title: Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy.
abstract: The first study of pseudo-bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time-resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo-gem-divinyl[2.2]paracyclophane. From X-ray crystal diffraction measurements we found that the ground-state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S(1) lifetimes we assigned a clear relation between the conformation and the excited-state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited-state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo-para-divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S(1) state.
authors.affiliation: Steacie Institute for Molecular Sciences, National Research Council (Canada), Ottawa, ON, Canada.
authors.forename: Rasmus Y
authors.initials: RY
authors.lastname: Brogaard
authors.forename: Andrey E
authors.initials: AE
authors.lastname: Boguslavskiy
authors.forename: Oliver
authors.initials: O
authors.lastname: Schalk
authors.forename: Gary D
authors.initials: GD
authors.lastname: Enright
authors.forename: Henning
authors.initials: H
authors.lastname: Hopf
authors.forename: Vitaly A
authors.initials: VA
authors.lastname: Raev
authors.forename: Peter G
authors.initials: PG
authors.lastname: Jones
authors.forename: Ditte L
authors.initials: DL
authors.lastname: Thomsen
authors.forename: Theis I
authors.initials: TI
authors.lastname: Sølling
authors.forename: Albert
authors.initials: A
authors.lastname: Stolow
mesh_terms: D003500:Cyclization; D008958:Models, Molecular; D008968:Molecular Conformation; D056951:Photoelectron Spectroscopy / Q000379:methods; D011083:Polycyclic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D014753:Vinyl Compounds / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D011083:Polycyclic Compounds; D014753:Vinyl Compounds; C558832:pseudo-para-divinyl(2.2)paracyclophane