Mode-dependent H atom tunneling dynamics of the S₁ phenol is resolved by the simple topographic view of the potential energy surfaces along the conical intersection seam.

This article focuses on the mode-dependent hydrogen atom tunneling dynamics in the S₁ excited state of phenol, emphasizing how vibrational mode excitations influence tunneling rates and nonadiabatic transitions near the S₁(ππ*)/S₂(πσ*) conical intersection. Using semi-classical Wentzel–Kramers–Brillouin (WKB) calculations on two-dimensional potential energy surfaces extended along specific vibrational normal modes, the study rationalizes experimental observations that certain vibrational modes (e.g., ν₁) strongly couple to the tunneling coordinate and expedite tunneling, while others (e.g., ν₉a) act as spectators with minimal effect. The work highlights that the topography of the conical intersection seam coordinates significantly shapes the effective tunneling barrier and thus governs the mode-specific tunneling dynamics, energy disposal, and nonadiabatic transition probabilities. These findings underscore the importance of considering multi-dimensional potential energy surface features, including conical intersection seam coordinates, to understand nonadiabatic photochemical processes in phenol and related systems.