title: Analysis of a Compound Class with Triplet States Stabilized by Potentially Baird Aromatic [10]Annulenyl Dicationic Rings.
abstract: The low-lying triplet state of a recently published compound (TMTQ) was analyzed quantum chemically in light of suggestions that it is influenced by Baird aromaticity. Two mesomeric structures describe this state: 1) a zwitterionic Baird aromatic structure with a triplet diradical 8π-electron methano[10]annulene (M10A) dicationic ring and 2) a Hückel aromatic with a neutral closed-shell 10π-electron ring. According to charge and spin density distributions, the Hückel aromatic structure dominates the triplet state (the Baird aromatic contributes at most 12 %), and separation of the aromatic fluctuation index (FLU) into α and β electron contributions emphasizes this finding. The small singlet-triplet energy gap is due to Hückel aromaticity of the M10A ring, clarified by comparison to the smaller analogues of TMTQ. Yet, TMTQ and its analogues are Hückel-Baird hybrids allowing for tuning between closed-shell 4n+2 Hückel aromaticity and open-shell 4n Baird aromaticity.
authors.affiliation: Department of Chemistry-BMC, Uppsala University, Box 576, 75123, Uppsala, Sweden.
authors.forename: Kjell
authors.identifier: http://orcid.org/0000-0002-4191-6790
authors.initials: K
authors.lastname: Jorner
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC) and, Departament de Química, Universitat de Girona, Campus de Montilivi s/n, 17071, Girona, Catalonia, Spain.
authors.forename: Ferran
authors.identifier: http://orcid.org/0000-0001-5147-0000
authors.initials: F
authors.lastname: Feixas
authors.affiliation: Department of Chemistry-BMC, Uppsala University, Box 576, 75123, Uppsala, Sweden.
authors.forename: Rabia
authors.identifier: http://orcid.org/0000-0003-2128-6733
authors.initials: R
authors.lastname: Ayub
authors.affiliation: Department of Chemistry-Ångström Laboratory, Uppsala University, Box 518, 75120, Uppsala, Sweden.|Uppsala Center for Computational Chemistry-UC3, Uppsala University, Box 518, 75120, Uppsala, Sweden.
authors.forename: Roland
authors.identifier: http://orcid.org/0000-0001-7567-8295
authors.initials: R
authors.lastname: Lindh
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC) and, Departament de Química, Universitat de Girona, Campus de Montilivi s/n, 17071, Girona, Catalonia, Spain. miquel.sola@udg.edu.
authors.forename: Miquel
authors.identifier: http://orcid.org/0000-0002-1917-7450
authors.initials: M
authors.lastname: Solà
authors.affiliation: Department of Chemistry-BMC, Uppsala University, Box 576, 75123, Uppsala, Sweden. henrik.ottosson@kemi.uu.se.
authors.forename: Henrik
authors.identifier: http://orcid.org/0000-0001-8076-1165
authors.initials: H
authors.lastname: Ottosson
keywords: Baird's rule; annulenes; aromaticity; diradical species; electronic structure