The gas phase reaction of C2H+2 with H2 below 3 K: The reopening of the bimolecular C2H+3 channel at low energy

The bimolecular reaction of C2H+2 with H2 is found to undergo a surprising change of mechanism to one of inverse energy dependence below temperatures of 3 K as observed in a free jet flow reactor. The observed rate coefficient for C2H+3 production is found to be k=5.6×10−11 T−2 cm3/s in this energy regime. Contrary to previous belief, the reaction is exothermic and results are described regarding kinetic isotope effects, the competitive three body association process and internal C2H+2 vibrational state dependency of the chemistry. A mechanism is suggested which involves a low temperature switching to a tunneling process through a small barrier and is intimately coupled to the long lived complex properties of very low energy collisions.