title: Doubly protonated benzene in the gas phase.
abstract: Structural aspects and the unimolecular fragmentations of doubly protonated benzene are studied by means of tandem-mass spectrometry. The corresponding dications are generated by electron ionization (EI) of 1,3- and 1,4-cyclohexadienes, respectively. It is suggested that EI of 1,3-cyclohexadiene leads to the singlet state of doubly protonated benzene, whereas EI of 1,4-cyclohexadiene yields a mixture of singlet and triplet states. Unimolecular fragmentation of doubly protonated benzene exclusively proceeds via dehydrogenation leading to the benzene dication. The proton affinities (PAs) of protonated benzene amount to PA(C(6)H(7)(+))(meta) = 1.9 +/- 0.3 eV for protonation taking place at the meta-position, PA(C(6)H(7)(+))(ortho) = 1.5 +/- 0.2 eV, and PA(C(6)H(7)(+))(para) = 0.9 +/- 0.2 eV, respectively. Various facets of the experiments are compared with density functional theory calculations and generally good agreement is found.
authors.affiliation: Institut für Chemie der Technischen Universität Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany. jana.roithova@jh-inst.cas.cz
authors.forename: Jana
authors.initials: J
authors.lastname: Roithová
authors.forename: Detlef
authors.initials: D
authors.lastname: Schröder
authors.forename: Robert
authors.initials: R
authors.lastname: Berger
authors.forename: Helmut
authors.initials: H
authors.lastname: Schwarz