A catalytic mechanism is proposed by the present ab initio study for HCN isomerization. This mechanism has to be considered in the context of the reactivity in cometary atmospheres because it supposes the interaction of a cold electron with a hydrated species HCN.H2O. The mechanism implies the capture of an electron by the hydrated species and the transfer of the excess energy gained into its internal vibrational modes. The resulting geometry deformation along with the increasing stability against autodetachment for the anionic species can lead to the transition-state structure along the hydrated anionic potential energy surface and can finally lead to the anionic or neutral separated products. The efficiency of this HNC formation mechanism appears then to be dependent on both the water molecule and cold electron densities.