Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 22, Issue 8, Pages 2793-2800Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201504924
Keywords
annulenes; aromaticity; Baird's rule; diradical species; electronic structure
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Funding
- Swedish Research Council [621-2011-4177]
- Ministerio de Economia y Competitividad (MINECO) of Spain [CTQ2014-54306-P]
- Generalitat de Catalunya [2014SGR931]
- Beatriu de Pinos programme from AGAUR [BP_A_00339, BP_A2_00022]
- European Union (EU) [UNGI10-4E-801]
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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 8-electron methano[10]annulene (M10A) dicationic ring and 2)a Huckel aromatic with a neutral closed-shell 10-electron ring. According to charge and spin density distributions, the Huckel aromatic structure dominates the triplet state (the Baird aromatic contributes at most 12%), and separation of the aromatic fluctuation index (FLU) into and electron contributions emphasizes this finding. The small singlet-triplet energy gap is due to Huckel aromaticity of the M10A ring, clarified by comparison to the smaller analogues of TMTQ. Yet, TMTQ and its analogues are Huckel-Baird hybrids allowing for tuning between closed-shell 4n+2 Huckel aromaticity and open-shell 4n Baird aromaticity.
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