Coupling constants heteroaromatics

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

Table 2.5. Typical HH coupling constants (Hz) of aromatic and heteroaromatic compounds...

Single-bond 13C- H coupling constants for six-membered heteroaromatic compounds lie in the approximate range 150-220 Hz, the magnitude varying with substituent electronegativity. Data for... 

Although single bond carbon-13 proton coupling constants (Vch) show a dependency on the geometry of the ring, a value of 200 Hz is considered typical for a carbon attached to oxygen in a heteroaromatic system (B-72MI22203). Values for 2- and 4-pyranone and furan are presented in Table 11. 

Table 3.10. One-, Two-, Three-, and Four-Bond Carbon-Proton Coupling Constants (Hz) of Benzene (a), Pyridine (b), Pyrimidine (c), Monosubstituted Benzenes [128] (d) and Five-Membered Heteroaromatics [130] (e).

The y//NMR spectrum contains five signals with integral levels in the ratios 1 1 1 1 3 four lie in the shift range appropriate for aromatics or heteroaromatics and the fifth is evidently a methyl group. The large shift values (up to Sh = 9.18, aromatics) and typical coupling constants (8 and 5 Hz) indicate a pyridine ring, which accounts for four out of the total five double-bond equivalents. 

High-level nonempirical quantum chemical calculations of C- C coupling constants (/cc) of heteroaromatic ketoximes are well consistent with the experimental data, the effect of the nitrogen lone electron pair on /cc values being strongly predominant over conformational and electronic effects (07RJO872). 

---