title: Forward and backward pericyclic photochemical reactions have intermediates in common, yet cyclobutenes break the rules.
abstract: Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes.
authors.affiliation: Max-Planck-Institut für Quantenoptik, 85741 Garching, Germany. w.fuss@mpq.mpg.de
authors.forename: Werner
authors.initials: W
authors.lastname: Fuss
authors.forename: Wolfram E
authors.initials: WE
authors.lastname: Schmid
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Trushin
authors.forename: Paul S
authors.initials: PS
authors.lastname: Billone
authors.forename: William J
authors.initials: WJ
authors.lastname: Leigh