2014MNRAS.445.3610V

Cyanopolyynes (HCnN, n = 1, 3, 5, …) with as many as 11 carbon atoms have already been identified in the interstellar medium. However, only the two smallest isocyanopolyynes (HCn-1NC) were observed in such environment. This fact motivated the first kinetic study of the isomerization reaction between such long-chain species with n = 7, 9 and 11. In this sense, thermodynamic data, transition-state geometries and rate constants for these reactions were estimated by means of advanced quantum chemistry calculations at temperatures from 298.15 to 3000K. We show that the general conclusions are similar to the ones observed in a previous investigation for n = 1, 3 and 5. Thus, this reaction is exothermic and faster in the direction that leads to cyanopolyynes, favouring these isomers in colder environments. However, the ratio between forward and reverse rate constants for this isomerization approaches unit as the temperature is raised, suggesting that warmer environments, such as those found in the atmospheres of evolved carbon stars, are probable candidates for the detection of long-chain isocyanopolyynes. This study also indicates that the kinetic data at the highest temperatures considered show some unexpected variations with respect to chain-size increments caused by entropic contributions. Finally, parameterized equations were fitted to reproduce our rate constant data for reactions with n = 7-11 along the temperature range considered.