SELECTED-ION FLOW DRIFT-TUBE STUDIES OF THE KINETICS OF THE REACTIONSOF SI-2) WITH C2H2 AND C6H6((P)

The reaction rate coefficients, k, for the reactions of ground state S i+(P-2) With C2H2 and C6H6 have been measured as a function of reactan t ion/reactant neutral centre-of-mass kinetic energy, KE(CM), in a sel ected-ion flow drift tube (SIFDT) apparatus, operated with helium at a temperature 298+/-2 K. The values k of the studied reactions have ver y pronounced negative energy dependences; k decreases by about one ord er of magnitude when KE(CM) increases from near thermal values to abou t 2 eV. The results are interpreted in terms of a simple model assumin g that the reactions proceed via the formation of long-lived complexes , These intermediate complexes decompose back to reactants or forward to products, the unimolecular decomposition rate coefficients for thes e reactions being k(-1) and k(2) respectively. It is found that a powe r law of the form k(-1)/k(2) = constant (KE(CM))(m) closely describes each reaction. The negative energy dependence of the reaction rate coe fficient of the reaction of Si+(P-2) with C2H2 is rationalized in term s of the negative entropy change in the reaction. The dependence of th e reaction rate coefficient of the reaction of Ar+ with C6H6 on KE(CM) is also reported.