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Machine Learning Photodynamics Unveils a Controlled H$_2$ Loss Channel in Methaniminium Cation

Daniil N. Chistikov, Pavel M. Radzikovitsky, Dmitry S. Popov, Ivan V. Dudakov, Vadim V. Korolev, Vladimir E. Bochenkov, Anastasia V. Bochenkova

Abstract

The methaniminium cation, CH$_2$NH$_2^+$, plays an important role in Titan's N$_2$--CH$_4$ atmospheric chemistry. As the simplest protonated Schiff base (PSB), it also serves as a model for studying the nonadiabatic dynamics of 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 CH$_2$NH$_2^+$, 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 S$_1$/S$_0$ conical intersection that mediates the concerted double H-atom elimination from the carbon center of CH$_2$NH$_2^+$, yielding carbene CNH$_2^+$ as a direct precursor to HCNH$^+$. On-the-fly trajectory surface hopping dynamics confirm the presence of direct H$_2$ loss following excitation to either the S$_2$ or S$_1$ 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 S$_1$ state, enabling targeted control of the photochemical outcome.

Machine Learning Photodynamics Unveils a Controlled H$_2$ Loss Channel in Methaniminium Cation

Abstract

The methaniminium cation, CHNH, plays an important role in Titan's N--CH atmospheric chemistry. As the simplest protonated Schiff base (PSB), it also serves as a model for studying the nonadiabatic dynamics of 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 CHNH, 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 S/S conical intersection that mediates the concerted double H-atom elimination from the carbon center of CHNH, yielding carbene CNH as a direct precursor to HCNH. On-the-fly trajectory surface hopping dynamics confirm the presence of direct H loss following excitation to either the S or S 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 S state, enabling targeted control of the photochemical outcome.

Paper Structure

This paper contains 5 figures, 1 table.

Figures (5)

  • Figure 1: Excited-state decay pathways of CH$_{2}$NH$_{2}^{+}$, showing XMCQDPT2 energy scans from geodesic interpolation zhu2019geodesic between the S$_2$ Franck-Condon point and three minimum-energy conical intersections. The natural orbitals of the valence $\pi\pi^*$ and $\sigma\pi^*$ states, primarily involved in the transitions, illustrate the change in electronic character.
  • Figure 2: XMCQDPT2 topographies of the potential energy surfaces around the minimum-energy conical intersections: (a) S$_2$($\pi\pi^*$)/S$_1$($\sigma\pi^*$) ; (b) S$_1$($\sigma\pi^*$)/S$_0$; (c) S$_1$($\pi\pi^*$)/S$_0$. The surfaces are plotted in the corresponding branching planes book:CI spanned by the gradient difference (g) and nonadiabatic coupling (h) vectors.
  • Figure 3: CASSCF/FSSH nonadiabatic dynamics following photoexcitation at 0 K. Shown are the electronic state populations over time for excitation to S$_1$ (a) and S$_2$ (b) and the branching ratios of the resulting photodissociation channels starting from S$_1$ (c) and S$_2$ (d).
  • Figure 4: Projections of the initial-condition geometries onto the molecular normal modes, color-coded by dynamical outcome. The red distribution represents all 13,000 initial conditions from the ML/LZBL simulations, while the blue distribution corresponds specifically to those trajectories that decay via the $\sigma\pi^{\ast}$/S$_0$ conical intersection.
  • Figure 5: Schematic overview of the major excited-state decay pathways in CH$_{2}$NH$_{2}^{+}$, showing branching ratios and the key conical intersections that mediate the nonadiabatic dynamics. Branching ratios are determined from CASSCF/FSSH dynamics (analyzing hops via the nearest MECI) and ML/LZBL dynamics (analyzing products formed within 100 fs). The Franck-Condon active modes and the pre-excited mode are indicated.