Benzenoid Signals

The proper assignment of benzenoid signals, especially when there is no substitution in the benzenoid ring, is often a difficult task owing to the 

The heteroatom exerts its slight, but consistent, effect on the chemical shifts and coupling constants of the adjacent ortho (heterocyclic ring) 

Assignment of Benzenoid Signals and Elucidation of Benzenoid Substitution 

The appearance of benzenoid signals in analyzable patterns, the coupling constants of benzenoid protons, the comparison of spectra with those of analogous compounds, the solvent and concentration dependence of the 7-proton resonance, spin decoupling experiments, and some additional factors which are summarized below, may lead to the proper assignment of the benzenoid signals, and thus to the substitution pattern. 

7-Halogeno49,50 and 7-nitro51 substitution shifts the 1-Me resonance downfield. 7-Alkyl derivatives reveal a mutual paramagnetic shift of the 1-alkyl and 7-alkyl resonances.52 

---

B. Assignment of Benzenoid Signals and Elucidation of Benzenoid Substitutions. 290... 

Since two quaternary atoms and four CH atoms appear in the C NMR spectrum, the latter with a benzenoid coupling constant of 7-9 Hz, this is a disubstituted benzene ring, and the C signal with 5c = 162.2 fits a phenoxy C atom. The keto carbonyl (5c = 204.9) and methyl (5c = 26.6) resonances therefore point to an acetyl group as the only meaningful second substituent. Accordingly, it must be either o- or m-hydroxyacetophenone A or B the para isomer would show only four benzenoid C signals because of the molecular symmetry. 

The C NMR spectrum of the metabolite shows 16 signals instead of 8 as expected from the elemental composition determined by high-resolution mass spectrometry. Moreover, aromaticity of the 2,6-xylenol is obviously lost after metabolism because two ketonic carbonyl carbon atoms (5c = 203.1 and 214.4) and four instead of twelve carbon signals are observed in the shift range of trigonal carbon nuclei (5c = 133.1, 135.4, 135.6 and 139.4) in the C NMR spectra. To conclude, metabolism involves oxidation of the benzenoid ring. 

Precursors are assigned the same letters as the corresponding ring positions of the related adducts. c Data in CC14. d Data from Reference 93. e Data from Reference 94. f Signal concealed by benzenoid ring multiplets. 

The amide ion attacks position 1 of isoquinoline, yielding adduct 24. The observed upheld shifts are 4.1 ppm at the reaction center and nearly 3 ppm at position 4. The latter is likely to indicate an increased electron density at the ring position it is detected as a doublet (J = 5.5 Hz) because of the coupling with H-3. The signal of the latter is not distinguishable from the protons of the benzenoid ring, which resonate at S 6.35-7.3, as a broad multiplet. The upheld shift can be estimated to be of nearly 0.65 ppm for the H-3 proton and is even lower for the protons of the benzenoid ring. 

The benzenoid C-l resonance of styrene units in acrylonitrile-styrene copolymers is particularly sensitive to the sequence of the chain relative configurations of triad sequences can be determined by quantitative evaluation of carbon-13 signals [524], Micro-structures of other vinyl polymers such as polystyrene [525], polypropylene oxide [526], and polyalkyl acrylates [527] have also been investigated by 13C NMR. 

The 13C chemical shifts of the parent compound were assigned by comparison with the spectrum of cinnamic acid (see Section 4.11.2) and from the knowledge that for benzenoid methine carbons introduction of an acyloxy function into the benzene ring causes small shifts of the meta carbons, and shifts the signals of the para carbons to higher field, while the ortho carbons become even more shielded (Fig. 5.16) [635]. 

With its doublet deriving from the four equatorial fluorines appearing at +51.6 ppm and its pentet deriving from the single axial fluorine at +77.9 ppm, both signals exhibit significant shielding compared to those of SF5-benzenoid aromatics, which appear at +62.3 and +84.1 ppm, respectively. This trend is in the same direction as that exhibited in the comparison of the fluorine chemical shifts of 2-(trifluoromethyl)pyridine (-68 ppm) with those of (tri-fluoromethyl)benzene (-63 ppm), but the SF5 difference is more dramatic. 

---