Phase equilibria and critical phenomena in the cesium nitrate–water–pyridine ternary system

The solubilities of components, phase equilibria, and critical phenomena in the cesium nitrate–water–pyridine ternary system are studied in the 5–100°C temperature range by the visual–polythermal method. Cesium nitrate is found to exhibit a salting-out effect at temperatures above 79.9°C causing phase separation in homogeneous water–pyridine solutions. The temperature of formation of the critical monotectic tie line (79.9°C) and the compositions of solutions corresponding to the liquid–liquid critical points at three temperatures are determined. The pyridine distribution coefficients between the aqueous and organic phases of the monotectic state at 85.0, 90.0, and 100.0°C are calculated. Their values demonstrate that salting-out of pyridine from aqueous solutions by cesium nitrate increases at higher temperatures. The plotted isotherms of phase diagrams confirm the fragment of the scheme of topological transformation of the phase diagrams of salt–binary solvent ternary systems with salting-in and salting-out phenomena.     

Deuterium NMR stereochemical analysis of threo–erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents

We report the proton-decoupled deuterium NMR study of labelled diastereomers with remote stereogenic carbons dissolved in various mixtures of poly-γ-benzyl-l-glutamate (PBLG) and poly-γ-benzyl-d-glutamate (PBDG) liquid crystalline solutions. The evolution of quadrupolar splitting as well as the diastereomeric and the enantiomeric discrimination versus the proportion of PBLG and PBDG in the liquid crystalline phase is studied. It is shown that racemic liquid crystalline solutions of PBLG and PBDG may be used to measure diastereomeric excess (de). Thereafter the spectrum in PBLG solution allows for measuring the enantiomeric excess (ee) of each diastereomer. These first results suggest substantial prospects in the field of the analysis of diastereomers with remote stereogenic carbons.     

The adequacy of the observed kinetic order in catalyst and the differential selectivity patterns to the hypothesis of the cooperative mechanism of catalysis of the Suzuki—Miyaura reaction

The generally accepted mechanism of the Suzuki—Miyaura reaction suggests a sequential activation of the substrate (aryl halide) and the reagent (arylboronic acid) by a palladium catalyst with the formation of unsymmetric biaryl as a result of a single turnover of the catalytic cycle, i.e., it is linear from the kinetic point of view. At the same time, the use of an unconventional kinetic approach based on the analysis of the differential selectivity of the reaction, rather than the regularities of catalytic activity, indicates the inadequacy of the linear mechanism, that is consistent with the hypothesis of a nonlinear (the so-called cooperative) mechanism of catalysis, in which the product is formed as a result of the substrate and reagent activation by two different palladium-containing intermediates in two parallel catalytic cycles. The experimentally observed low kinetic orders of the Suzuki—Miyaura reaction with respect to the concentration of the palladium catalyst precursor under the ligand-free conditions of catalysis are also consistent with the cooperative mechanism and can be due to the changes in the relative amount of the catalyst in two parallel catalytic cycles and/or to the process of catalyst deactivation.

     

Calorimetric investigation of order–disorder transition in Cu0.6Pd0.4 and Cu0.85Pd0.15 alloys

Order–disorder phase transitions in Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys have been investigated using differential scanning calorimetry and drop calorimetry. The differential scanning calorimetry measurements show that the transition in both these alloys are reversible in nature and the enthalpy increment measurements reveal that these transitions are first order in nature. The transition temperature of first-order phase transition in Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys have been evaluated to be 884(±2) and 799(±2) K, respectively, from drop calorimetric measurements. The latent heat of first-order phase transition in Cu_0.6Pd_0.4 alloy were evaluated to be 31.2(±0.6) and 28.9(±0.5) J g^?1, by enthalpy increment and differential scanning calorimetry measurements, respectively. Similarly, the latent heat of first-order phase transition in Cu_0.85Pd_0.15 alloy were evaluated to be 23.1(±0.6) and 21.3(±0.5) J g^?1, by enthalpy increment and differential scanning calorimetry measurements, respectively. The solidus temperatures of Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys were found to be 1,457(±2) and 1,360 K, respectively.     

Reaction mechanism in crystalline solids: kinetics and conformational dynamics of the Norrish type II biradicals from α-adamantyl-p-methoxyacetophenone

In an effort to determine the details of the solid-state reaction mechanism and diastereoselectivity in the Norrish type II and Yang cyclization of crystalline α-adamantyl-p-methoxyacetophenone, we determined its solid-state quantum yields and transient kinetics using nanocrystalline suspensions. The transient spectroscopy measurements were complemented with solid-state NMR spectroscopy spin-lattice relaxation experiments using isotopically labeled samples and with the analysis of variable-temperature anisotropic displacement parameters from single-crystal X-ray diffraction to determine the rate of interconversion of biradical conformers by rotation of the globular adamantyl group. Our experimental findings include a solid-state quantum yield for reaction that is 3 times greater than that in solution, a Norrish type II hydrogen-transfer reaction that is about 8 times faster in crystals than in solution, and a biradical decay that occurs on the same time scale as conformational exchange, which helps to explain the diastereoselectivity observed in the solid state.     

The error due to neglecting the core spin–orbit splitting in valence ZORA calculations with frozen core approximation and its elimination

In valence zeroth-order regular approximation (ZORA) calculations with frozen core approximation, when the basis set optimized to the related scalar relativistic ZORA calculations is used, neglecting the core spin–orbit splitting may result in additional basis set truncation errors. It is found that the error is negligible for most elements except the 6p-block elements. When the basis set is extended by a p-type STO function put on the 6p element atoms with the ζ value proper to 5p_1/2 orbitals, the error can be reduced to be negligible. The calculated atomic properties related to valence orbitals can be improved greatly by use of this extended basis set. The frozen core approximation calculations of some molecules containing Tl, Pb and Bi with closed shells show that neglecting the core spin–orbit splitting only slightly affects the calculated bond lengths and bond energies, and the calculated molecular property can also be improved slightly by use of the extended basis sets.     

The role of disclinations in two phase changes induced by temperature and pressure in crystalline solids: melting and the brittle–ductile transition

It has long been known that the melting temperature T _m of close-packed metals correlates well with the mono-vacancy formation energy. However, with the possible exception of the face-centered-cubic metals, there is a prior phase transition from a mechanically brittle solid phase to a ductile phase. Here the likely role of disclinations in the brittle-ductile phase change is stressed. The present picture may help to understand the brittle–ductile transition not only in crystalline materials but also in amorphous phases. The structure of such phases can probably be characterized in terms of a disordered disclination network. As examples of elemental crystalline solids, Si and graphite are finally discussed, with the melting under pressure of graphite being quantified.     

Deuteron NMR study of ammonium ion mobility near the order–disorder phase transition in (ND4)2PbCl6

Deuteron NMR spectroscopy was applied to study ammonium ion mobility below 50 K in fully deuterated ammonium hexachloroplumbate. Tunnelling frequencies from the spectra and correlation times from spin-lattice relaxation were measured. The spectra down to about 41 K show also a motionally narrowed component. The order–disorder phase transition takes place at 38.4 K. Below the phase transition two components are found and attributed to ions in ordered domains and transition regions between them, characterised below 25 K by tunnelling frequencies equal 0.35 and 1 MHz, respectively. A gradual change of the potential experienced by ammonium ions was observed.     

The Tafel–Heyrovsky route in the kinetic mechanism of the hydrogen evolution reaction

The examination of the dependence of surface coverage on overpotential for the kinetic mechanism of the hydrogen evolution reaction (HER) through the Volmer–Heyrovsky route shows that, under certain conditions, the Tafel re-adsorption can be produced. On this basis, the HER could be described with the contribution of Tafel and Heyrovsky elementary steps. The present communication derives and analyses the kinetic mechanism of the HER for the Tafel–Heyrovsky route, under Frumkin adsorption conditions.     

Kinetics and nucleation mechanism of carbamazepine–saccharin co-crystals in ethanol solution

Journal of Thermal Analysis and Calorimetry - This study aimed to investigate the metastable zone width (MSZW) and the nucleation order of carbamazepine–saccharin (CBZ–SAC) co-crystals...     

Contribution to cation–π interaction of the univalent silver cation with racemic [7]helicene in the solid state

Structural Chemistry - Yellow crystals of the complex [7]helicene–silver perchlorate (i.e., C30H18–AgClO4 or more accurately [Ag(η4-C30H18)]ClO4, respectively), crystallizing in...     

Computer-assisted study on the reaction between pyruvate and ylide in the pathway leading to lactyl–ThDP

In this study the formation of the lactyl-thiamin diphosphate intermediate (L-ThDP) is addressed using density functional theory calculations at X3LYP/6-31++G(d,p) level of theory. The study includes potential energy surface scans, transition state search, and intrinsic reaction coordinate calculations. Reactivity is analyzed in terms of Fukui functions. The results allow to conclude that the reaction leading to the formation of L-ThDP occurs via a concerted mechanism, and during the nucleophilic attack on the pyruvate molecule, the ylide is in its AP form. The calculated activation barrier for the reaction is 19.2?kcal/mol, in agreement with the experimental reported value.     

Chemical inequivalence behavior of the protons attached to the same carbon in ethylene-propylene copolymer（EPC）

Six ethylene-propylene copolymer samples with different structural parameters were analyzed by ^1H NMR,^13C NMR and ^1H-^13C COSY in this paper.With the aid of the ^1H-^13C COSY spectra of ethylene-propylene copolymer,the chemical inequivalence behavior of the protons attached to the same carbon in the ethylene-propylene copolymer could be observed. Four basic types of chemical bonds were divided,and two kinds of main conformations were proposed to explain this phenomenon. Based on the calculation of conformation distribution,it can be proved that the explanation of this phenomenon was reasonable.     

Study on the excitation mechanism 1. Non-Boltzmann–Saha distributions in the inductively coupled plasma

This work has further shown that the collisional–radiative multi-level model for the analyte atom and ion could be approximated by the step-wise series model. The non-Boltzmann and non-Saha factors of Ca for 48 levels corresponding to radiative decay, radiative recombination, Penning ionization and absorption processes, respectively, as well as to their mutual processes occurring at the axial channel of inductively coupled plasma and at an observation height of 15 mm above the load coil, were calculated. It was found that the high level relative to low level is over populated.     

Orientation order in monoethanolamine at 25–75°C, according to data from dielcometry

The static permittivity (?_s) of monoethanolamine (MEA) is measured at a frequency of 1 MHz in the temperature range of 20–90°C. The found data were analyzed in the context of the Onsager-Kirkwood-Fröhlich theory. The values of the correlation factor (g) are calculated in the investigated temperature range. Similar calculations using literature data are performed for 3-amino-1-propanol (AP) and N,N-dimethylethanolamine (DMEA) (temperature range, 15–35°C). The difference between the correlation factors of MEA, AP, and DMEA and their temperature dependences are explained by the structures of these amino alcohols and their molecules. A conclusion is drawn on the orientation order in liquid MEA, based on the obtained results made. In addition, the data from our study of the equilibrium dielectric properties of MEA are compared to the similar properties of ethylene glycol (EG), and a conclusion is drawn on features of the structure of amino alcohol relative to diol.