Conformer-Specific Tunneling Dynamics Dictated by the Seam Coordinate of the Conical Intersection

The dynamic role of the conical intersection “seam” coordinate has been first revealed in the H fragmentation reaction of ortho(o) -cresol conformers. One of the (3 N – 8) dimensional seam coordinates of the S 1 (ππ*)/S 2 (πσ*) conical intersection has been identified as the CH 3 torsional potential function. The tunneling dynamics of the reactive flux is dictated by its nuclear layout with respect to the CH 3 torsional angle, as the multidimensional tunneling barrier is dynamically shaped along the conical intersection seam. The effective tunneling-barrier weight-averaged over the quantum-mechanical probability along the CH 3 torsional angle perfectly explains the experimental finding: the sharp variation of the tunneling rate ((700–400) ps –1 ) with the CH 3 torsional mode excitations within the narrow (0–100 cm –1 ) energetic window. The much longer S 1 lifetime of cis compared to trans is ascribed to the higher-lying S 1 /S 2 conical intersection of the former. With the use of distinct lifetimes, vibronic bands of each conformer could be completely separated.