Solvent and Concentration Dependence of the 7-Proton Resonance

Recent studies33 of solvent and concentration effects on the NMR of indoles have established that, in addition to the 2-proton, the 7-proton resonance in certain substituted indoles also undergoes downfield shifts in more polar and more dilute solutions. The chemical shifts of protons of indole and several methylindoles in two solvents, carbon tetrachloride and tetrahydrofuran, are listed in Ref. 33. Proton chemical shifts of the 

For all the indoles investigated, except 1-methylindole, the concentration dependence curves for the 2- and 7-protons or 2- and 7-methyl groups were essentially similar in shape, but different from those of the protons or methyl groups at other positions. Thus, the 2- and 7-protons distinguish themselves from the remaining protons of the indole nucleus, a fact of substantial assistance in their NMR detection. 

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The appearance of benzenoid signals in analyzable patterns, the coupling constants of benzenoid protons, the comparison of spectra with those of analogous compounds, the solvent and concentration dependence of the 7-proton resonance, spin decoupling experiments, and some additional factors which are summarized below, may lead to the proper assignment of the benzenoid signals, and thus to the substitution pattern. 

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