EPR Studies on Branched High‐Spin Arylnitrenes |
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Authors: | Dr. Sergei V. Chapyshev Dr. Denis V. Korchagin Dr. Mikhail F. Budyka Dr. Tatiana N. Gavrishova Dr. Patrik Neuhaus Prof. Dr. Wolfram Sander |
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Affiliation: | 1. Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region (Russia), Fax: (+7)?496‐534‐9676;2. Lehrstuhl für Organischen Chemie II, Ruhr‐Universit?t, 44780 Bochum (Germany), Fax: (+49)?234‐32‐14353 |
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Abstract: | The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, DT=0.92 cm?1, ET=0 cm?1), quintet dinitrene 6 (g=2.003, DQ=0.204 cm?1, EQ=0.035 cm?1), and septet trinitrene 8 (g=2.003, DS=?0.0904 cm?1, ES=?0.0102 cm?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ~1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(EQ)=180 MHz for dinitrene 6 and Γ(ES)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations. |
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Keywords: | azides EPR spectroscopy high‐spin systems nitrenes photolysis |
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