Variation of CH stretch frequencies with CH4 orientation in the CH4 − F− complex: multiple resonances as vibrational conical intersections

In the \chem{CH_4 - F^-} complex, an adiabatic separation of the CH stretch frequencies from the \chem{CH_4} orientational coordinates allows the calculation of the four adiabatic CH stretch surfaces. These ab initio calculations reveal (i) a large variation of CH stretch frequencies (\textgreater 100 \wn) in the orientational space and (ii) the existence of four symmetrically equivalent sets of vibrational conical intersections (CIs). Two sets of symmetry-allowed CIs are identified in addition to the symmetry-required CIs at the front- and back-side C$_{3v}$ geometries. These results have implications for the evolution of excited CH vibrations in methane during its approach to a potentially reactive surface