<p>Despite its profound significance, the molecular structural changes near the transition state, driven by the vibronic coupling, have remained largely unexplored, leaving a crucial aspect of chemical reactions shrouded in uncertainty. Herein, the dynamical behavior of the reactive flux on the verge of chemical bond breakage was revealed through the spectroscopic characterization of a large amplitude vibrational motion. Highly excited internal rotor states of S₁ methylamine (CH₃ND₂) report on the structural change as the molecule approaches the transition state, indicating that the quasi-free internal rotation is strongly coupled to the reaction coordinate as their energies near the maximum of the reaction barrier for the N-D chemical bond predissociation. Energy-dependent behavior of the rate constant perfectly correlates with that of the molecular structural change in the N-D bond length, providing unprecedented crucial information about how vibrational energy flows into the reaction coordinate on the adiabatic potential energy surfaces.</p>