Electronic state dependence of the ion–molecule reaction CH3CN+ + CH3CN → CH4CN+ + CH2CN: threshold electron–secondary ion coincidence (TESICO) and direct ab initio molecular dynamics study

Abstract

The ion–molecule reaction, CH₃CN⁺ + CH₃CN → CH₃CNH⁺ + CH₂CN, has been investigated using the threshold electron–secondary ion coincidence (TESICO) technique. Relative reaction cross sections for two microscopic reaction mechanisms, i.e., proton transfer (PT) from the acetonitrile ion CH₃CN⁺ to neutral acetonitrile CH₃CN and hydrogen atom abstraction (HA) by CH₃CN⁺ from CH₃CN, have been determined for two low-lying electronic states, ²E and ²A₁ of the CH₃CN⁺ primary ion. The cross section for PT of the ²A₁ state was smaller than that of the ²E state, whereas that of HA are almost the same in the two states. Ab initio calculations showed that the dissociation of the C–H⁺ bond of CH₃CN⁺ is easier in the ²E state than that in the ²A₁ state. The direct ab initio molecular dynamics (MD) calculations showed that two mechanisms, direct proton transfer and complex formation, contribute the reaction dynamics.