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The barrierless, exothermic reactions of H ^– with HC_n N cyanopolyynes, with n = 1 and 3, are analyzed using ab initio calculations of the interaction forces. The shape of the reactive potential energy surface suggests the most efficient approach of H ^– to be on a nearly collinear arrangement on the H-side of HC_nN. Using simple transition state formulation of the reaction rates, which are obtained via calculation of the partition functions of each transition state configuration, provides a new non-Langevin behavior of the reaction which can help explain the unexpectedly large density of CN ^– formation found in observations. A similar procedure is also employed for the reaction of H ^– with HC₃N and the differences in the results, indicating a lower efficiency of the latter reactivity compared with that for CN ^–, are discussed in this paper.