Infrared spectra of pyrroles and dipyrrylmethanes
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The IR spectra of 18 substituted pyrroles and 6 substituted dipyrrylmethanes are investigated. It is shown that the position of the valence vibrations frequency of the free NH group, νfree NH depends on the number of substituents in the ring. The frequency is influenced by the electronegativity of a substituent, the effect of an α- being much more marked than that of a β-substituent. The stability of the hydrogen bond increases with increase in substituent electronegativity. When two electronegative substituents occupy the α and β positions, they have a mutually weakening effect. The νC=O (acetyl) absorption bands in pyrrolecontaining compounds are situated 40–60 cm−1 lower than in aromatic ketones and greatly depend on the position and nature of the substituent in the pyrrole ring.
KeywordsHydrogen Hydrogen Bond Absorption Band Ketone Acetyl
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