Different types of hydrogen bonds in 2‐substituted pyrroles and 1‐vinyl pyrroles as monitored by 1H, 13C and 15N NMR spectroscopy and ab initio calculations

Abstract

According to the ¹H, ¹³C and ¹⁵N NMR spectroscopic data and ab initio calculations, the strong NH···O intramolecular hydrogen bond in the Z‐isomers of 2‐(2‐acylethenyl)pyrroles causes the decrease in the absolute size of the ¹J(N,H) coupling constant by 2 Hz in CDCl₃ and by 4.5 Hz in DMSO‐d₆, the deshielding of the proton and nitrogen by 5–6 and 15 ppm, respectively, and the lengthening of the NH link by 0.025 Å. The NH···N intramolecular hydrogen bond in the 2(2′‐pyridyl)pyrrole leads to the increase of the ¹J(N,H) coupling constant by 3 Hz, the deshielding of the proton by 1.5 ppm and the lengthening of the NH link by 0.004 Å. The CH···N intramolecular hydrogen bond in the 1‐vinyl‐2‐(2′‐pyridyl)‐pyrrole results in the increase of the ¹J(C,H) coupling constant by 5 Hz, the deshielding of the proton by 1 ppm and the shortening of the CH link by 0.003 Å. Different behavior of the coupling constants and length of the covalent links under the hydrogen bond influence originate from the different nature of the hydrogen bonding (predominantly covalent or electrostatic), which depends in turn on the geometry of the hydrogen bridge. The Fermi‐contact mechanism only is responsible for the increase of the coupling constant in the case of the predominantly electrostatic hydrogen bonding, whereas both Fermi‐contact and paramagnetic spin‐orbital mechanisms bring about the decrease of coupling constant in the case of the predominantly covalent hydrogen bonding Copyright © 2005 John Wiley & Sons, Ltd.