D. V. Pavlov

Study of conformations and hydrogen bonds in the configurational isomers of pyrrole‐2‐carbaldehyde oxime by 1H, 13C and 15N NMR spectroscopy combined with MP2 and DFT calculations and NBO analysis

The 1H, 13C and 15N NMR studies have shown that the E and Z isomers of pyrrole‐2‐carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole‐2‐carbaldehyde oxime is stabilized by the NH···N and NH···O intramolecular hydrogen bonds, respectively. The NH···N hydrogen bond in the E isomer causes the high‐frequency shift of the bridge proton signal by about 1 ppm and increase the 1J(N, H) coupling by ∼3 Hz. The bridge proton shows further deshielding and higher increase of the 1J(N, H) coupling constant due to the strengthening of the NH···O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by ∼3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of 1H shielding and 1J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the NH···N and NH···O hydrogen bondings to be estimated. The NBO analysis suggests that the NH···N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the NH bond through the NH···O hydrogen bond occurs in the Z isomer. Copyright

Mesomechanics of shear resistance along a filled crack

The paper reports on laboratory experiments with the aim of studying the effect of microstructural and macromechanical properties of a crack filled with discrete material on the formation of a sliding mode. It is shown that the spectrum of possible deformation events on the discontinuity is governed by both the macroscopic characteristics of the gouge and its mesoscale structure. The evolution of force bridges which are formed and collapsed in shear along the crack, their length and number fully control the type of deformation—stable sliding, stick-slip, and intermediate modes with low-velocity motion of the crack edges. The variation of the Coulomb strength affects mainly the stress drop value in dynamic failure or a slip event with low displacement velocity and little affects the deformation mode. Consideration is also given to the regularities by which the macroscopic characteristics of contact vary in shear.

Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

In the 1H and 13C NMR spectra of 1‐(2‐selenophenyl)‐1‐alkanone oximes, the 1H, the 13C‐3 and 13C‐5 signals of the selenophene ring are shifted by 0.1–0.4, 2.5–3.0 and 5.5–6.0 ppm, respectively, to higher frequencies, whereas those of the 13C‐1, 13C‐2 and 13C‐4 carbons are shifted by 4–5, ∼11 and ∼1.7 ppm to lower frequencies on going from the E to Z isomer. The 15N chemical shift of the oximic nitrogen is larger by 13–16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the 77Se resonance positions is revealed in the studied oxime isomers, the 77Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the 1H, 13C, 15N and 77Se shielding constants in the energy‐favorable conformation with the syn orientation of theCNOH group relative to the selenophene ring. Copyright

Stereospecificity of 1H, 13C and 15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1H chemical shifts

In the 13C NMR spectra of methylglyoxal bisdimethylhydrazone, the 13C‐5 signal is shifted to higher frequencies, while the 13C‐6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the 1H‐6 chemical shift and 1J(C‐6,H‐6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the –CH═N– bond does not change. This paradox can be rationalized by the C–H⋯N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum‐chemical calculations including Baders quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ(1H‐6) and 1J(C‐6,H‐6) parameters. The effect of the C–H⋯N hydrogen bond on the 1H shielding and one‐bond 13C–1H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The 1H, 13C and 15N chemical shifts of the 2‐ and 8‐(CH3)2N groups attached to the –C(CH3)═N– and –CH═N– moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8‐(CH3)2N group conjugate effectively with the π‐framework, and the 2‐(CH3)2N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N‐2– and N‐8– nitrogen lone pairs to the π‐framework varies, which affects the 1H, 13C and 15N shieldings. Copyright

Experimental and theoretical study of the intramolecular C–H···N and C–H···S hydrogen bonding effects in the 1H and 13C NMR spectra of the 2-(alkylsulfanyl)-5-amino-1-vinylpyrroles: a particular state of amine nitrogen

In the 1H NMR spectra of the 1‐vinylpyrroles with amino‐ and alkylsulfanyl groups in 5 and 2 positions, an extraordinarily large difference between resonance positions of the HA and HB terminal methylene protons of the vinyl group is discovered. Also, the one‐bond 1J(Cβ,HB) coupling constant is surprisingly greater than the 1J(Cβ,HA) coupling constant in pyrroles under investigation, while in all known cases, there was a reverse relationship between these coupling constants. These spectral anomalies are substantiated by quantum chemical calculations. The calculations show that the amine nitrogen lone pair is removed from the conjugation with the π‐system of the pyrrole ring so that it is directed toward the HB hydrogen. These factors are favorable to the emergence of the intramolecular C–HB•••N hydrogen bonding in the s‐cis(N) conformation. On the other hand, the spatial proximity of the sulfur to the HB hydrogen provides an opportunity of the intramolecular C–HB•••S hydrogen bonding in the s‐cis(S) conformation. Presence of the hydrogen bond critical points as well as ring critical point for corresponding chelate ring revealed by a quantum theory of atoms in molecules (QTAIM) approach confirms the existence of the weak intramolecular C–H•••N and C–H•••S hydrogen bonding. Therefore, an unusual high‐frequency shift of the HB signal and the increase in the 1J(Cβ,HB) coupling constant can be explained by the effects of hydrogen bonding. Copyright

Study of stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the configurational isomers of the selenophene-2-carbaldehyde azine by NMR spectroscopy and MP2-GIAO calculations

In the 1H and 13C NMR spectra of selenophene‐2‐carbaldehyde azine, the 1H‐5, 13C‐3 and 13C‐5 signals of the selenophene ring are shifted to higher frequencies, whereas those of the 1H‐1, 13C‐1, 13C‐2 and 13C‐4 are shifted to lower frequencies on going from the EE to ZZ isomer or from the E moiety to the Z moiety of EZ isomer. The 15N chemical shift is significantly larger in the EE isomer relative to the ZZ isomer and in the E moiety relative to the Z moiety of EZ isomer. A very pronounced difference (60–65 mg/g) between the 77Se resonance positions is revealed in the studied azine isomers, the 77Se peak being shifted to higher frequencies in the ZZ isomer and in the Z moiety of EZ isomer. The trends in the changes of the measured chemical shifts are reasonably reproduced by the GIAO calculations at the MP2 level of the 1H, 13 C, 15 N and 77Se shielding constants in the energy‐favorable conformation with the syn orientation of both selenophene rings relative to the C = N groups. The NBO analysis suggests that such an arrangement of the selenophene rings may take place because of a higher energy of some intramolecular interactions. Copyright

Some questions of geomechanics of the faults in the continental crust

We present the results of laboratory experiments on studying the formation of different slip modes on the interfaces in a rock massif such as aseismic creep, stick-slip, and periodic slow-slip events. It is shown that the way of releasing the accumulated elastic energy is determined by the mesoscale structure of the gouge rather than by its macroscopic strength characteristics. The evolution of the stress chains which are formed and broken during the displacement on the fracture, as well as the length and number of these chains, completely determines the regularities of the deformation. The role of these load-bearing elements in nature can be played, e.g., by the “contact spots,” which determine the regularities of stress concentration near the interblock boundary. We consider the effects of low-amplitude vibrations on stressed fractures. It is shown that, depending on the mode of deformation, the vibration impact can either reduce or boost the amplitude of separate events and the fraction of energy that is released dynamically. In the conclusion of the paper, we discuss the possibility of using the shear strength of the fault zone as a geomechanical parameter controlling the mode of deformation.

Structural peculiarities of configurational isomers of 1-styrylpyrroles according to 1Н, 13С and 15N NMR spectroscopy and density functional theory calculations: electronic and steric hindrance for planar structure.

Comparative analysis of the 1Н and 13С NMR data for a series of the E and Z‐1‐styrylpyrroles, E and Z‐1‐(1‐propenyl)pyrroles, 1‐vinylpyrroles and styrene suggests that the conjugation between the unsaturated fragments in the former compounds is reduced. This is the result of the mutual influence of the donor p–π and π–π conjugation having opposite directions. According to the NMR data combined with the density functional theory calculations, the Z isomer of 1‐styrylpyrrole has essentially a nonplanar structure because of the steric hindrance. However, the E isomer of 1‐styrylpyrrole is also an out‐of‐plane structure despite the absence of a sterical barrier for the planar one. Deviation of the E isomer from the planar structure seems to be caused by an electronic hindrance produced by a mutual influence of the p–π and π–π conjugation. The structure of the E isomer of the 2‐substituted 1‐styrylpyrroles is similar to that of the 2‐substituted 1‐vinylpyrroles. The steric effects in the Z isomer of the 2‐substituted 1‐styrylpyrroles result in the large increase of the dihedral angle between planes of the pyrrole ring and double bond. Copyright

Study of acoustic emission signals during fracture shear deformation

We study acoustic manifestations of different regimes of shear deformation of a fracture filled with a thin layer of granular material. It is established that the observed acoustic portrait is determined by the structure of the fracture at the mesolevel. Joint analysis of the activity of acoustic pulses and their spectral characteristics makes it possible to construct the pattern of internal evolutionary processes occurring in the thin layer of the interblock contact and consider the fracture deformation process as the evolution of a self-organizing system.

Application of chemical shifts in 1H and 13C NMR spectra to configuration assignment of Schiff bases in the liquid phase

In 1H and 13C NMR spectra of N-substituted dimethylketimines chemical shifts of protons and carbon atoms of the methyl groups in the cis-position with respect to the unshared electron pair of the nitrogen are larger than those of the CH3 groups in the trans-position by 0.2–0.4 and 8–11 ppm respectively. This effect is accompanied by the reduction of the corresponding direct spin-spin coupling constant 13C-13C by 10 Hz. The experimental trends in the variation of the spectral parameters are well reproduced by ab initio quantum-chemical calculations. The discovered stereochemical dependence of the chemical shifts of 1H and 13C may underlie a simple and efficient method of the configuration assignment in various compounds with a C=N bond.