Insights into facile methane activation by a spin forbidden reaction with Ta⁺ ions in the gas phase

The activation of methane (CH₄) by transition-metal cations in the gas phase provides a model for understanding the impact of electronic spin on reactivity, with implications in single atom catalysis. In this work, we present a mixed quantum-classical trajectory surface hopping study on the nominally spin-forbidden reaction Ta⁺ + CH₄ → TaCH₂ ⁺ + H₂. To facilitate the dynamics calculations, full twelve-dimensional PESs for three low-lying spin (quintet, triplet, and singlet) states are constructed using a machine learning method from density functional theory data. Furthermore, we report the temperature dependence of the rate coefficients for the Ta⁺ + CH₄ → TaCH₂ ⁺ + H₂ reaction measured using the selected ion flow tube (SIFT) technique. The measured rate coefficient has a near zero temperature dependence and is approximately 50% of the capture limit at room temperature. Our theoretical results with a Gaussian-binning treatment of the product zero-point energy reproduced the experimental rate coefficient and the temperature dependence. Satisfactory agreement is also obtained between theory and differential cross sections measured recently using molecular beams combined with velocity map imaging. Specifically, our multi-state calculations confirm the indirect mechanism of this reaction with long-lived reaction intermediate after passing through the initial barrier and reveal that the kinetic bottleneck in this reaction is intersystem crossing between the quintet and triplet states. Furthermore, the energy disposal in the TaCH₂ ⁺ (both singlet and triplet) and H₂ products is found to be largely statistical due to the long lifetime of the exit-channel complex.