  We present an extensive range of quantum calculations for the state-changing
rotational dynamics involving two simple molecular anions which are expected to
play some role in evolutionary analysis of chemical networks in the
Interstellar environments, C$_2$H$^-$($X^1\Sigma^+$) and C$_2$N$^-$ ($X^3
\Sigma^-$) but for which inelastic rates are only known for C$_2$H$^-$. The
same systems are also of direct interest in modelling selective
photo-detachment (PD) experiments in cold ion traps where the He atoms function
as the chief buffer gas at the low trap temperatures. This study employs
accurate, \textit{ab initio} calculations of the interaction potential energy
surfaces (PESs) for these anions, treated as Rigid Rotors (RR) and the He atom
to obtain a wide range of state-changing quantum cross sections and rates at
temperatures up to about 100 K. The results are analysed and compared for the
two systems, to show differences and similarities between their rates of
state-changing dynamics.
