title: Structures of Metalloporphyrin-Oligomer Multianions: Cofacial versus Coplanar Motifs as Resolved by Ion Mobility Spectrometry.
abstract: We have combined ion mobility mass spectrometry with quantum chemical calculations to investigate the gas-phase structures of multiply negatively charged oligomers of meso-tetra(4-sulfonatophenyl)metalloporphyrins comprising the divalent metal centers ZnII, CuII, and PdII. Sets of candidate structures were obtained by geometry optimizations based on calculations at both the semiempirical PM7 and density functional theory (DFT) levels. The corresponding theoretical cross sections were calculated with the projection approximation and also with the trajectory method. By comparing these collision cross sections with the respective experimental values we were able to assign oligomer structures up to the tetramer. In most cases the cross sections of the lowest energy isomers predicted by theory were found to agree with the measurements to within the experimental uncertainty (2%). Specifically, we find that for a given oligomer size the structures are independent of the metal center but depend strongly on the charge state. Oligomers in low charge states with a correspondingly larger number of sodium counterions tend to form stacked, cofacial structures reminiscent of H-aggregate motifs observed in solution. By contrast, in higher charge states, the stack opens to form coplanar structures.
authors.affiliation: Institute of Physical Chemistry, Karlsruhe Institute of Technology , Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
authors.forename: Katrina
authors.initials: K
authors.lastname: Brendle
authors.affiliation: Institute of Physical Chemistry, Karlsruhe Institute of Technology , Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Schwarz
authors.affiliation: Institute of Nanotechnology, Karlsruhe Institute of Technology , Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Jäger
authors.affiliation: Institute of Physical Chemistry, Karlsruhe Institute of Technology , Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Weis
authors.affiliation: Institute of Physical Chemistry, Karlsruhe Institute of Technology , Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.|Institute of Nanotechnology, Karlsruhe Institute of Technology , Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Kappes