Reactivity of HNC with small hydrocarbon radicals

We report a theoretical study of the potential energy surfaces relevant to reaction of hydrogen isocyanide (HNC) with the hydrocarbon radicals R (R = H, CH₃, C₂H, C₂H₃, C₂H₅). Stationary points on these surfaces have been obtained at the B3-LYP/6-311 +G** level of theory; relative energies of the stationary points have been determined by implementation of the CBS-RAD "model chemistry" methodology on the B3-LYP/6-311+G** optimized geometries. Product channels considered are nitrile formation (RCN + H), R-catalyzed tautomerization (HCN + R), and, when exothermic, H-atom abstraction (RH + CN). We find that the barriers to the HCN + R channel universally exceed those for the RCN + H channel, thus effectively inhibiting the former process at low temperatures. Nevertheless (with the exception of R=C₂H), a moderate barrier (E_a < 35 kJ mol^-1) also exists to RCN + H production. The comparatively low reactivity thus inferred for HNC, in the context of its reactions with radicals in general, is consistent with the apparent longevity of HNC in cold astrophysical environments, which contain such radicals. However, none of the reactions surveyed here seems to fulfill the requirements of the unidentified "200 K" barrier to HNC removal in interstellar clouds.