Interstellar and circumstellar reaction kinetics of Na⁺, Mg ⁺, and Al⁺ with cyanopolyynes and polyynes

We report calculated rate coefficients for the radiative association reactions of M⁺ (Na⁺, Mg⁺, or Al⁺) with cyanopolyynes HC_2n+1N (n = 0-4) and polyynes HC_2nH (n = 1-5), at temperatures relevant to conditions within dense interstellar clouds and the outer envelopes of mass-losing stars. These reaction rate coefficients were determined via variational transition state theory kinetic modeling calculations employing structural, energetic, and spectroscopic parameters obtained from density functional theory calculations. We have performed simple kinetic modeling of these reactions within a cold dark cloud (TMC-1) and within the outer envelope of a C-rich, post-asymptotic giant branch mass-losing star (IRC +10216), with molecular compositions and other parameters of these environments taken from current chemical models. This modeling indicates that the highly efficient radiative association reactions of Mg ⁺ and Al⁺ with HC₅N and HC₇N, and of Na⁺ with HC₇N, give a drastic reduction in the lifetime of the atomic metal ions in these environments compared with a model in which radiative recombination of M⁺ with e^- is the sole metal-ion loss process. These results also provide further support to the hypothesis that M⁺/cyanopolyyne reactions are the route to metal cyanide radicals such as MgCN and MgNC in cold astrophysical environments.