“Conical Intersections Can Ruin a Perfectly Good Approximation- the Born Oppenheimer Approximation”
Dr. David R. Yarkony
Chair and D. Mead Johnson Professor of Chemistry
Johns Hopkins University
December 9, 2020, 7:00 PM EST
ZOOM Event
Once thought of as an arcane theoretical concept, over the last 30 years conical intersections have become known be ubiquitous, efficient funnels for radiation less decay of excited electronic states. But what about chemical processes that take place on the ground or a single isolated, electronic state. Surely if the conical intersection is at high energy (energetically inaccessible) the single state Born-Oppenheimer(BO) approximation should hold.
In this talk we will show using two representative photodissociation processes that, that simple energetic imperative is simplistic. We consider:
C6H5OH(S0)+hv → C6H5OH(S1) → C6H5O(1,22A)+H
which represents photodissociation on an excited but at the energies employed isolated electronic state , S1 and
CH2OH(12A)+hv → CH2OH(12A, vOH = 4) → CH2O(12A, J)+H
where J denotes the rotational quantum number of the formaldehyde formed by the high overtone pumped photodissociation.
We will explain how in each case the standard BO approximation fails as a consequence of energetically inaccessible conical intersections. We will explain why in the case of phenol the barrier inferred from experiment must differ appreciably from that computed using standard electronic structure methods.