MO study of the effect of α-substituents on the13C hfs constants of acyl radicals: an electronic rather than structural effect

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Abstract
The experimental 13C hyperfine splitting (hfs) constants of the acyl radicals Y–C˙O, (Y = H, Me, OBut, NH2 and F) have been well reproduced by ab initio calculations at the UMP2/DZP//UHF/6-311G** level of theory. The large increase observed experimentally in a(13C) as the electronegativity of the α-substituents increases is due to the electronic influence of the α-substituents rather than to an increase in bending at the radical centre as previously suggested. There is a large breakdown in orbital following in the singly occupied molecular orbital (SOMO) which strongly depends on the electronegativity of the α-substituents. This confirms still further that in localized radicals reliable structural information cannot be obtained by taking into account the trend in the experimental isotropic hfs constants at the radical centre.