Machine Learning Photodynamics Unveils a Controlled H₂ 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.