Proton and Heavy Atom Motions during the Stepwise Proton Tautomerism of Various Oxalamidines. A Semiempirical PM3-MNDO Study

The tautomerism of oxalamidine (OA) and the substituted analogs tetraphenyloxalamidine (TPOA), 2,2’-bis-(4,5-dihydro-1,3-diazole) (OA5), 2,2\u27-bis-(3)4,5,6-tetrahydro-l,3-diazixine) (OA6) and 2,2\u27-bis-(4,5,6,7-tetra- hydro-1,3-diazepine) (OA7) has been studied theoretically using the semiempirical PM3-MNDO method. According to a previous experimental study, this process consists of an intramolecular degenerate double proton transfer in weak intramolecular N-H- N hydrogen bonds, where the two protons are transferred stepwise involving a zwitterionic intermediate. In addition, evidence was obtained for a substantial heavy atom reorganisation which is strongly dependent on the chemical structure. In the present study, this interpretation is confirmed theoretically by calculation of the energies and geometries of the initial states, transition states and intermediate states of the tautomerism. For A06 and A07, sin- and anti-forms are obtained which differ in the confirmation of methylene bridges. The geometry of anti-OKI agrees remarkably well with the crystal geometry of OA7. Therefore, although the calculated barrier heights of the tautomerism highly exceed the experimental values, the calculated molecular geometries seem to be reliable. Especially interesting is the nature of reorganisation of the molecular skeleton during proton transfer in various oxalamidines. This reorganisation mainly involves a decrease of the nitrogen-nitrogen distances of the hydrogen bond in which the proton transfer takes place, thus lowering the barrier for the tautomerism. The reorganisation energy strongly depends on the chemical structure. In the case of OA and TPOA, compression of the proton transferring hydrogen bond is possible without major changes in the geometry of the other hydrogen bond. By contrast in bicyclic oxalamidines, compression of one hydrogen bond involves elongation of the other bond or a ring deformation, requiring additional energy. Ring deformation requires less energy in OA7 as compared to OA5 and OA6, in accordance with experimental findings

Solid State NMR for Nonexperts: An Overview of Simple but General Practical Methods

There are varieties of methods available for the exploration of solids using nuclear magnetic resonance (NMR) spectroscopy. Some of these methods are quite sophisticated, others require specialized equipment. This review is addressed to those for whom NMR is not the main research method. It discusses simple methods that can be applied to solids with little or no adaptation to a specific system. Despite their technical simplicity and ease of use, these methods are powerful analytical tools that provide unique insights into the structure, dynamics, and noncovalent interactions in homo- and heterogeneous systems. Particular attention is paid to the characterization of porous materials and solids containing phosphorus. 31P NMR of organometallic compounds has been used as an example of how theoretical calculations can help in deeper analysis of experimental data

Water and small organic molecules as probes for geometric confinement in well- ordered mesoporous carbon material

Mesoporous carbon materials were synthesized employing polymers and silica gels as structure directing templates. The basic physico-chemical properties of the synthetic mesoporous materials were characterized by 1H and 13C MAS solid-state NMR, X-ray diffraction, transmission electron microscopy (TEM) and nitrogen adsorption measurements. The confinement effects on small guest molecules such as water, benzene and pyridine and their interactions with the pore surface were probed by a combination of variable temperature 1H-MAS NMR and quantum chemical calculations of the magnetic shielding effect of the surface on the solvent molecules. The interactions of the guest molecules depend strongly on the carbonization temperature and the pathway of the synthesis. All the guest-molecules, water, benzene and pyridine, exhibited high-field shifts by the interaction with the surface of carbon materials. The geometric confinement imposed by the surface causes a strong depression of the melting point of the surface phase of water and benzene. The theoretical calculation of 1H NICS maps shows that the observed proton chemical shifts towards high-field values can be explained as the result of electronic ring currents localized in aromatic groups on the surface. The dependence on the distance between the proton and the aromatic surface can be exploited to estimate the average diameter of the confinement structures

К вопросу о контроле и автоматизации шахтных дегазационных систем

Розглянуті існуючі системи контролю та автоматизації шахтної газопровідної мережі, а також засобі контролю газодинамічних параметрів метано-повітряної суміші у дільничному дегазаційному трубопроводі.The existing system of control and automation of mine degassingnetwork, and controls the gasdynamic parameters of methane-air mixture in the precinct line degassing

Dirhodium Coordination Polymers for Asymmetric Cyclopropanation of Diazooxindoles with Olefins: Synthesis and Spectroscopic Analysis

A facile approach is reported for the preparation of dirhodium coordination polymers [Rh₂(L1)₂]n (Rh₂-L1) and [Rh₂(L2)₂]n (Rh₂-L2; L1=N,N’-(pyromellitoyl)-bis-L-phenylalanine diacid anion, L2=bis-N,N’-(L-phenylalanyl) naphthalene-1,4,5,8-tetracarboxylate diimide) from chiral dicarboxylic acids by ligand exchange. Multiple techniques including FTIR, XPS, and ¹H→¹³C CP MAS NMR spectroscopy reveal the formation of the coordination polymers. ¹⁹F MAS NMR was utilized to investigate the remaining TFA groups in the obtained coordination polymers, and demonstrated near-quantitative ligand exchange. DR-UV-vis and XPS confirm the oxidation state of the Rh center and that the Rh-single bond in the dirhodium node is maintained in the synthesis of Rh₂-L1 and Rh₂-L2. Both coordination polymers exhibit excellent catalytic performance in the asymmetric cyclopropanation reaction between styrene and diazooxindole. The catalysts can be easily recycled and reused without significant reduction in their catalytic efficiency