Collision-Induced Dissociation Dynamics of the [OCS · C₂H₂]⁺ Complex. A Combined Experimental and Theoretical Study

Collision-induced dissociation (CID) of the [OCS · C₂H₂]⁺ complex ion with both Xe and Ar over an energy range of 0 to 10 eV in the center of mass frame is studied using a guided ion beam tandem mass spectrometer. The cross sections of the ionic products observed (C₂H₂S⁺, OCS⁺, C₂H₂⁺, and S⁺) are analyzed by taking into account reactant energy distributions, multiple collisions, lifetime effects and competition. A recently devised statistical model for the simultaneous analysis of competitive product channels is used to analyze three channels for the first time, with good results. Thresholds for product formation at 0 Kelvin are 0.33±0.07 eV for C₂H₂S⁺, 0.95±0.07 eV for OCS⁺, 1.22±0.08 eV for C₂H₂⁺, and an upper limit of 4.26 eV for S⁺. These results are comparable to available literature thermochemical data within experimental errors. Competitive shifts are significant, about 0.3 eV for both OCS⁺ and C₂H₂⁺. Ab initio calculations at the QCISD/6-311+G**//MP2/6-311+G** and CCSD//6-31G*//CCD/6-31G* levels are performed on the system. The reaction coordinates of the potential energy surface of the system is quantitatively mapped using results from CID and ab initio calculations. The identity of the C₂H₂S⁺ product is suggested to be the cyclic ethylene sulfide cation on the basis of the results of calculations and previous kinetic energy release measurements. Product branching ratios as a function of energy are analyzed and compared to those determined in previous photodissociation and bimolecular reaction experiments.