculations and is corroborated by the experimentally determined ionization threshold of 8.9 ( 0.3 eV for the internally warm HCCS produced from the title reaction.