  The rates of numerous activated reactions between neutral species increase at
low temperatures through quantum mechanical tunneling of light hydrogen atoms.
Although tunneling processes involving molecules or heavy atoms are well known
in the condensed phase, analogous gas-phase processes have never been
demonstrated experimentally. Here, we studied the activated CH + CO2 -> HCO +
CO reaction in a supersonic flow reactor, measuring rate constants that
increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by
CH insertion into the C-O bond, with rate calculations accurately reproducing
the experimental values. To exclude the possibility of H-atom tunneling, CD was
used in additional experiments and calculations. Surprisingly, the equivalent
CD + CO2 reaction accelerates at low temperature as zero point energy effects
remove the barrier to product formation. In conclusion, heavy-particle
tunneling effects might be responsible for the observed reactivity increase at
lower temperatures for the CH + CO2 reaction, while the equivalent effect for
the CD + CO2 reaction results instead from a submerged barrier with respect to
reactants.
