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Dynamic and substituent effects in the ESR spectra of triplet biradical intermediates in lithiation reactions of 2‐alkylbenzotriazoles: A cautionary note
Author(s) -
Dalal Nar S.,
Xu Ruixin,
Katritzky Alan R.,
Wu Jing,
Jesorka Aldo
Publication year - 1994
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260321204
Subject(s) - unpaired electron , chemistry , benzotriazole , substituent , spectral line , photochemistry , radical , electron , electron paramagnetic resonance , dipole , oxygen , nitrogen , ground state , triplet state , computational chemistry , crystallography , stereochemistry , atomic physics , nuclear magnetic resonance , molecule , organic chemistry , physics , quantum mechanics , astronomy
Investigations of intermediates generated in the course of lithiation of four 2‐alkylbenzotriazoles revealed largescale temperature‐dependent changes in their ESR spectra. A cautionary observation was that the ESR spectra obtained under synthesis conditions exhibited lineshapes that were assignable to a monoradical, while additional, lower temperature measurements clearly showed the species to be a biradical. The electron–electron dipolar couplings yield an average distance between the two unpaired electrons in the biradical, R av , of about 5.3 ± 0.16 Å. These R av values are consistent with the unpaired electrons being localized on the nitrogen atoms of the two benzotriazole moieties in the biradical fragment. This conclusion is supported by oxygen trapping (nitroxide radical formation) studies. The biradicals are shown to have triplets as the ground state with a single‐triplet separation of about 26±4 kJ mol −1 .