We report new results from quantum calculations of energy-transfer processes
taking place in interstellar environments and involving two newly observed
molecular species: C5N− and C7N− in collision with He atoms and the
p-H2 molecules. These species are part of the anionic molecular chains
labeled as cyanopolyynes which have been observed over the years in
molecule-rich Circumstellar Envelopes and in molecular clouds. In the present
work, we first carry out new abinitio calculations for the C7N−
interaction potential with He atom and then obtain state-to-state rotationally
inelastic cross sections and rate coefficients involving the same transitions
which have been observed experimentally by emission in the interstellar medium
(ISM) from both of these linear species. For the C5N−/He system we extend
the calculations already published in our earlier work (see reference below) to
compare more directly the two molecular anions. We extend further the quantum
calculations by also computing in this work collision rate coefficients for the
hydrogen molecule interacting with C5N−, using our previously computed
interaction potential. Additionally, we obtain the same rate coefficients for
the C7N−/H2 system by using a scaling procedure that makes use of the
new C7N−/He rate coefficients, as discussed in detail in the present
paper. Their significance in affecting internal state populations in ISM
environments where the title anions have been found is analyzed by using the
concept of critical density indicators. Finally, similarities and differences
between such species and the comparative efficiency of their collision rate
coefficients are discussed. These new calculations suggest that, at least for
the case of these longer chains, the rotational populations could reach local
thermal equilibrium conditions within their observational environments