  Quantum tunneling reactions play a significant role in chemistry when
classical pathways are energetically forbidden, be it in gas phase reactions,
surface diffusion, or liquid phase chemistry. In general, such tunneling
reactions are challenging to calculate theoretically, given the high
dimensionality of the quantum dynamics, and also very difficult to identify
experimentally. Hydrogenic systems, however, allow for accurate
first-principles calculations. In this way the rate of the gas phase proton
transfer tunneling reaction of hydrogen molecules with deuterium anions, H_2 +
D^- --> H^- + HD, has been calculated, but has so far lacked experimental
verification. Here we present high-sensitivity measurements of the reaction
rate carried out in a cryogenic 22-pole ion trap. We observe an extremely low
rate constant of (5.2 +- 1.6) x 10^(-20) cm^3/s. This measured value agrees
with quantum tunneling calculations, serving as a benchmark for molecular
theory and advancing the understanding of fundamental collision processes. A
deviation of the reaction rate from linear scaling, which is observed at high
H_2 densities, can be traced back to previously unobserved heating dynamics in
radiofrequency ion traps.
