Step Towards Modeling the Atmosphere of Titan: State-Selected Reactions of O⁺ with Methane

Methane conversion and in particular the formation of the C-O bond is one of fundamental entries to organic chemistry and it appears to be essential for understanding parts of atmospheric chemistry of Titan, but, in broader terms it might be also relevant for Earth-like exoplanets. Theoretical study of the reactions of methane with atomic oxygen ion in its excited electronic states requires treating simultaneously at least 19 electronic states. Development of a computational strategy that would allow chemically reasonable and computationally feasible treatment of the CH₄ (X)/O⁺ (²D, ²P) system is by far not trivial and it requires careful examination of all the complex features of the corresponding 19 potential energy surfaces. Before entering the discussion of the rich (photo) chemistry, inspection of the long range behavior of the system with focus on electric dipole transition moments is required. Our calculations show nonzero probability for the reactants to decay before entering the multiple avoided crossings region of the [CH₄ + O → products]⁺ reaction. For the CH₄/O⁺ (²P) system non-zero transition moment probabilities occur over the entire range of considered C-O distances (up to 15 Å), while for the CH₄/O⁺ (²D) system these probabilities are lower by one order of magnitude and were found only at C-O distances smaller than 6 Å.