Substituent effects on hyperfine coupling constants
27 monosubstituted benzenes ØX were measured either as pure liquids or in concentrated solutions. We use toluene as a model case to discuss the observed features and the assignment procedure. Three spectra are shown in Figure 5.1. At 3 kG three pairs of lines are observed (top). They correspond to radicals with coupling constants of A’μ = 153.8 MHz, 155.9 MHz, and 160.0 MHz, i.e. within 5% the same as the value of 161.7 MHz obtained for C6H6Mu in benzene. This is obviously typical for cyclohexadienyl type radicals. As was demonstrated for C6H6Mu one expects line splitting in a field of 1 kG when the second methylene hydrogen is H, and no splitting when it is D. This is seen for o-dideutero toluene (middle) where the absence of splitting of the two intense lines identifies the ortho Mu adduct. For p-deutero toluene (bottom) the smallest two lines are unsplit and therefore assigned to the para adduct. By default, the lines of intermediate intensity are assigned to the meta isomer. The ipso adduct is not observed. Because it would have no H in the methylene position it would not show splitting in a field of 1 kG.
KeywordsSubstituent Effect Meta Position Hyperfine Coupling Constant Spin Density Distribution Spin Population
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