title: Near ultraviolet photochemistry of 2-bromo- and 2-iodothiophene: Revealing photoinduced ring opening in the gas phase?
abstract: Velocity map imaging methods, with a new and improved ion optics design, have been used to explore the near ultraviolet photodissociation dynamics of gas phase 2-bromo- and 2-iodothiophene molecules. In both cases, the ground (X) and spin-orbit excited (X*) (where X = Br, I) atom products formed at the longest excitation wavelengths are found to recoil with fast, anisotropic velocity distributions, consistent with prompt C-X bond fission following excitation via a transition whose dipole moment is aligned parallel to the breaking bond. Upon tuning to shorter wavelengths, this fast component fades and is progressively replaced by a slower, isotropic recoil distribution. Complementary electronic structure calculations provide a plausible explanation for this switch in fragmentation behaviour—namely, the opening of a rival C-S bond extension pathway to a region of conical intersection with the ground state potential energy surface. The resulting ground state molecules are formed with more than sufficient internal energy to sample the configuration space associated with several parent isomers and to dissociate to yield X atom products in tandem with both cyclic and ring-opened partner fragments.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
authors.forename: Barbara
authors.initials: B
authors.lastname: Marchetti
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
authors.forename: Tolga N V
authors.initials: TN
authors.lastname: Karsili
authors.affiliation: Photek Ltd., 26 Castleham Road, St. Leonards-on-Sea, East Sussex TN38 9NS, United Kingdom.
authors.forename: Orla
authors.initials: O
authors.lastname: Kelly
authors.affiliation: Photek Ltd., 26 Castleham Road, St. Leonards-on-Sea, East Sussex TN38 9NS, United Kingdom.
authors.forename: Panos
authors.initials: P
authors.lastname: Kapetanopoulos
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
authors.forename: Michael N R
authors.initials: MN
authors.lastname: Ashfold