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Cycloheptatrienyl radicals: An EPR study of substituent effects
Author(s) -
MacCorquodale Finlay,
Walton John C.
Publication year - 1990
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.1260280415
Subject(s) - chemistry , substituent , radical , electron paramagnetic resonance , ring (chemistry) , hyperfine structure , photochemistry , resonance (particle physics) , computational chemistry , molecular orbital , atomic orbital , hydrogen atom abstraction , spectroscopy , benzene , crystallography , stereochemistry , molecule , nuclear magnetic resonance , organic chemistry , atomic physics , electron , physics , quantum mechanics
Cycloheptatrienyl radicals with donor and acceptor substituents were generated by hydrogen abstraction from the corresponding cycloheptatrienes and studied by EPR spectroscopy. The barrier to rotation about the C‐1CO bond in ethoxycarbonylcycloheptatrienyl radicals was found to be ca. 7.5 kcal mol −1 (31.4 kJ mol −1 ). The observed hyperfine splittings of the ring hydrogens were interpreted in terms of a model in which the substituent causes an energy separation Δ E between the frontier orbitals. Populations of the symmetric and antisymmetric orbitals were deduced, and Δ E values were estimated. A similar analysis was carried out from EPR data for substituted benzene radical anions. These energy separations show a linear correlation with Hammett constants. The p‐values from plots of this type for [n]annulene radicals decrease in magnitude as the size of the ring increases.