“…Thus, for example, the 3-, 4-, and 5-protons of 2-acetylpyrrole are shifted downfield by 0-67, 0-03, and 0-41 p.p.m. Variable temperature NMR measurements have permitted the calculation of the energy barrier to rotation of the 2-formyl group (440) and it has been found that, with the exception of the tert-butyl group, 1-alkyl substituents have little effect upon the magnitude of the energy barrier, which lies within the range 451-46-8 kJmol -1 . (274) Electron-withdrawing substituents on the 1-position produce large downfield shifts on signals of the 2-and 5-protons but leave the signals for the 3-and 4-protons virtually unchanged/ 185 ' 274) From observations of the *Η NMR spectra of a wide range of substituted pyrroles, the coupling constants appear to fall within four ranges comparable with those for pyrrole: J 34 = 3-40-3-80; J 2)3 (Λ,5) = 2-40-3-10; J 2)5 = 1-95-2-30; and J 2>4 (J 3f5 ) = 1-35-1-80 Hz.…”