The values of the downfield chemical shift of the bridge hydrogen atom were estimated for a series of compounds containing an intramolecular hydrogen bond O-HO, O-HN, O-HHal, N-HO, N-HN, C-HO, C-HN and C-HHal. Based on these values, the empirical estimation of the hydrogen bond energy was obtained by using known relationships. For the compounds containing an intramolecular hydrogen bond, the DFT B3LYP/6-311++G(d,p) method was used both for geometry optimization and for QTAIM calculations of the topological parameters (electron density ρ and the density of potential energy V in the critical point of the hydrogen bond). The calculated geometric and topological parameters of hydrogen bonds were also used to evaluate the energy of the hydrogen bond based on the equations from the literature. Comparison of calibrating energies from the H NMR data with the energies predicted by calculations showed that the most reliable are the linear dependence on the topological ρ and V parameters. However, the correct prediction of the hydrogen bond energy is determined by proper fitting of the linear regression coefficients. To obtain them, new linear relationships were found between the calculated ρ and V parameters and the hydrogen bond energies obtained from empirical H NMR data. These relationships allow the comparison of the energies of different types of hydrogen bonds for various molecules and biological ensembles.

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.

Using the molecular tailoring and function‐based approaches allows one to divide the energy of the O─H⋯O═C resonance‐assisted hydrogen bond in a series of the β‐diketones into resonance and hydrogen bonding components. The magnitude of the resonance component is assessed as about 6 kcal mol−1. This value increases by ca. 1 kcal mol−1 on going from the weak to strong resonance‐assisted hydrogen bonding. The magnitude of the hydrogen bonding component varies in the wide range from 2 to 20 kcal mol−1 depending on the structure of the β‐diketone in question.

The energy of the intramolecular hydrogen bond has been recognized and may be estimated by the molecular fragmentation method and the method based on the functional dependence on the hydrogen bond descriptor. Both basic methods for estimating the energy of intramolecular hydrogen bonds are compared using a series of the hydroxycarbonyl aliphatic compounds as the benchmark. The functional dependencies are established that provide the best fit with the molecular fragmentation method. As both methods for estimating the energy of an intramolecular hydrogen bond have disadvantages, a new hybrid approach is proposed. In this approach, the average value of the intramolecular hydrogen bond energy calculated by the molecular fragmentation method and functional dependence method is used as the estimated parameter. The hybrid approach provides a compensation of the error in the evaluation of the interaction strength compared to the molecular fragmentation method or the functional dependencies method. Hence, it allows one to more reliably rank the strength of the intramolecular hydrogen bond in homogeneous series of molecules. Based on this approach, the strength of the О−Н···О=C intramolecular hydrogen bond in the studied compounds are classified in the energy scale in terms of weak, moderate, and strong interactions.

The introduction of a substituent in the $position of the pyrrole ring in N-vinyl-Zarylpyrroles and -2-heteroarylpyrroles results in an increase in the dihedral angles between the planes of the pyrrole ring and the aryl (or heteroaryl) residue, and a decrease in the dihedral angle between the planes of the pyrrole ring and the vinyl group. Introduction of the substituent in the %position of the pyrrole ring has the opposite effect on these angles.For N-vinyl-2-(2'-furyl)pyrrole, some spectral data suggest the formation of a C-H -* 0 hydrogen bond between the a-hydrogen atom of the vinyl group and the oxygen atom of the fury1 ring. This is in accord with the results of quantum chemical calculations.