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1H chemical shifts in NMR. Part 18. Ring currents and π-electron effects in hetero-aromatics

The 1H chemical shifts of a number of hetero-aromatics and related compounds were obtained by the assignment of the NMR spectra in CDCl3 solution or from the literature. These included furan, pyrrole, thiophene, oxazole, imidazole and thiazole, various methyl and 4,5-dihydro derivatives, the benzo derivatives benzofuran, indole and benzothiophene plus the related compounds vinyl methyl ether, phenol, anisole, aniline, vinyl methyl sulfide and thiophenol. The six membered hetero-aromatics pyridine, pyrimidine, pyrazine, pyridazine, quinoline and isoquinoline and a number of their methyl derivatives were also investigated. The 1H chemical shifts in these molecules were analysed in terms of the ring currents and π-electron effects together with a model (CHARGE7h) for the calculation of the two-bond and three-bond electronic effects. This model gives the first comprehensive calculation of the proton chemical shifts in these compounds. For the 
data set considered (215 proton chemical shifts) ranging from δ 
= 1.9 to 9.4 ppm the rms error of observed vs. calculated shifts was 0.096 ppm. The model also allows the interpretation of the chemical shifts in terms of the separate interactions calculated in the programme. This showed the large effects of the ring currents and π-electron densities on the 1H chemical shifts. Methyl substitution has a large effect on the chemical shifts which is due to increased π-electron densities in the methyl compounds. The ring currents in furan, pyrrole and thiophene were found to be equal to the benzene ring current, but the introduction of an aza nitrogen decreased the ring current by ca. 10% in both the five and six-membered heterocyclics. The effect was cumulative in the diazabenzenes.