Tunneling in hydrogen and deuterium atom addition to CO at low temperatures

The hydrogen and deuterium atom addition reactions of CO to form HCO and DCO are addressed by Harmonic Quantum Transition State Theory calculations. Special attention is paid to the reactions at very low temperatures (5–20 K) where it is found that quantum tunneling leads to substantial rates of reaction. This supports experiments in the solid phase, which conclude that these reactions are driven by tunneling at low temperatures. The calculated kinetic isotope effect of kD/kH = 1/250 is found to be lower than the experimentally deduced value of 0.08 for the surface reaction. Possible reasons for this discrepancy are discussed.