Branching ratio in the photodissociation of (C₆H₅NH₂)⁺-H₂O–H₂¹⁸O

Molecular systems including clusters often manifest multiple photodissociation pathways upon absorption of photon energy enough to break down chemical bonds. This certainly raises fundamental questions to chemists: which pathway will be most favored and how can we predict it with precision? To address these issues, we had previously introduced a rather crude but highly simplified and straightforward calculation method, Rice-Ramsperger-Kassel-Marcus (RRKM) calculation method complemented by the concept of extreme loose transition state (eLTS). This approach has proven effective in estimating branching ratios in photodissociation of C₆H₄BrCl⁺. Here, we have extended this method to interpret results in IR photodissociation of (C₆H₅NH₂)⁺-H₂O–H₂¹⁸O for further evaluation and refinement of this method. We compared branching ratios derived from RRKM-eLTS with those obtained via phase-space theory (PST) to find that our calculation results through RRKM-eLTS were quite in line with the experimental data while those from PST calculation fluctuated significantly depending on the calculation levels and basis sets. This indicates that RRKM-eLTS model not only aligns well with experimental observations giving insights into the relevant rate constants but also intuitively explains these results. We, hereby, suggest that RRKM-eLTS model is a robust and user-friendly method for computing branching ratios, with possible applications to other molecular systems.