Vol 11, No 3 (2017)

Analytical expressions for describing the dynamics of the populations of the states of a five-level model molecule during fluorescence reflecting the quantum beats of the populations of its two excited degenerate states coupled by dynamic interaction are derived. The cases of excitation of the molecule by a short light pulse in a state with an energy higher than the energy of the degenerate states and into one of the degenerate states are considered. The populations of states are determined by solving the Schrödinger equation for the amplitudes of probability of populating the states of the composite system comprised of the molecule and a quantized radiation field. It was found that the resulting expressions satisfy the optical Bloch equations for the considered dynamics of the populations of the molecule’s states. Significant distinctions of the present results on the simulation of quantum beats from the corresponding simulation results reported in some works, performed based on different approaches, are revealed. The reasons for these differences are discussed.

The current study involves the measurements of T₁ relaxation time of complexes three different crown ethers (4-aminobenzo-15-Crown-5, 1-Aza-18-Crown-6, 1-Aza-15-Crown-5) and two different ions sodium perchlorate (NaClO₄), lithium perchlorate (LiClO₄) by a 400 MHz NMR spectrometer. The association constants (K_a) of the complexes were determined on the basis of the evaluation of these measurements using the Benesi-Hildebrand method that we have modified. These association constants were consistent with those obtained from ¹H NMR titration technique. As a result, it can be speculated that binding constants could be measured using T₁ relaxation time when chemical shifts are very small or cannot be observed.

It is shown that the initiated oxidation of deuterated esters of linoleic acid proceeds as a radical chain process. The value of the kinetic isotope effect for hydrogen atom abstraction from the bisallyl group of linoleic acid esters is determined: 3.0 for tert-butyl peroxy and cumylperoxy radicals.

The occurrence of oscillating modes at specific process parameters was demonstrated by the numerical analysis of the kinetics of methane oxidation in a continuous stirred tank reactor. These parameters characterize the composition and pressure of a reaction mixture, the feed rate of reagents, the thermal properties of the system, and the reactor geometry. The oscillating process is related to both the reaction kinetics and the evolution and removal of heat and reactants. In this case, oscillations occur only in a relatively small range of changes in the parameters that characterize the properties of the system; however, they can be accompanied by a considerable change in the yield of products.

This article is an overview, containing both new research data and results of published works. Using a layered system model that takes into account chemical processes and heat and mass transfer, proceeding in the one and two-zone combustion modes, we considered various types of combustion systems comprised of two components: base propellant and various additives, such as organic materials (decomposable, evaporable, oxidizeable), energetic materials, coolants, catalysts, and metals. Based on analytical-form results, the combustion characteristics are calculated as functions of the pressure, size of the components, and physicochemical properties of the system. In a number of cases, complex dependences are observed, for example, areas with an accelerated or retarded growth of the burning rate with increasing pressure, and extrema in the dependences. A comparison of calculation results with experimental data demonstrated a good qualitative agreement. The results can be used in developing solid rocket propellants with the required ballistic properties.

The results of investigation of the characteristics of thermal explosion in a semibatch reactor filled with a liquid−solid heterogeneous exothermic system are presented. Two successive macroscopic stages in this reacting system are considered. The focus was on studying the effects of the rate of supply of solid reagent and the rate of its dissolution on the critical condition of thermal explosion. It is shown that a thermal explosion can occur in both the first (during solid reagent supply into the reactor) and second macroscopic stages (after supply cessation).

Parameters of transverse hotspot-forming have been investigated from the standpoint of the socalled hotspot-pulsating combustion mechanism for condensed energetic substances (propellants, explosives, etc.). The propagation of a transverse wave over the lateral surface of the sample has been videorecorded. A frame-by-frame analysis of the video record has made it possible to determine the geometric characteristics and local velocities of the propagating wave. Combustion wave velocity distribution data obtained by other methods are presented, and all velocity data are analyzed in relation to the sample-average velocity and pressure.

It is shown that, in a wide class of physicochemical systems described by a system of conservation equations, including filtration combustion, the autowave mode of the process is impossible if the thermodynamic forces and flows are linearly related and the boundary conditions are stationary.

An approach to determining the nucleation barrier and interfacial free energy (surface tension) based on molecular dynamics simulations of structural transformations, in particular, the formation of new phase nuclei, is reported. The approach is based on the thermodynamic integration method, wherein key elements are trajectories characterizing the potential energy change, which are obtained from independent numerical experiments. An important feature of the approach is its applicability to both equilibrium and nonequilibrium systems, as well as the possibility of determining the above thermodynamic characteristics for small new-phase nuclei, with a significant curvature of the surface. For example, we present the temperature dependencies of the surface tension of water droplet nuclei for water vapor nucleation and of the nucleation barrier to crystal nucleation in two model glassy systems, which are computed within the framework of the proposed approach. The calculated values of the surface tension coefficient of water droplet nuclei are compared with the available experimental data.

The metal–insulator transition in finite one-dimensional lattices with diagonal disorder is considered. Although the true phase transition does not occur in such systems, a conditional criterion can be introduced to distinguish the regions of localization and delocalization of eigenvectors and wave functions, depending on the degree of disorder W. A relationship between the degree of disorder W and the characteristic length separating the localized and delocalized states is obtained: with high accuracy, ξ_cr ~ W^–2. The calculations are performed by integrating the evolutional Schrodinger equations and through analyzing the spectral properties of the corresponding random matrices. The probability of charge transfer is estimated within the framework of a simple model of DNA.

The formation of various fullerenyl radicals during the polymerization of methyl methacrylate (MMA) and styrene in the presence of C₆₀ fullerene is studied using the density functional method. It is shown that the interaction of an initiating radical with the fullerene molecule, yielding fullerenyl radical, is less favorable in energy terms than the initiation of the chain, i.e., the interaction of the same radical with a monomer. For radical polymerization of styrene, the termination of polystyrene chains on the fullerene is thermodynamically more probable than the growth of polystyrene chains, with the reactions being equiprobable in the case of MMA polymerization. The addition of monomer molecules to fullerenyl radicals is characterized by an enthalpy of activation exceeding that for conventional free radical chain growth by no more than 1.4 to 2.3 times; the thus formed growth radicals with fullerene molecules incorporated into the backbone polymer feature a reactivity not inferior to conventional polymer growth radicals.

A model of the physicochemical hydrodynamics of the synthesis of butadiene rubber on the TiCl₄−Al(i-C₄H₉)₃ catalytic system modified by turbulizing flows is developed. The model includes a hydrodynamics block for describing the dispersion of TiCl₄−Al(i-C₄H₉)₃ catalytic system particles in a tubular turbulizing apparatus of diffuser–confuser type, a block for calculating the concentrations of active sites of all types on TiCl₄−Al(i-C₄H₉)₃ particles proceeding from their size and taking into account the disjoining effect, and a macrokinetics block for simulating the synthesis of butadiene rubber in a stirred reactor. The kinetic constants of the process at 298 K are determined from the condition of best match between the kinetic curves and molecular- weight characteristics of the butadiene rubber calculated by the model and measured in experiment.

The effect of preparation conditions for positively and negatively charged silver iodide sols on the particle size and number concentration has been determined. It has been found that the preparation of a mixture of precursors with a concentration of 0.01 mol/L at a KI to AgNO₃ volume ratio of 10: 7 or 7: 10 leads to the formation of a large number of small particles (on the order of 50–60 nm) corresponding to the range of true colloidal systems with a maximum interfacial area. The presence of a polymer stabilizer–chitosan–leads to a significant improvement of the stability of the sol particles, which makes it possible to use the resulting polymer-colloidal dispersions containing nanoparticles of silver iodide sols to design hybrid materials for biomedical applications.

The Br₂ molecule adsorption on BP-NC and AlP-NC surfaces was investigated by using density functional theory calculations in gas phase. The effects of NH₂ functionalization and ethanol on the adsorption of Br₂ gas on BP-NC and AlP-NC surfaces were investigated. Results reveal that adsorptions of Br₂ on studied nanocones are exothermic and experimentally possible from the energetic viewpoint. Results show that, E_ad value of Br₂ on AlP-NC surface are more negative than corresponding values of BP-NC. Results reveal that, NH₂ functionalization and ethanol increase and decrease the absolute E_ad values of Br₂ on studied nanocones, respectively. These results show that, there are good linearity dependencies between E_ad and orbital energy values of studied nanocones. Therefore we can conclude the E_ad and orbital energy values of studied nanocones can consider as important parameters to propose suitable nanocones with enhanced Br₂ adsorption potential.

The coefficient of diffusion of distilled and sea water in polylactide of two grades at 23°C was calculated. The diffusion coefficient of water was (0.63–0.85) × 10^–10 cm²/s for both polylactide samples, which corresponds to hydrophobic polymers. The temperature dependence of the water diffusion coefficient at 23, 30, and 50°C was obtained. The activation energy of the diffusion of distilled water was determined to be 38 and 36 kJ/mol, respectively, for both polylactide samples. A differential scanning calorimetry (DSC) study showed that after exposure to distilled and sea water at 23°C for 120 days, the melting point decreased by 2–3°C, and the degree of crystallinity increased by 5–9% in the experiment with distilled water for both samples. In an electron paramagnetic resonance (EPR) study it was determined that after hydrolysis, the correlation time of radical rotation in amorphous regions slightly decreased for both samples.

The results of quantum chemical DFT calculations of energy barriers during the diffusion of vacancies on the Cu(001) surface at increased concentrations of vacant sites were described. The formation and destruction of dimers, trimers, and configuration of four vacancies were considered. For vacancies located at the neighboring sites of the surface lattice, there is effective attraction, which promotes the formation of vacancy clusters.