Rate of reaction between CN radicals and vibrationally excited HCl

Abstract

Experiments have been performed to measure the increase in the rate of the reaction CN(v₁)+ HCl(v₂)→ HCN + Cl at room temperature when the HCl reagent is excited vibrationally using the output from a pulsed HCl chemical laser. CN radicals are generated by pulsed laser photolysis of NCNO and the decay in their concentration is measured by laser-induced fluorescence. The fraction of HCl which is excited by output from the HCl chemical laser is estimated by observing the infrared fluorescence from excited molecules in the presence and absence of a ‘cold gas filter’. The results of the experiments indicate that HCl(v₂= 1) reacts (160 ± 70) times faster than HCl(v₂= 0) at 298 K. This increase is much larger than the essentially negligible enhancement observed previously for CN(v₁= 1)(I. R. Sims and I. W. M. Smith, J. Chem. Soc., Faraday Trans. 2, 1989, 85, 915), demonstrating that the energy requirements for this reaction are vibrationally selective. The experimental results are compared with the results of calculations using vibrationally adiabatic transition state theory and ab initio information about the nature of the minimum energy path. Finally, calculations based on the angle-dependent line-of-centres model suggest that relative translational energy would be even more effective in promoting this reaction, as would be expected for an exothermic reaction with a relatively early potential-energy barrier.