Vol 11, No 2 (2017)

A mathematical modeling of the time evolution of the populations of the states of a five-level molecule during transformation of resonant monochromatic irradiation and spontaneous emission from the highest-energy state excited by a short pulse of light is performed. The formalism of the optical Bloch equations and quantum theory of radiation are applied a composite system consisting of a molecule and a quantized radiation field. The results of simulation of the evolution of the population of the states of the molecule in the case of spontaneous emission are similar for both of these two approaches, but differ significantly in the case of conversion by the molecule of monochromatic radiation. These differences are the greater, the higher the intensity of resonance Rayleigh scattering or (and) relaxed fluorescence, as a result of which the molecule returns to the initial ground state. An explanation of the nature of these differences is given.

The latest results of density matrix theory are summarized in terms of analysis of off-diagonal components. For a given reduced density matrix (RDM) of arbitrary order, the full density matrix is explicitly constructed, from which the RDM is derived by means of the corresponding reduction procedure. Both the matrices satisfy all the necessary conditions, such antisymmetry, normalization, hermiticity, except that the non-negative definiteness of the full DM is not guaranteed. If an RDM satisfies the necessary inequalities with the same number of indices as it has itself, then, as proved here, there is an additive correction to the full DM matrix that turns it into a non-negative definite matrix leading to the same RDM and satisfying the other requirements. This establishes a fundamental fact that these inequalities are sufficient as well, providing the N-representability of the given RDM. The construction is based on the classification, introduced in the article, of the multi-index elements of the RDM and full DM according to their degree of off-diagonality. RDM elements with a predetermined degree of off-diagonality are expressed through full DM elements with the same off-diagonality. This shows that the conventional variational approach to calculating the energy of the system by convex programming methods under the conditions of N-representability (before and even after reaching the minimum energy) generally does not lead to constructing a mixed quantum state composed of solutions to the Schrödinger equation. Therefore, except for the energy, it does not guarantee a correct description of the other properties so long as the conditions of correctness for each particular property are not expressed in terms of the RDM and not incorporated into the specific conditions of N-representability (regardless of the requirement of energy minimization).

The carbon fibers obtained by carbonization of polyacrylonitrile fibers were studied by electron paramagnetic resonance and X-ray diffraction analysis in the range of small and wide scattering angles. Their elastic and strength characteristics were also studied. The concentration of the paramagnetic centers was correlated with the mechanical properties of carbon fibers. The wide-angle X-ray diffraction study did not reveal essential structural differences in the carbon fiber samples with different mechanical properties. At the same time, the small-angle X-ray scattering study showed that the fiber nanostructures with different mechanical properties differ substantially.

In molecular zinc-porphyrin-based donor–acceptor systems, the electron transfer from the second singlet excited state S₂ is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized charge-separated state (20%). It is demonstrated that the quantum yield of ultrafast charge separation in donor–acceptor triads D–A₁–A₂ can be close to 100% in molecular systems with lifetimes of the S₂ state longer than 150 ps. As prototypes of such systems, donor–acceptor diads D–A₁ and triads D–A₁–A₂ are considered, wherein the xanthione molecule plays the role of a donor. The ranges of the model parameters are determined in which the efficiency of charge separation is high. The twostage photoinduced charge transfer is studied within the framework of a multichannel stochastic model that takes into account the reorganization of a polar solvent and a high-frequency intramolecular vibrational mode.

For a model reaction of tetralylhydroperoxide decomposition, kinetic data for evaluating the catalytic activity of Cu(II) complexes in solvents of different natures were obtained. The experiments were performed using chlorobenzene (aprotic solvent), ethanol (capable of forming intermolecular hydrogen bonds), and ethyl acetate (having no movable proton). The passivation of the catalysts in the presence of benzoic acid and N,N'-bis(o-hydroxybenzoyl) hydrazine (inhibitor) in these solvents was studied. It was found that the tested additives produce different effects on the catalytic activity of the initial and reaction-modified forms of Cu(II).

The entry of a shock wave from air into water containing reactive gas (stoichiometric acetylene–oxygen mixture) bubbles uniformly distributed over the volume of the liquid has been numerically investigated using equations describing two-phase compressible viscous reactive flow. It has been demonstrated that a steady-state supersonic self-sustaining reaction front with rapid and complete fuel burnout in the leading shock wave can propagate in this bubbly medium. This reaction front can be treated as a detonation-like front or “bubble detonation.” The calculated and measured velocities of the bubble detonation wave have been compared at initial gas volume fraction of 2 to 6%. The observed and calculated data are in satisfactory qualitative and quantitative agreement. The structure of the bubble detonation wave has been numerically studied. In this wave, the gas volume fraction behind the leading front is approximately 3–4 times higher than in the pressure wave that propagates in water with air bubbles when the other initial conditions are the same. The bubble detonation wave can form after the penetration of the shock wave to a small depth (~300 mm) into the column of the bubbly medium. The model suggested here can be used to find optimum conditions for maximizing the efficiency of momentum transfer from the pressure wave to the bubbly medium in promising hydrojet pulse detonation engines.

An improved model of the laser initiation of the explosive decomposition of energetic materials containing light-absorbing nanoparticles is investigated. The model takes into account how the light absorption efficiency factor changes with an increasing temperature. It is demonstrated that, as the temperature of an aluminum nanoparticle in pentaerythritol tetranitrate increases from 300 to 700 K, the light absorption efficiency factor increases by a factor of over 2. It is also shown that, for each particular nanoparticle radius in the 40–150 nm range, the temperature dependence of the light absorption efficiency factor over the relevant temperature range can be interpolated well by a second-order polynomial. Taking into account the variation of the efficiency of light absorption by the aluminum nanoparticle in the initiation of the explosive decomposition of pentaerythritol tetranitrate by a 12-ns-long neodymium laser pulse reduces the calculated critical energy density by a factor of 2.11 and decreases the optimum nanoparticle radius from 98 to 92 nm.

The collisional alignment of macromolecule ions in an external electric field was studied based on the scaling of the dimensions and properties of a real macromolecule and its environment into the domain of classical mechanics. The calculations were carried out within the framework of computational fluid dynamics for a streamlined prolate spheroid macromolecule; they showed that the collisional alignment of ions occurred in an oscillatory mode with a characteristic time of several nanoseconds.

A correlation between the microscopic adhesion characteristics of Aspergillus niger fungus and its growth rate on different polymeric materials was determined. Regression equations relating these characteristics and making it possible to assess the fungal resistance of materials based on the results of experimental measurements of the adhesion of Aspergillus niger spores were obtained.

Biosorption equilibrium and kinetics of oxalic acid on wheat straw, Triticum aestivum, in an aqueous solution was investigated. Among the models tested, namely the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms, the biosorption equilibrium was best described by Freundlich model. It was also found the biosorption of oxalic acid by wheat straw followed second-order kinetics. The equilibrium amount of oxalic acid adsorbed onto the wheat straw increased with increasing the initial oxalic acid concentration and decreasing adsorbent weight. On the other hand, an increase of temperature decreased the biosorption of oxalic acid. Mean free energy of biosorption (10 kJ/mole) indicates that biosorotion of oxalic acid by wheat straw might follow a chemisorptions mechanism.

The radical reactions of copolymerization with vinyl monomers and homopolymerization of synthesized fullerene-containing acrylates are studied. In contrast to the homopolymers, the fullerene-containing copolymers are readily soluble in typical organic solvents. The kinetic parameters of the copolymerization process and molecular-weight characteristics of the resulting products are determined.

The enzymatic destruction of chitosan in an acetic acid solution in the presence of sodium chloride and sulfate was considered. It was shown that the addition of these low-molecular electrolytes is accompanied by a tightening of the macromolecular coil, as indicated by a decrease in the α constant in the Mark–Kuhn–Houwink equation and the intrinsic viscosity of chitosan. As a consequence, the chitosan units become less accessible for interaction with the enzyme, and the kinetic parameters of the process (Michaelis constant K_M and the maximum rate of enzymatic destruction V_max) change accordingly. This suggests that the resistance of this polymer to the enzyme action can be enhanced by introducing low-molecular electrolyte salts in the solution or film coating to suppress the polyelectrolyte swelling of chitosan.

At present, the problem of spontaneous segregation has no satisfactory theoretical description. The main difficulty is a very low probability of the spontaneous formation of a chirally pure nucleus in a racemic mixture if the enantioselectivity energy is below (10–12)kT, which does not correspond to reality. In this paper, we propose a mechanism of the enhancement of enantioselectivity in the chiral phase nucleus, which circumvents the problem of the low probability of its formation.

Here, we studied the sensing ability of pure C₂₄, C₃₂, C₆₀, B-, and AlC₅₉ nano-cages toward the CNCl molecule using DFT calculations. Noticeably, the adsorption energies of CNCl–AlC₅₉ and ClCN–BC₅₉ with–1.174 and–0.382 eV which were more than other adsorption configurations (C₂₄, C₃₂, C₆₀) standing for higher detection of these systems. On the other hand, concerning dipole change detection, the CNCl–AlC₅₉ and ClCN–BC₅₉ were recognized as the most promising sensor systems introduced at the present study with the dipole moment values of 13.87 and 10.17 Debye. The recovery time for all adsorption configurations were negligible.

Changes in the mobility of water protons in the chemically modified starches (CMS)–water system are studied by differential scanning calorimetry and NMR relaxation. The amounts of unfrozen water at negative temperatures and additional (after gelation) unfrozen for CMS are lower than those for native starch. The proton spin–spin relaxation time T₂ for CMS samples, conventionally attributed to the water fraction in starch granules, decreases monotonically with increasing temperature, whereas for native starch, this dependence exhibits an extreme behavior. Studying the dispersion dependences for 7 wt % gels, which characterize the rate of chemical exchange of water protons with protons of hydroxyl groups of polysaccharides, showed the absence of this kind of dependence for the CMS studied when the instrument operated at a frequency of 20 MHz. This data indicate the significant destructive changes in the structure of the CMS.

Hydrogen interaction with gold nanoparticles deposited on the surface of a highly oriented pyrolytic graphite is determined by the external potential ϕ of the sample with respect to the ground. It is demonstrated that, at ϕ > +1 V, molecular hydrogen does not practically interacts with hydrogen, whereas at ϕ ≤ 0 V, such interaction is possible.

The concept of “odd oxygen” is considered and its atmospheric lifetime is evaluated. It is demonstrated that the modern interpretation of this concept precludes obtaining reliable data on the atmospheric lifetime of odd oxygen, which is of fundamental importance in ozonosphere chemistry. An algorithm for correctly assessing the atmospheric lifetime of odd oxygen is presented and relevant estimates for the end of the XX century are made. It is also shown that, if the lifetime of odd oxygen is comparable to or greater than the time of vertical eddy transport, the former should be replaced by a combined lifetime, taking into account the action of both photochemical and dynamic factors.