title: A Thermally Populated, Perpendicularly Twisted Alkene Triplet Diradical.
abstract: Variable-temperature NMR and ESR spectroscopic studies reveal that bis(dibenzo[a,i]fluorenylidene) 1 possesses a singlet ground state, 1(S0), while the 90° twisted triplet 1(T1) is populated to a small extent already at room temperature. Analysis of the increasing amount of paramagnetic 1(T1) at temperatures between 300 and 500 K yields the exchange interaction Jex/h c=3351 cm-1and a singlet-triplet energy splitting of 9.6 kcal mol-1, which is in excellent agreement with calculations (9.3 kcal mol-1at the UKS BP86/B3LYP/revPBE level of theory). In contrast, the zero-field splitting parameter D is very small (calculated value -0.018 cm-1) and unmeasurable.
authors.affiliation: School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland, 4072, Australia.
authors.forename: Curt
authors.identifier: 0000-0003-0874-7144
authors.initials: C
authors.lastname: Wentrup
authors.affiliation: School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland, 4072, Australia.
authors.forename: Michèle J
authors.initials: MJ
authors.lastname: Regimbald-Krnel
authors.affiliation: Anorganisch-Chemisches Institut and IWR, Universität Heidelberg, Im Neuenheimer Feld 270, 69120, Heidelberg, Germany.
authors.forename: Dennis
authors.initials: D
authors.lastname: Müller
authors.affiliation: Anorganisch-Chemisches Institut and IWR, Universität Heidelberg, Im Neuenheimer Feld 270, 69120, Heidelberg, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Comba
keywords: diradicals; exchange coupling; singlet-triplet splitting; strained molecules; twisting