CH···N and CH···O intramolecular hydrogen bonding effects in the ¹H, ¹³C and ¹⁵ N NMR spectra of the configurational isomers of 1‐vinylpyrrole‐2‐carbaldehyde oxime substantiated by DFT calculations

Abstract

According to the (1)H, (13)C and (15)N NMR spectroscopic data and DFT calculations, the E-isomer of 1-vinylpyrrole-2-carbaldehyde adopts preferable conformation with the anti-orientation of the vinyl group relative to the carbaldehyde oxime group and with the syn-arrangement of the carbaldehyde oxime group with reference to the pyrrole ring. This conformation is stabilized by the C-H...N intramolecular hydrogen bond between the alpha-hydrogen of the vinyl group and the oxime group nitrogen, which causes a pronounced high-frequency shift of the alpha-hydrogen signal in (1)H NMR (approximately 0.5 ppm) and an increase in the corresponding one-bond (13)C-(1)H coupling constant (ca 4 Hz). In the Z-isomer, the carbaldehyde oxime group turns to the anti-position with respect to the pyrrole ring. The C-H...O intramolecular hydrogen bond between the H-3 hydrogen of the pyrrole ring and the oxime group oxygen is realized in this case. Due to such hydrogen bonding, the H-3 hydrogen resonance is shifted to a higher frequency by about 1 ppm and the one-bond (13)C-(1)H coupling constant for this proton increases by approximately 5 Hz.

Direct Evidence for Methyl Group Coordination by Carbon-Oxygen Hydrogen Bonds in the Lysine Methyltransferase SET7/9

“…1) (23, 66 -69); thus, we sought to verify our experimental chemical shift change using quantum chemistry calculations. Although this combination of techniques has been used to identify CH⅐⅐⅐O hydrogen bonds in small organic molecules (37,70,71) and in computational biology (68), it has, to our knowledge, not yet been applied experimentally in biological macromolecules. Using this combination of techniques, we reasoned that it should be possible to solve for the hydrogen positions and, thus hydrogen bonding patterns, of the AdoMet methyl group within the SET7/9 active site.…”

“…Previous studies of CH⅐⅐⅐O hydrogen bonding in small organic molecules showed that the chemical shift calculations of hydrogen were usually accurate to 0.1 ppm of the experiment (37,70,71). Error in biological molecules could arise from many sources, including but not limited to implicit solvation modeling and large or truncated molecules used in calculations.…”

15N chemical shifts of a series of isatin oxime ethers and their corresponding nitrone isomers

“…[9] The other major factor that might contribute to the downfield shift is the extension of the conjugation system to include a double bond or an aromatic system. For example, the 15 N chemical shift of cyclopentanone oxime [10] (δN = 324.3 or −55.9 ppm) is 25 ppm higher field than those of 1-vinylpyrrole-2-carbaldehyde oxime [11] (δN = 349.1. or −31.1 ppm).…”

Intramolecular hydrogen bonds C-H⋯N in bisheterocyclic compounds according to 1H and 13C NMR data

“…It is indicated by the signifi cant downfi eld shift of the 1 Н Х signal (by 0.6 ppm) and by the increase in 1 J(C α ,Н Х ) by 3 Hz with respect to the isomeric 1-vinyl-3-(2-furyl)-5-vinylsulfanyl-1,2,4-triazole (VI) accompanied by the insignifi cant changes in the chemical shifts of atoms Н А , Н В , 1 J(C β ,Н А ) and 1 J(C β ,Н В ) of the vinyl group (Tables 1, 2). The formation of stronger intramolecular hydrogen bonds С-H ... N in vinyl compounds resulted in downfi eld shift of the signal of proton Н Х up to 1-1.5 ppm, and 1 J(C α ,Н Х ) grew by 5-7 Hz [1][2][3][4][5][6]. The above cited hydrogen bonds С-H ... N between the heterocycles in bisheterocyclic compounds II, III, V, VI cause considerably smaller variations in δ(Н) and 1 J(C,Н), consequently, they are weaker.…”