title: Nucleophilic Aromatic Substitution Between Halogenated Benzene Dopants and Nucleophiles in Atmospheric Pressure Photoionization.
abstract: In a preceding work with dopant assisted-atmospheric pressure photoionization (DA-APPI), an abundant ion at [M + 77](+) was observed in the spectra of pyridine and quinoline with chlorobenzene dopant. This contribution aims to reveal the identity and route of formation of this species, and to systematically investigate structurally related analytes and dopants. Compounds containing N-, O-, and S-lone pairs were investigated with APPI in the presence of fluoro-, chloro-, bromo-, and iodobenzene dopants. Computational calculations on a density functional theory (DFT) level were carried out to study the reaction mechanism for pyridine and the different halobenzenes. The experimental and computational results indicated that the [M + 77](+) ion was formed by nucleophilic aromatic ipso-substitution between the halobenzene radical cation and nucleophilic analytes. The reaction was most efficient for N-heteroaromatic compounds, and it was weakened by sterical effects and enhanced by resonance stabilization. The reaction was most efficient with chloro-, bromo-, and iodobenzenes, whereas with fluorobenzene the reaction was scarcely observed. The calculated Gibbs free energies for the reaction between pyridine and the halobenzenes were shown to increase in the order I < Br < Cl < F. The reaction was found endergonic for fluorobenzene due to the strong C-F bonding, and exergonic for the other halobenzenes. For fluoro- and chlorobenzenes the reaction was shown to proceed through an intermediate state corresponding to [M + dopant](+), which was highly stable for fluorobenzene. For the bulkier bromine and iodine, this intermediate did not exist, but the halogens were shown to detach already during the approach by the nucleophile.
authors.affiliation: Faculty of Pharmacy, University of Helsinki, Helsinki, 00014, Finland. tiina.kauppila@helsinki.fi.
authors.forename: Tiina J
authors.initials: TJ
authors.lastname: Kauppila
authors.affiliation: Department of Physical and Theoretical Chemistry, University of Wuppertal, Wuppertal, 42119, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Haack
authors.affiliation: Department of Physical and Theoretical Chemistry, University of Wuppertal, Wuppertal, 42119, Germany.
authors.forename: Kai
authors.initials: K
authors.lastname: Kroll
authors.affiliation: Department of Physical and Theoretical Chemistry, University of Wuppertal, Wuppertal, 42119, Germany.
authors.forename: Hendrik
authors.initials: H
authors.lastname: Kersten
authors.affiliation: Department of Physical and Theoretical Chemistry, University of Wuppertal, Wuppertal, 42119, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Benter
mesh_terms: D001274:Atmospheric Pressure; D001969:Bromobenzenes / Q000737:chemistry*; D002722:Chlorobenzenes / Q000737:chemistry*; D054879:Halogenation; D007462:Iodobenzenes / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D013058:Mass Spectrometry; D008958:Models, Molecular; D055668:Photochemical Processes; D011725:Pyridines / Q000737:chemistry*; D011804:Quinolines / Q000737:chemistry*
chemical_list: D001969:Bromobenzenes; D002722:Chlorobenzenes; D007462:Iodobenzenes; D007477:Ions; D011725:Pyridines; D011804:Quinolines; C031905:iodobenzene; C037219:quinoline; C031294:chlorobenzene; C023666:pyridine
keywords: Atmospheric pressure photoionization; Dopant; Gas-phase ion/molecule reactions; Nucleophilic aromatic substitution; ipso-substitution