Analyses of the N.M.R. spectra of pyrrole derivatives: pyrrole-2-carboxylic acid and pyrrole-2-aldehyde

By the N-decoupling technique the spectrum of the N-H proton in pyrrole-2-carboxylic acid and pyrrole-2-aldehyde was clearly unveiled. Using the full spectra including the N-H band, the N.M.R. parameters of the two pyrrole derivatives were determined with a high degree of accuracy by computer simulation. It was demonstrated that two types of long-range coupling exist for the aldehyde proton of pyrrole-2-aldehyde. From partial decoupling experiments by using the third r.f. field the relative signs of all coupling constants were determined to be the same for the two pyrrole derivatives, except for a long-range coupling, J cho-nh , in pyrrole-2-aldehyde. Based on an assumption that the long-range couplings of the aldehyde proton originate from the pi electrons, it was concluded that the absolute signs of all coupling constants among the ring protons of pyrrole-2-aldehyde were positive.