Rate constants for the slow Mu + propane abstraction reaction at 300 K by diamagnetic RF resonance

The study of kinetic isotope effects for H-atom abstraction rates by incident H-atoms from the homologous series of lower mass alkanes (CH₄, C₂H₆ and, here, C₃H₈) provides important tests of reaction rate theory on polyatomic systems. With a mass of only 0.114 amu, the most sensitive test is provided by the rates of the Mu atom. Abstraction of H by Mu can be highly endoergic, due to the large zero-point energy shift in the MuH bond formed, which also gives rise to high activation energies from similar zero-point energy corrections at the transition state. Rates are then far too slow near 300 K to be measured by conventional TF-μSR techniques that follow the disappearance of the spin-polarised Mu atom with time. Reported here is the first measurement of a slow Mu reaction rate in the gas phase by the technique of diamagnetic radio frequency (RF) resonance, where the amplitude of the MuH product formed in the Mu + C₃H₈ reaction is followed with time. The measured rate constant, k_Mu = (6.8 ± 0.5) × 10⁻¹⁶ cm³ s⁻¹ at 300 K, is surprisingly only about a factor of three slower than that expected for H + C₃H₈, indicating a dominant contribution from quantum tunneling in the Mu reaction, consistent with elementary transition state theory calculations of the k_Mu/k_H kinetic isotope effect.