Effect of chlorine atoms in chlorinated ethylene on the rate and mechanism of its reaction with ozone

The UQCISD, UB3LYP, UMP2, and MRMP2 methods in conjunction with the 6-31+G**/6-311+G** and aug-cc-PVDZ basis sets are used to study the primary reaction of ozone with chlorinated ethylene derivatives: tetrachloroethylene, trichloroethylene, 1,2-trans-dichloroethylene, 1,2-cis-dichloroethylene, 1,1-dichloroethylene, and chloroethylene. The reaction is studied for both concerted and nonconcerted ozone addition. The UB3LYP DFT method in conjunction with the 6-31+G** basis set is used to examine various modes of addition of ozone to these chlorinated ethylenes by the Criegee and DeMore mechanisms. The geometry and energy of the transition states, the enthalpy and entropy, and the rate constants and ratios thereof for all the reactions are calculated. The UB3LYP method generally satisfactorily describes the two reaction pathways and, largely correctly predicts the rate constants, in agreement with the available experimental data. At the same time, this method appears to be inapplicable to modeling the interaction of ozone with 1,1-dichloroethylene. In this case, the single-determinant approximation turns out to be unsuitable, and, therefore, MCSCF methods should be used. The MRMP2 method yields reasonable values of the rate constants for the DeMore mechanism, whereas in the case of the Criegee mechanism, the MP2 method does well. The UB3LYP/6-31+G** and UQCISD/aug-cc-PVDZ methods give similar values of the ratio between the rate constants for the two pathways, a result that demonstrates the versatility of the first one.     

Electron paramagnetic resonance detection of the giant concentration of nitrogen vacancy defects in sintered detonation nanodiamonds

A giant concentration of nitrogen vacancy defects has been revealed by the electron paramagnetic resonance (EPR) method in a detonation nanodiamond sintered at high pressure and temperature. A high coherence of the electron spins at room temperature has been observed and the angular dependences of the EPR spectra indicate the complete orientation of the diamond system.     

EXAFS study of copper complexes with azomethinic ligand environment

A series of copper metallochelates C₂₂H₁₈CuN₄O₂ X (X = Se, S, O) as models of active centers of natural metalloproteins have been synthesized on the basis of new azomethine ligand systems. The structure of the complexes has been studied by extended X-ray absorption fine-structure spectroscopy. It is shown that, in the metallochelates with X = Se or S, one azomethine chalcogen-containing ligand undergoes tridentate interaction with copper ions, while the other ligand is an acetate group. As a result, a complex with the N₂O₂ X environment is formed, where one of the oxygen atoms of the acetate group is at a large distance from the metal ion: R = 2.56–2.68 Å. For the metallochelate with X = O, coordination of the acetate group by a copper ion is found to be absent, and only interaction with azomethine ligands having average Cu-N/O distances R = 1.96–2.04 Å is observed.     

Study of the effects of hydroxyapatite nanocrystal codoping by pulsed electron paramagnetic resonance methods

The effect of codoping of hydroxyapatite (HAP) nanocrystals with average sizes of 35 ± 15 nm during “wet” synthesis by CO₃^2? carbonate anions and Mn²⁺ cations on relaxation characteristics (for the times of electron spin–spin relaxation) of the NO₃^2? nitrate radical anion has been studied. By the example of HAP, it has been demonstrated that the electron paramagnetic resonance (EPR) is an efficient method for studying anion–cation (co)doping of nanoscale particles. It has been shown experimentally and by quantummechanical calculations that simultaneous introduction of several ions can be energetically more favorable than their separate inclusion. Possible codoping models have been proposed, and their energy parameters have been calculated.     

Localized charge inhomogeneities and phase separation near a second-order phase transition

Localized charge inhomogeneities and phase separation are described in the framework of the phenomenological theory of phase transitions. It is shown that Coulomb interaction determines the charge distribution and the characteristic size of the emerging inhomogeneities. Phase separation associated with charge segregation becomes possible because of a high dielectric constant and a low excess charge density in the localization region. The phase diagram of the system is calculated, and estimates are obtained for the gain in energy associated with the emerging state. The role of Coulomb interaction is exposed, and corresponding estimates are given.     

To the theory of phase transitions in relaxors

It is shown that the process of charge carrier localization on impurity centers is of importance. Localized carriers produce local electric fields, thereby stimulating the appearance of an induced polarization near the phase transition point. The direction of this polarization is dictated by the spatial distribution of the centers occupied by charge carriers. The temperature dependence of the dielectric constant is determined by the dynamics of the local-center occupation with decreasing temperature. The dispersion of the dielectric constant is determined by the vibrational characteristics of the local states forming near a localized charge. The phase transitions in relaxors are considered in the framework of the thermodynamic approach.     

Identification of the La6F37 cubooctahedral clusters in mixed crystals (BaF2)1 ? x(LaF3)x by the electron paramagnetic resonance method

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF₂)_{1 − x} (LaF₃) _x (x = 0, 0.001, 0.002, 0.005, 0.010, 0.020) doped with Ce³⁺ ions (0.1%) are investigated at a frequency v ≈ 9.5 GHz in magnetic fields up to 1.45 T at temperatures T = 10 and 15 K. The EPR spectrum of “pure” barium fluoride BaF₂ (x = 0) is characterized by a single Ce³⁺-F⁻ center with tetragonal symmetry (i.e., the O center with g _‖ = 2.601 and g _⊥ = 1.555). For a lanthanum trifluoride concentration x ≠ 0, the spectrum exhibits new lines due to the presence of the clusters containing Ce³⁺ and La³⁺ ions. The intensity of EPR signals from the O centers decreases rapidly as the lanthanum trifluoride concentration x increases. The lines attributed to a paramagnetic center with tetragonal symmetry and strongly anisotropic g factors (i.e., the K center with g _‖ = 0.725 and g _⊥ = 2.52) are separated in the complex EPR spectrum with the use of the angular dependence of the EPR signal intensity measured for the samples with x ≥ 0.002. This center is identified as a cubooctahedral cluster of the La₆F₃₇ type in which one of the La³⁺ ions is replaced by the Ce³⁺ ion. Original Russian Text ? L.K. Aminov, I.N. Kurkin, S.P. Kurzin, I.A. Gromov, G.V. Mamin, R.M. Rakhmatullin, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 11, pp. 1990–1993.     

Manifestations of Electron–Nuclear Interactions in the High-Frequency ENDOR/ODMR Spectra for Triplet Si–C Divacancies in 13C-Enriched SiC

JETP Letters - The frequencies of electron–nuclear interactions with 13C and 29Si nuclei on remote coordination spheres are determined in triplet spin centers in the form of neutral...