Calculations of interstellar radiative association rates including tunnelling: the strange case of C2H +2+ H2

Abstract

Both the radiative association reaction and the ion–molecule reaction between the reactants C₂H⁺₂ and H₂ occur appreciably at temperatures under 100 K and must be included in models of interstellar and circumstellar chemistry. In this paper, we consider the mechanism of the association reaction to form C₂H⁺₄. Ab initio quantum chemical calculations have been undertaken to determine the pathway of minimum potential energy. This pathway is distinct from another pathway we have calculated for the ion–molecule reaction, which forms C₂H⁺₃+ H. For the association channel, our ab initio calculations show that there is a small entrance channel barrier under which low-temperature reactants must tunnel to access the deep potential well associated with C₂H⁺₄. Other than redissociation into reactants, the C₂H⁺₄ reaction complex can dissociate only to the classical (higher energy) form of C₂H⁺₃+ H; this channel is endothermic and does not occur at low temperatures, enhancing the prospect that the C₂H⁺₄ complex can be stabilized by collision with a third body or via emission of a photon. Phase space calculations extended to include tunnelling for both the formation and redissociation of the C₂H⁺₄ complex do not reproduce low-temperature experimental measurements of the rate of the association channel. Rather, a phase space calculation in which it is assumed that the entrance channel barrier does not exist is in agreement with most of the experimental data. It is likely, however, that a small barrier does exist. Accurate radiative association rate coefficients, largely unaffected by the existence of such a small barrier, are tabulated in the temperature range 2–80 K.