Natural abundance 17O NMR spectroscopy of heteroaromatic Nitro compounds

The ¹⁷O chemical shift data, at natural abundance, for selected nitroquinolines, nitroindoles, nitroindazoles and nitrothiophenes are reported. In the absence of a peri or a lone-pair repulsion effect, the nitroquinolines' chemical shifts differ little from those of their carbocyclic analogs. However, the signal for 5-nitroquinoline, 2 , is deshielded by 25 ppm compared to 6-nitroquinoline, 1 , and the ¹⁷O nucleus in 8-nitroquinoline, 3 , is deshielded by 49 ppm compared to that in 1 . Both these shifts are attributed to rotation of the nitro group from the plane of the heteroaromatic ring arising from peri hydrogen interaction and lone pair repulsion, respectively. The signals for nitro groups on electron excessive ring systems (e.g., indoles and thiophenes) are shielded relative to corresponding ones in electron deficient heterocyclic ring system analogs. The chemical shifts for the π-excessive systems are interpreted in terms of electronic effects.