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Carbon, nitrogen and oxygen NMR spectra of some nitro derivatives of pyrrole and imidazole have been investigated. The 13C chemical shifts of para-carbons and the 170 chemical shifts of the nitro group correlate qualitatively with the electron densities on these carbon and oxygen atoms, which in turn depend upon the degree of conjugation of the nitro groups with the heterocyclic ring. Conjugation of several nitro groups with the benzene ring is in most cases not impaired by mutual interactions and the 13C shifts show good additivity. Such additivity is much worse in pyrrole and imidazole derivatives. Taken together with the diamagnetic nature of these deviations from additivity, this leads to a possible conclusion about the less pronounced conjugation of the nitro groups with the heterocyclic ring in heterocyclic dinitro derivatives. I N T R O D U C T I O NHETERONUCLEAR resonance is a useful new technique that considerably widens the possibilities of proton resonance for the investigation of the electron structure of organic compounds. We recently used 13C, 14N and 170 NMR spectra for the study of charge distribution in aliphatic nitro compounds and the corresponding charged species.l In this paper we present similar data about some heterocyclic nitro compounds, derivatives of pyrrole and imidazole.The 13C and 14N spectra have been studied by the direct method in the case of the unsubstituted parent compounds2 to and methylpyrr~les.~*~ Recently the 14N shifts of pyrrole, 2-acetylpyrrole and 2,5-diacetylpyrrole were measured from proton spectra by the indirect double resonance technique.7 EXPERIMENTAL MethodThe absorption spectra of 13C and 15N as well as the wide-line single resonance absorption spectra of 14N and 170 were registered at 4.32, 6.08, 8.11 and 15.08 MHz, respectively, on the frequency sweep, time sharing NMR spectrometer which has been described earlier.' 15 mm O.D. nonrotating sample tubes (2 to 4 ml sample volume) were used. Nitropyrroles were studied as 30 to 40% by weight solutions in acetone (for lSC, 14N and 15N spectra) or as saturated solutions in ethyl alcohol (for 1 7 0 spectra). Nitroimidazoles were dissolved in dimethyl sulphoxide and the dinitro derivatives in acetone (15 to 30% by weight). In many cases the restricted solubility and line broadening, resulting from the quadrupole relaxation (I4N and I'O), or scalar interaction with nitrogen of the ' SC nuclei, as well as the limited amounts of material available, made the use of spectrum accumulation unavoidable. This is particularly true of "0 NMR spectra which are extremely weak as a rule. Most samples were studied at room temperature but the less soluble imidazole derivatives were measured at 70"C, which also leads to a sharpening of the otherwise broad 14N lines.All chemical shifts are given in the &scale (paramagnetic shifts positive) and measured from external reference compounds-tetramethylsilane for 13C and nitromethane for 14N, I5N and "0. 153

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