SCF Xα DW nuclear magnetic screening tensor calculations

Conclusions Calculations have been performed on the nuclear magnetic screening tensors for molecules via perturbation theory and the X DW method, and a comparison is made with results from other methods. Although the absolute errors in the for the molecules are appreciable, the relative errors in determining the NR chemical shifts are much less (see for example, the shift for CO relative to CH₄), which is a hopeful factor leading one to expect that X DW can be widely used in analyzing NMR spectra. At the same time, X DW can closely reproduces the anisotropy, which is particularly important for compounds such as V₂O₅ or its derivatives, which show very large anisotropy in the magnetic screening tensors.Catalysis Institute, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 5, pp. 32–36, September–October, 1988.     

ON THE NATURE OF THE INTERMEDIATE ELECTRON ACCEPTOR (A1) IN THE PHOTOSYSTEM I REACTION CENTER

Abstract— A sodium dodecyl sulfate-Photosystem I (PSI) complex has been prepared and characterized with respect to its electron acceptors. Component X and iron-sulfur centers A and B are absent from this preparation but the intermediate electron acceptor (A₁) is present. Flash-induced absorbance changes at 25°C show charge separation, followed by a back-reaction with a half-time of 5 µs. The spectrum of the flash-induced change from 350 to 550 nm indicates a contribution from the intermediate electron acceptor, A₁, as well as from P700⁺. EPR studies show that A₁ is associated with a free-radical signal having a g-value of 2.0025 and a linewidth of 12 gauss. A, would appear to be associated with a monomeric form of either Chi α or pheophytin a.     

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.     

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.     

Determination of the quantum yields of the photochromism of spiropyrans with two chromene fragments

Conclusions The quantum yields of the photochromism of spiropyrans with two chromene functions have been determined. The combining of nitrochromene and benzochromene fragments in a single molecule results in an increase in the quantum yields of the photochromism of the benzochromene part of the molecules.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 773–776, April, 1988.We thank P. P. Levin for carrying out the measurements by the method of laser photolysis and A. S. Kholmanskii for determining the extinction coefficients of a number of compounds.     

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.     

Zur Stabilitätsfrage Rotierender Elastischer Stäbe

Ohne ZusammenfassungDie nachstehende Arbeit bildet eine namentlich nach der variationstheoretischen Seite hin orientierte Weiterentwickelung einer von der philosophischen Fakultät der Friedrich-Wilhelm-Universität Berlin genehmigten Doktor-Dissertation (Referenten: Prof. Dr. R. v. Mises, Geheimrat Prof. Dr. M. Planck.)     

5d–4f luminescence of Ce3+, Gd3+ and Lu3+ in LiCaAlF6

The emission and excitation spectra as well as decay kinetics of luminescence due to 5d–4f transitions in Ce³⁺, Gd³⁺ and Lu³⁺ ions doped into LiCaAlF₆ crystals have been analyzed with high spectral and time resolution using synchrotron radiation for excitation. The rich fine structure originating from electronic origins of transitions and their phonon replica has been well resolved and identified. Experimental data are compared with the spectra simulated in the framework of the semiphenomenological models of the crystal field and the crystal lattice dynamics.