Quantitative study of the photostability of spiropyrans

A quantitative method of determining the quantum yield of irreversible photodegradation (_C) of photochromic spiropyrans (SPP) was proposed for the first time. The _C of a large number of SPP of different types (with a different heterocyclic part) were determined. It was shown that for all of the SPP studied (except for dithiolane SPP), _C is almost independent of the heterocyclic part and is determined by the structure of the chromene part of the SPP. In addition to photodegradation, photooxidation takes place as a result of the reaction of the triplet state of the closed form of the SPP with O₂ for nitrosubstituted SPP of all types. The value of _C is determined by the efficiency of the reaction of isomer X of the colored form with the solvent or impurities, i.e., the lifetime of this isomer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 290–298, February, 1990.     

Photodissociation of aromatic thiols and disulfides

For the first time, the quantum yields of cleavage of SS and SH bonds in a disulfide and a thiol and also the extinction coefficient of the photoinduced aminophenylthiyl radical were determined, on the basis of which the relation of the photodissociation quantum yield to the energy of an absorbed quantum was observed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 540–543, March, 1990.     

On continuous relaxation spectrum. Method of calculation

A method of continuous relaxation spectrum calculations based on the Mellin integral transform has been proposed. It was demonstrated that the equivalent results were obtained regardless an experimental viscoelastic function used a base for calculations. It proves the correctness of the proposed method. The obtained relaxation spectrum provides right predictions in calculation of different viscoelastic functions for various polymeric materials regardless their relaxation state.     

On rheological properties of polydisperse polymers

An investigation has been carried out into the effect of the fractional composition on the rheological (flow and elastic) properties of a system, using mixtures of polybutadienes with narrow molecularweight distribution (MWD). For mixtures of high-molecular-weight components, the initial Newtonian viscosity is determined by the weight-average molecular weight: $\text{η}{}_0\text{～}\text{M}{}^{\mathrm{\alpha }}{}_{\mathrm{<mtext>w</mtext>}}$; when low-molecular weight components are introduced, it is also determined by the MWD moment ratio. The characteristic relaxation time of a system is determined by the z-average molecular weight: , and in the general case α₁ ≠ α. A new model has been proposed to explain the non-Newtonian phenomenon as a consequence of the existence of a molecular-weight distribution. According to this model, as the shear rate increases the high-molecular-weight components gradually (at their critical rates) pass over to the high-elastic state. Therefore, at high shear rates, their contribution to viscous losses of a polydisperse polymer is associated with their behaviour as a viscoelastic filler in a viscous liquid.     

Density functional calculations of electronic g-tensors for semiquinone radical anions. The role of hydrogen bonding and substituent effects

A recently developed density functional approach has been used to carry out a systematic computational study of electronic g-tensors for a series of 1,4-semiquinone radical anions. Good agreement with high-field EPR data in frozen 2-propanol is achieved only after taking into account the significant reduction of g-tensor anisotropy caused by hydrogen bonding to solvent molecules. The comparison of various model systems for the first solvation shell suggests two hydrogen bonds from 2-propanol molecules to each of the carbonyl groups of the radical anions, and one additional hydrogen bond to each of the methoxy groups in ubiquinone systems. 2-Propanol makes stronger hydrogen bonds than water and thus influences g-tensor anisotropy more strongly. Substituent effects at the semiquinone are reproduced quantitatively by the calculations. The g-tensor anisotropy is influenced significantly by the conformations of methyl and methoxy substituents, with opposite contributions. Analyses and interpretations of the interrelations between structure, bonding, and spectroscopic data are provided. The relevance of the computational results for the EPR spectroscopy of semiquinone radical anions in photosynthetic reaction centers is discussed.     

HYPERICIN DERIVED TRIPLET STATES and TRANSIENTS IN ALCOHOLS and WATER

Abstract— Direct pulse photoexcitation of an antivirally active compound, hypericin sodium salt in ethanol, results in a short-lived transient, attributed to a triplet state. In the presence of reducing agents, a long-lived transient is observed, which indicates a radical anion species. In isopropanol solution, an identical triplet state is formed, accompanied by a long-lived intermediate that consists of a semiquinone-type radical. Laser excitation of hypericin sodium salt aggregates dispersed in water produces a very short-lived transient, also assigned to a triplet state, which decays, leaving an absorption spectrum, indicating a radical anion species. The latter reacts with oxygen with a rate constant of k ∼ 6 × 10⁷ M^-1 s^-1, suggesting the formation of superoxide.     

Non-linearity in rheology —an essay of classification

Different non-linear phenomena (such as non-Newtonian flow, large elastic deformations, instabilities of different types and many others) are the heart of rheology. Therefore many attempts were carried out to find quantitative, or at least qualitative, models of non-linear behavior. The general or perhaps most attractive way of developing rheological constitutive equations consists in the search for the most general method to describe everything in the framework of a single approach. Naturally, this leads to very complicated and ambiguous equations. Meanwhile, it is reasonable to try another way based on separating observed phenomena into different types depending on observed phenomena into different types depending on their physical origin. An attempt to propose such their physical origin. An attempt to propose such classification of nonlinear rheological effects is made.According to the assumed scheme three levels of non-linearity are distinguished. There is a group of phenomena which originate as a consequence of finite elastic deformations. This is weak non-linearity related to equilibrium properties of a matter. The second level can be characterized as strong non-linearity. It is related to reversible structure changes, developing in time and connected with changes in relaxation properties of a matter. This group of effects can be treated as kinetic phenomena. Lastly, the third level of non-linearity is connected with breaking or phase transitions induced by deformations. This leads to the most severe consequences and can be treated as effects of thermodynamic nature. It is shown that some well known rheological effects can be explained if we consider them as a superposition of non-linearity of different types.Presented as keynote lecture at the European Rheology Meeting, September 4–9, 1994, Sevilla, Spain     

Relativistic two-component calculations of electronic g-tensors that include spin polarization

The first two-component relativistic density-functional approach for the calculation of electronic g-tensors is reported that includes spin polarization using noncollinear spin-density functionals. The method is based on the relativistic Douglas-Kroll-Hess Hamiltonian and has been implemented into the ReSpect program package. Using three self-consistent-field calculations with orthogonal orientations of total magnetization J, the full g-matrix may be obtained. In contrast to previous spin-restricted two-component treatments, results with the new approach agree excellently with spin-polarized one-component calculations for light-atom radicals. Additionally, unlike one-component approaches, the method also reproduces successfully the negative deltag(parallel)-values of heavy-atom 2sigma radicals and the negative deltag(perpendicular) components in cysteinyl. The new method removes effectively the dilemma existing up to now regarding the simultaneous inclusion of spin polarization and higher-order spin-orbit effects in g-tensor calculations. It is straightforwardly applicable to higher than doublet spin multiplicities and has been implemented with hybrid functionals.     

Chemical transformations and phase transitions in polymer rheology and technology

Fundamental rheokinetic effects observed in processes of linear polymerization and three-dimensional oligomer curing are discussed. It was shown that changes in rheological properties during linear polymerization can be described by relationships based on treating a reactive mass as a polymer solution, if a newly formed polymer is soluble in reactive medium. Phase separation limits this approach and leads to some new rheological consequences such as change in the rate of viscosity growth (in some cases viscosity can even decrease in the course of polymerization). Shearing influences temperature of a phase transition and kinetics of chemical reaction if it proceeds in heterogeneous conditions. The same phenomena were also observed for oligomer curing which can occur in heterogeneous manner with microgelation and formation of colloid particles of a new phase before the gelation of a system as a whole.     

Effect of mechanical activation on the thermal stability of polytetrafluoroethylene powder

It was found that processing in a planetary activator leading to amorphization of the structure of polytetrafluoroethylene powder was not accompanied by a noticeable decrease in its thermal stability. It was established that the temperature at the onset of melting drops slightly, the melting rate increases, and the enthalpy of the endoeffect declines noticeably heating.