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 (1)H, (13)C and (15)N 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 (1)J(N,H) coupling constant by 2 Hz in CDCl(3) and by 4.5 Hz in DMSO-d(6), 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 A. The N--H...N intramolecular hydrogen bond in the 2(2'-pyridyl)pyrrole leads to the increase of the (1)J(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 A. The C--H...N intramolecular hydrogen bond in the 1-vinyl-2-(2'-pyridyl)-pyrrole results in the increase of the (1)J(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 A. 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.

Comparative analysis of hydrogen bonding with participation of the nitrogen, oxygen and sulfur atoms in the 2(2′‐heteroaryl)pyrroles and their trifluoroacetyl derivatives based on the 1H, 13C, 15N spectroscopy and DFT calculations

“…As the energy difference between the conformers with the syn and anti orientation of the pyridine ring in 2(2 -pyridyl)pyrrole (1) and its trifluoroacetyl derivative 4 is large (4.8 and 3.3 kcal/mol, respectively), the spatial forms with the anti orientation of pyridine ring for these compounds should be negligibly populated at room temperature because of the Boltzmann distribution. Stabilization of the rotamers with the syn arrangement of the pyridine and pyrrole rings occurs due to intramolecular N-H· · ·N hydrogen bonding [4,5] in 2(2 -furyl)pyrrole (2) and its trifluoroacetyl derivative 5 is also considerable (1.4 and 1.0 kcal/mol, respectively). However, the stabilization of the syn forms in the pyrroles 2 and 5 is not so important as in the case of the 2(2 -pyridyl)pyrroles 1 and 4 owing to the weakening of the N-H· · ·O intramolecular hydrogen bonding in the 2(2 -furyl)pyrroles 2 and 5 (Scheme 4).…”

“…Also, the N-H· · ·N intramolecular hydrogen bond in the 2(2 -pyridyl)pyrrole and the bifurcated N-H· · ·N, N-H· · ·O intramolecular hydrogen bonds in the 2-trifluoroacetyl-5-(2 -pyridyl)-pyrrole were investigated. [4,5] These investigations show that GIAO DFT calculations of shielding constants and coupled perturbed finite perturbation theory (CP FPT) DFT calculations of coupling constants reproduce adequately the effect of hydrogen bonding on the NMR parameters in the series of the substituted pyrroles. [3 -5] In order to develop a comparative analysis of hydrogen bondings with participation of the nitrogen, oxygen and sulfur atoms and their influence on the NMR parameters, we studied the N-H· · ·O and N-H· · ·S intramolecular hydrogen bonds in the 2(2 -furyl)-and 2(2 -thienyl)pyrrole as well as the bifurcated N-H· · ·O, N-H· · ·O and N-H· · ·O, N-H· · ·S intramolecular hydrogen bonds in the 2-trifluoroacetyl-5-(2 -furyl)-and -5-(2 -thienyl)pyrrole.…”

“…Depending on the nature of the hydrogen bonding, the one-bond coupling constant can either decrease or increase. [1,2,4,5,8] Hence, the change in the one-bond coupling constant for a proton donor is an essential indicator of hydrogen-bond formation.…”

Bifurcated hydrogen‐bonding effect on the shielding and coupling constants in trifluoroacetyl pyrroles as studied by 1H, 13C and 15N NMR spectroscopy and DFT calculations

“…7 Therefore, the N-HÐ Ð ÐO hydrogen bond in pyrroles 4-6 is weaker than the N-HÐ Ð ÐN hydrogen bond in pyrroles 1 and 4; these hydrogen bonds are predominantly electrostatic in nature. 1 It should also be stressed that the N-H bond length is extended by several thousandths of an angström because of the N-HÐ Ð ÐN and N-HÐ Ð ÐO hydrogen bonds. The length of the free N-1-H-1 bond in the anti conformer of 1, and in the syn and anti conformers of 2 and 3, ranges from 1.005Å to 1.006Å, whereas the length of this bond is increased to 1.009-1.011Å in the syn conformer of 1 and 6 (Table 2), as well as in the syn/syn and anti/syn conformers of 4 and 5, in which N-HÐ Ð ÐN or/and N-HÐ Ð ÐO hydrogen bonds are present.…”

“…1 On the one hand, an increase in the one-bond 1 J N,H coupling constant is observed if the hydrogen bonding is predominantly an electrostatic interaction;…”