Tunneling Enhancement of the Gas-Phase CH + CO2 Reaction at Low Temperature

TL;DR

This study demonstrates that quantum tunneling significantly enhances the gas-phase CH + CO2 reaction rate at low temperatures, with experimental evidence and mechanistic insights revealing heavy-particle tunneling effects.

Contribution

First experimental demonstration of heavy-particle tunneling in a gas-phase reaction, specifically for CH + CO2 at low temperatures, supported by rate measurements and theoretical calculations.

Findings

Reaction rate increases rapidly below 100 K.

Tunneling occurs via CH insertion into C-O bond.

Heavy-particle tunneling likely causes reactivity increase at low temperatures.

Abstract

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.