"Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction"

Andrew R. Attar: Aditi Bhattacherjee, C. D. Pemmaraju, Kirsten Schnorr, Kristina D. Closser, David Prendergast, Stephen R. Leone; Science , 04/07/17.

Additional Authors: Aditi Bhattacherjee, C. D. Pemmaraju, Kirsten Schnorr, Kristina D. Closser, David Prendergast, Stephen R. Leone

Abstract:

The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a table-top apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-tovalence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs.