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.
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Die Tetraaza‐bicyclodione (I) bzw. (III) werden mit Lithium‐aluminiumhydrid in Dioxan bei 95°C während 24 Stdn. zu den bicyclischen Tetraminen (II) bzw. (IV) reduziert.
Changes in 13C and 14N chemical shifts of the nitro derivatives of nitrogen heterocycles upon ionization (anion or cation formation) are twofold-first a uniform paramagnetic or in the case of protonation, a uniform diamagnetic shift of all the ring resonances that parallels the changes in the respective ultraviolet spectra and must be caused by changes in the molecular excited states, and second-the influence of the conjugated nitro group. About one third of the total negative anion charge may be localized on the nitro group, which causes unusually large shifts of the ring 13C resonances in this case.
I N T R O D U C T I O NIN THE first part of this work nitro derivatives of pyrrole and imidazole were studied' and the main trends in the influence of the nitro groups as conjugated substituents were established. The corresponding charged species (anions and cations) present different possibilities for conjugation with the nitro group, but hitherto the 13C, 14N and lH spectra of only the unsubstituted pyrrole anion and both the anion and cation of imidazole have been d e s~r i b e d .~*~*~ The relationship between the electron structure and the 13C and 14N chemical shifts is rather complicated. It is clear that both the n-and o-charge densities are important, but the change in charge density with protonation is in the wrong direction to explain the experimental shifts. These shifts can be rationalized through the use of atomic charge densities, together with the N-C n-bond overlap populations which decrease when the anionic species is protonated and thus lead to upfield 13C and 14N shifts,2 or conversely-through charge densities and an increase of the average excitation energy on protonation which also leads to a diamagnetic shift.3 The calculation methods are so well parametrized that a good fit with the experiment can be achieved both ways. Quite possibly all these factors actually play a role plus other additional factors, for example the diamagnetic part of the 13C screening constant that can also vary within fairly wide limits (-10 to +2 ppm from ben~ene).~The object of this work was to study the conjugation of the nitro groups with the charged nitrogen heterocycles and to use the resonances of other nuclei, nitrogen and oxygen, to establish, even qualitatively, the true mechanisms for the unexpected displacement of the 13C and 14N chemical shifts to higher field on protonation of the nitrogen lone pairs.
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