✍ Scribed by Takashi Iikubo; Tetsuji Itoh; Katsuyuki Hirai; Yasutake Takahashi; Masaki Kawano; Yuji Ohashi; Hideo Tomioka
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X‐ray crystallographic structures were determined for five diphenyldiazomethanes (DDMs) with various ortho substituents, namely, bis(2,4,6‐trichlorophenyl)diazomethane, bis(2,4,6‐tribromophenyl)diazomethane, bis(4‐tert‐butyl‐2,6‐dibromophenyl)diazomethane, (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,4,6‐tribromophenyl)diazomethane, and 4‐tert‐butyl‐2,6‐bis(trifluoromethyl)phenyldiazomethane, and for bis(4‐bromophenyl)diazomethane, a DDM with no ortho substituents. The correlation between the structural parameters, the ESR zero‐field splitting (ZFS) parameters and the lifetimes of the triplet diphenylcarbenes (DPCs) generated from these diazo compounds was examined. It is noted that as ortho substituents are introduced onto the phenyl rings of the DDMs, the interplanar angle between the two phenyl rings increases, while the angle of the diazo carbon changes very little. DDM 6‐N~2~, from which the longest‐lived triplet carbene is generated, is shown to have the largest interplanar angle of the DDMs examined. The bond distances between the aromatic carbons and the ortho substituents as well as the van der Waals radii of the substituents were also examined. These studies do not provide a quantitative correlation between the structural parameters of the precursor DDMs and the ZFS parameters and lifetimes of their photoproducts, triplet DPCs, but show how steric shielding of the ortho substituents affects the structures and stabilities of triplet DPCs. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
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