Kinetics and Product Branching in Dihydrogen Activation by Gaseous Manganese Monoxide Cations

The activation of dihydrogen by transition-metal monoxide cations (MO⁺) in the gas phase offers valuable mechanistic insight into multistate reactions. In this work, we present a mixed quantum-classical trajectory surface hopping study of the MnO⁺ + H₂ reaction, based on newly developed full-dimensional coupled potential energy surfaces (PESs) for both the lowest-lying quintet and septet spin states, trained using a machine learning approach with density functional theory (DFT) data. The calculated total rate coefficients show good agreement with experimental values at low temperatures but deviate somewhat at higher temperatures, presumably due to an underestimation of the reaction barrier by DFT and/or the neglect of quantum tunneling. While reactivity is controlled by an entrance channel barrier in the quintet state, post-transition state intersystem crossing as well as multiple adiabatic reaction channels lead to branching in product formation. The calculated product branching fractions for the MnOH⁺ + H and Mn⁺ + H₂O channels are in good agreement with experimental observations. Finally, isotope effects on branching fractions are obtained, although quantitative discrepancies with experiment remain.