Alexander V. Vashchenko

CH···N and CH···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

According to the 1H, 13C and 15N 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 α‐hydrogen of the vinyl group and the oxime group nitrogen, which causes a pronounced high‐frequency shift of the α‐hydrogen signal in 1H NMR (∼0.5 ppm) and an increase in the corresponding one‐bond 13C–1H 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 13C–1H coupling constant for this proton increases by ∼5 Hz. Copyright

Synthesis of 2‐(Selenophen‐2‐yl)pyrroles and Their Electropolymerization to Electrochromic Nanofilms

Bridging pyrrole and selenophene chemistries: Molecular assemblies have been developed that allow scrutiny of the electronic communication between pyrrole and selenophene nuclei. Divergent syntheses of 2-(selenophen-2-yl)pyrroles and their N-vinyl derivatives from available 2-acylselenophenes and acetylenes in a one-pot procedure have been devised (see scheme), which provide access to these exotic heterocyclic ensembles.The divergent syntheses of 2-(selenophen-2-yl)pyrroles and their N-vinyl derivatives from available 2-acylselenophenes and acetylenes in a one-pot procedure make these exotic heterocyclic ensembles accessible. Now we face a potentially vast area for exploration with a great diversity of far-reaching consequences including conducting electrochromic polymers with repeating of pyrrole and selenophene units (emerging rivalry for polypyrroles and polyselenophenes), the synthesis of functionalized pyrrole-selenophene assembles for advanced materials, biochemistry and medicine, exciting models for theory of polymer conductivity.

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

The NHċbondX (X = N,O,S) intramolecular hydrogen bond in the series of 2(2′‐heteroaryl)pyrroles and their trifluoroacetyl derivatives is examined by the 1H, 13C, 15N spectroscopy and density functional theory (DFT) calculations. The influence of the hydrogen bond on coupling and shielding constants is considered. It is shown that the NHċbondN intramolecular hydrogen bond causes a larger increase in the absolute size of the 1J(N,H) coupling constant and a larger deshielding of the bridge proton than the NHċbondO hydrogen bond. The effect of the NHċbondS interaction on the 1J(N,H) coupling constant and the shielding of the bridge proton is small. The NMR parameter changes in the series of the 2(2′‐heteroaryl)pyrroles due to NHċbondX hydrogen bond and the series of the 1‐vinyl‐2‐(2′‐heteroaryl)‐pyrroles due to CHċbondX hydrogen bond have the same order. The proximity of the nitrogen, oxygen or sulfur lone pair to the FċbondH hydrogen bridge quenches the trans‐hydrogen bond spin–spin couplings 1hJ(F,H‐1) and 2hJ(F,N). Copyright

GIAO, DFT, AIM and NBO analysis of the NH···O intramolecular hydrogen‐bond influence on the 1J(N,H) coupling constant in push–pull diaminoenones

In the series of diaminoenones, large high‐frequency shifts of the 1H NMR of the NH group in the cis‐position relative to the carbonyl group suggests strong NH···O intramolecular hydrogen bonding comprising a six‐membered chelate ring. The NH···O hydrogen bond causes an increase of the 1J(N,H) coupling constant by 2–4 Hz and high‐frequency shift of the 15N signal by 9–10 ppm despite of the lengthening of the relevant NH bond. These experimental trends are substantiated by gauge‐independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3‐bis(isopropylamino)‐1‐(aryl)prop‐2‐en‐1‐one (12) for conformations with the Z‐ and E‐orientations of the carbonyl group relative to the NH group. The effects of the NH···O hydrogen‐bond on the NMR parameters are analyzed with the atoms‐in‐molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the NH···O hydrogen bond as compared with that of 1,1‐di(pyrrol‐2‐yl)‐2‐formylethene (13) where NH···O hydrogen bridge establishes a seven‐membered chelate ring, and the corresponding 1J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) →σ*NH hyperconjugative interaction is weakened on going from the six‐membered chelate ring to the seven‐membered one due to a more bent hydrogen bond in the former case. A dominating effect of the NH bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the 1J(N,H) value as a consequence of the NH···O hydrogen bonding in the studied diaminoenones. Copyright

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles.

According to the density functional theory calculations, the X···H···N (XN, O) intramolecular bifurcated (three‐centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the 1hJ(N,H) and 2hJ(N,N) coupling constants across the NH···N hydrogen bond and an increase of the 1J(N,H) coupling constant across the NH covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the NH···N hydrogen bridge resulting in a lengthening of the N···H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge‐independent atomic orbital calculations of the shielding constants suggest that a weakening of the NH···N hydrogen bridge in case of the three‐centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms‐in‐molecules analysis shows that an attenuation of the 1hJ(N,H) and 2hJ(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density ρH···N at the hydrogen bond critical point and Laplacian of this electron density ∇2ρH···N. The natural bond orbital analysis suggests that the additional NH···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ*NH antibonding orbital across hydrogen bond weakening of the 1hJ(N,H) and 2hJ(N,N) trans‐hydrogen bond couplings through Fermi‐contact mechanism. An increase of the nitrogen s‐character percentage of the NH bond in consequence of the bifurcated hydrogen bonding leads to an increase of the 1J(N,H) coupling constant across the NH covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright

Estimating the energy of intramolecular hydrogen bonds from 1H NMR and QTAIM calculations

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 ρBCP 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 1H NMR data with the energies predicted by calculations showed that the most reliable are the linear dependence on the topological ρBCP 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 ρBCP and V parameters and the hydrogen bond energies obtained from empirical 1H NMR data. These relationships allow the comparison of the energies of different types of hydrogen bonds for various molecules and biological ensembles.

Synthesis of Acyl Terphenyls and Higher Polyaromatics via Base-Promoted C–H Functionalization of Acetylarenes with Arylacetylenes

KO(t)Bu/DMSO-promoted C-H functionalization of acetylarenes with arylacetylenes (100 °C, 30 min), generating β,γ-ethylenic ketones, triggers upon further heating (100 °C, 4 h, with or without acidifying additive) the cascade assembly of acyl terphenyls and higher polyaromatics in good yields.

Comparative estimation of the energies of intramolecular C-H…O, N-H…O, and O-H…O hydrogen bonds according to the QTAIM analysis and NMR spectroscopy data

The energies of intramolecular C-H…O, N-H…O, and O-H…O hydrogen bonds in model compounds are empirically estimated based on the values of the hydrogen bond induced weak-field shift of the bridging hydrogen atom signal in the 1H NMR spectrum. It is supported by a theoretical estimation of these energies based on the electron density value at the hydrogen bond critical point calculated within the QTAIM method. Good agreement between the empirical and theoretical estimates is found, which gives evidence of their reliability. It is shown that from the standpoint of their strength the intramolecular N-H…O and O-H…O hydrogen bonds can be classified as moderate whereas the intramolecular C-H…O hydrogen bonds must be classified as very weak interactions similar in their energy significance to van der Waals interactions.

H-H interaction in phenanthrene: Attraction or repulsion?

By the example of the structure of phenanthrene the character of the H-H interaction is studied by nonempirical quantum chemical methods. The calculations performed confirm Bader’s conclusions about the attractive character of the H-H interaction, which were made based on the QTAIM analysis, and disprove the repulsive character of the above interaction, which was derived from the EDA procedure.

The fine difference in electronic structure of heteroatoms in methyl vinyl ether and sulfide

Abstract Quantum chemical calculations carried out in D95++(d,p) basis set at B3LYP level of density functional theory were obtained for methyl vinyl ether and methyl vinyl sulfide to compare the electronic structure of heteroatom lone pairs. The molecules were observed in the framework of Baders theory ‘Atoms in Molecules’. Essential differences in electronic structure of oxygen and sulfur atoms are observed. An asymmetry of sulfur atom is supposed.