Activation of Methane by Zr⁺: A Deep-Dive into the Potential Surface via Pressure- and Temperature-Dependent Kinetics...

The kinetics of Zr⁺ + CH₄ are measured using a selected-ion flow tube apparatus over the temperature range 300-600 K and the pressure range 0.25-0.60 Torr. Measured rate constants are small, never exceeding 5% of the Langevin capture value. Both collisionally stabilized ZrCH₄⁺ and bimolecular ZrCH₂⁺ products are observed. A stochastic statistical modeling of the calculated reaction coordinate is used to fit the experimental results. The modeling indicates that an intersystem crossing from the entrance well, necessary for the bimolecular product to be formed, occurs faster than competing isomerization and dissociation processes. That sets an upper limit on the lifetime of the entrance complex to crossing of 10^-11 s. The endothermicity of the bimolecular reaction is derived to be 0.09 ± 0.05 eV, in agreement with a literature value. The observed ZrCH₄⁺ association product is determined to be primarily HZrCH₃⁺ not Zr⁺(CH₄), indicating that bond activation has occurred at thermal energies. The energy of HZrCH₃⁺ relative to separated reactants is determined to be -0.80 ± 0.25 eV. Inspection of the statistical modeling results under best-fit conditions reveals reaction dependences on impact parameter, translation energy, internal energy, and angular momentum. Reaction outcomes are heavily affected by angular momentum conservation. Additionally, product energy distributions are predicted.