Low-temperature rate enhancement of the OH plus methanol reaction
In plain words
The reaction of hydroxyl radicals with methanol runs nearly a hundred times faster at 63 K than at 200 K, the opposite of the usual slowing on cooling. Quantum tunneling of a hydrogen atom has been proposed, but whether the speed-up survives at the very low gas densities of interstellar clouds is disputed.
Precise statement
For OH + CH3OH in Laval-nozzle flows from about 20 to 200 K, explain the rise of $k(T)$ by about two orders of magnitude below 200 K and its pressure dependence from a calculation combining the hydrogen-bonded pre-reaction complex, deep tunneling through the H-abstraction barrier and collisional stabilization, and determine $k(T)$ in the zero-pressure limit. An answer is a master-equation plus tunneling calculation on an accurate potential that reproduces measured $k(T, P)$.
What would settle it
Pressure-resolved low-temperature kinetics compared with master-equation calculations that include instanton or quantum-scattering tunneling rates.
Status in the literature
Tunneling through a weakly bound complex was proposed in 2013 (Shannon, Blitz, Goddard, Heard, Nature Chemistry, https://doi.org/10.1038/nchem.1692); the size of the enhancement at interstellar densities is disputed.