Machine Learning Photodynamics Unveils a Controlled H 2 Loss Channel in the Methaniminium Cation

The methaniminium cation, CH2NH2+, plays an important role in Titan’s N2–CH4 atmospheric chemistry. As the simplest protonated Schiff base (PSB), it also serves as a model for studying the nonadiabatic dynamics of the retinal PSB, the chromophore central to vertebrate vision. While previous studies have established CN bond cleavage and photoisomerization as the primary pathways in the photochemistry of CH2NH2+, we now report a new UV-induced photochemical pathway to HCNH+, the dominant ion in Titan’s upper atmosphere. Through high-level XMCQDPT2 and CASSCF(12,12) calculations, we identify a novel S1/S0 conical intersection that mediates the concerted double H atom elimination from the carbon center of CH2NH2+, yielding carbene CNH2+ as a direct precursor to HCNH+. On-the-fly trajectory surface hopping dynamics confirm the presence of direct H2 loss following excitation to the S2 or S1 state. Furthermore, our large-scale, machine learning-accelerated simulations reveal that mode-specific pre-excitation can selectively funnel the dynamics into this new channel via the vibronically allowed S1 state, enabling targeted control of the photochemical outcome.