Modelling reaction kinetics of distonic radical ions: a systematic investigation of phenyl-type radical addition to unsaturated hydrocarbons

Gas phase ion-molecule reactions are central to chemical processes across many environments. A feature of many of these reactions is an inverse relationship between temperature and reaction rate arising from a submerged barrier (an early reaction barrier that is below the energy of the separated reactants), which often arises due to a stable pre-reactive complex. While the thermodynamics and kinetics of many ion-molecule reactions have been extensively modelled, the reaction kinetics of ion-molecule reactions involving radical ions are less explored. In this investigation, the target reactions involve distonic radical ions, where the charge and radical moieties are separated within the molecular structure. Experimental rate coefficients for the reaction of either C2H2 or C2H4 with a suite of eighteen distonic radical ions are reported. Rate coefficients are modelled using potential energy schemes combined with a statistical reaction-rate (RRKM-ME) model. Second-order rate coefficients are in good agreement with experimental values with an average RMS deviation of 37% across three orders of magnitude. These predictions are generally sensitive to the relative energetics of the pre-reactive complex forward transition state but are relatively insensitive to the overall exothermicity of the covalent-addition product