A contribution to the theory of the structural glass transition

The equations characterizing the temperature dependence of the mobility of molecules in the glass transition region are compared and analysed. A relationship between the parameters of the relaxation theory of the glass transition and the concept of free volume is established. The Williams-Landel-Ferry equation is treated in terms of the relaxation theory of glass transition.     

A contribution to the theory of OD EPR of spin-correlated radical pairs

A composite structure of OD EPR spectra is predicted. The OD EPR spectra consist of a broad component which manifests many features similar to conventional CW EPR spectra and a narrow component which appears as a hole in the broad spectrum. The width of the narrow component is estimated. The contribution of singlet-triplet dephasing to the shape of the OD EPR spectra is discussed comprehensively. It is shown that changes of the OD EPR spectra with an increase in the rate of singlet-triplet dephasing can be interpreted in terms of a redistribution of the populations of mixed (singlet and triplet) states of radical pairs and of exchange of the coherences corresponding to the EPR transitions for radical pairs.     

The performance concept: A contribution to the modern theory of capillary GC

An original theoretical approach, the “performance concept” is introduced. This concept enables the linear carrier gas velocity and column length to be optimized in order to obtain the shortest possible analysis time for a required separation. The optimized gas velocity and column length are expressed as functions of the B and C coefficients of the HETP equation, the extra-column equipment constant, σ²_EC, and the necessary plate number, N_ne, determined by the analytical problem. A new equation is given for predicting analysis time as a function of the same parameters. The validity of the concept is illustrated with an example of such an optimization. The fundamental relationships have been proved experimentally and excellent agreement found between predicted and measured values.     

A contribution to Semenov's general theory of thermal ignition regarding thermal analysis

The general theory of thermal ignition under the conditions of thermal analysis of flammable substances is discussed. For a linear heating rate of the specimen the ignition temperature is obtained from the relationship $$(dT/dt)_b - \frac{q}{{(dT/dt)_b }} = \frac{E}{{RT_b^2 }}(T_b - T_c^\prime )$$ whereT′_c is the temperature of the reactor wall (heated at the rateq) at the starting moment of the development of the thermal explosion.     

MNDO refinement of the Townes-Dailey theory. Calculation of NQR frequencies of chloro-containing compounds

Corrections accounting for the effect of the differences in p-electron energies on the electric field gradient are suggested and justified in the framework of Townes-Dailey theory. MNDO calculations of 31 chloro-containing molecules with full geometry optimization were used to evaluate the³⁵Cl NQR frequencies. The correlational relationships between experimental and calculated NQR frequencies are compared.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 38–42, January, 1995.This work was supported by the Russian Foundation for Basic Research (Project No. 93-03-5201).     

Symmetry in the design of NMR multiple-pulse sequences

The symmetry principles of NMR pulse-sequence design are summarized. The discussion is guided by an analogy with tiling schemes in the decorative arts. The symmetry operations for NMR pulse sequences are discussed in terms of excitation field modifiers and temporal modifiers. The quantum operators which describe the effect of these modifiers on the excitation field spin Hamiltonian are provided. The symmetry transformations of spin propagators, and the different types of pulse-sequence elements are discussed. The common types of symmetry expansion are treated using the propagator transformations and the Euler angles for the excitation field propagators. The selection rules associated with symmetrical pulse sequences are discussed using average Hamiltonian theory.     

Single crystal zeeman effect studies on 35Cl NQR lines of 2,6-dichlorophenol

Zeeman effect studies on the two ³⁵Cl NQR lines in cylindrical single crystals of 2,6-dichlorophenol were carried out using a self-quenched super-regenerative NQR spectrometer to obtain information on the nature of the crystalline unit cell and the effect of hydrogen bonding on the electric field gradient tensor. Analysis of the experimental data reveals: (1) the results are in good agreement with those reported from X-ray studies; (2) the crystal is unequivocally identified as belonging to the orthorhombic system; (3) there are two crystallographically equivalent and four physically nonequivalent directions for the principal field gradients for both the low and high frequency resonance lines; (4) the directions of the crystalline a, b, c axes are uniquely identified as (90°, 0°), (0°, −), and (90°, 90°); (5) the b-axis is identified as the growth axis; (6) there are a minimum of four molecules per unit cell, the four molecules lie in different planes, which are, however, connected by symmetry operations; (7)_there exists a weak intramolecular hydrogen bonding in the crystal; (8) the asymmetry parameters for the loci corresponding to the low frequency resonance line, which is affected by hydrogen bonding, are less than the asymmetry parameters of the loci corresponding to the high frequency resonance line, which is not affected by hydrogen bonding; (9) the single bond and ionic bond characters for the hish frequency line are less than that of the low frequency line, while the double bond character for the low frequency line is less than that of the high frequency line and (10) the small deviation between the single bond and double bond characters of the two resonance lines is attributed to the existence of weak hydrogen bonding in the crystal.     

Dubinin"s theory and its contribution to adsorption science

Dubinin"s theory for the volume filling of micropores (TVFM), originally developed for the adsorption of single vapours by microporous solids such as activated carbons and zeolites, has gradually been extended to other areas. They include immersion calorimetry, the adsorption of water vapour and of mixtures, as well as adsorption from aqueous solutions. Recent studies in the field of adsorption from aqueous solutions, by activated carbons, suggest that the principle of temperature invariance is fulfilled and in the case of phenolic compounds a modified DRK equation can be used to predict the adsorption equilibrium over a certain range of temperatures. Computer modelling of CO₂ adsorption by carbons at 273 K leads to micropore distributions, which are in good agreement with those derived from other techniques. It also appears that the model isotherms in single slit-shaped micropores can be fitted to the Hill-de Boer isotherm, in agreement with mathematical studies of the origin of the Dubinin—Astakhov equation.     

Measurement and theory of hydrogen bonding contribution to isosteric DNA base pairs

We address the recent debate surrounding the ability of 2,4-difluorotoluene (F), a low-polarity mimic of thymine (T), to form a hydrogen-bonded complex with adenine in DNA. The hydrogen bonding ability of F has been characterized as small to zero in various experimental studies, and moderate to small in computational studies. However, recent X-ray crystallographic studies of difluorotoluene in DNA/RNA have indicated, based on interatomic distances, possible hydrogen bonding interactions between F and natural bases in nucleic acid duplexes and in a DNA polymerase active site. Since F is widely used to measure electrostatic contributions to pairing and replication, it is important to quantify the impact of this isostere on DNA stability. Here, we studied the pairing stability and selectivity of this compound and a closely related variant, dichlorotoluene deoxyriboside (L), in DNA, using both experimental and computational approaches. We measured the thermodynamics of duplex formation in three sequence contexts and with all possible pairing partners by thermal melting studies using the van't Hoff approach, and for selected cases by isothermal titration calorimetry (ITC). Experimental results showed that internal F-A pairing in DNA is destabilizing by 3.8 kcal/mol (van't Hoff, 37 °C) as compared with T-A pairing. At the end of a duplex, base-base interactions are considerably smaller; however, the net F-A interaction remains repulsive while T-A pairing is attractive. As for selectivity, F is found to be slightly selective for adenine over C, G, T by 0.5 kcal mol, as compared with thymine's selectivity of 2.4 kcal/mol. Interestingly, dichlorotoluene in DNA is slightly less destabilizing and slightly more selective than F, despite the lack of strongly electronegative fluorine atoms. Experimental data were complemented by computational results, evaluated at the M06-2X/6-31+G(d) and MP2/cc-pVTZ levels of theory. These computations suggest that the pairing energy of F to A is ~28% of that of T-A, and most of this interaction does not arise from the F···HN interaction, but rather from the CH···N interaction. The nucleobase analogue shows no inherent selectivity for adenine over other bases, and L-A pairing energies are slightly weaker than for F-A. Overall, the results are consistent with a small favorable noncovalent interaction of F with A offset by a large desolvation cost for the polar partner. We discuss the findings in light of recent structural studies and of DNA replication experiments involving these analogues.     

A contribution to the ternary phase diagram Co-Sb-Te

The melting behaviour, some phase boundaries and the lattice parameters in the region of the NiAs-type phase in the ternary system cobalt-antimony-tellurium have been determined by differential thermal analysis and X-ray diffraction. A projection of the liquidus surface and the phase relationships in different sections with constant antimony/tellurium ratios are presented. The peculiar variation of the lattice parameters with the composition of the nonstoichiometric NiAs-type phase is discussed in terms of the defect structure.Dedicated to Prof. K. L. Komarek on the occasion of his 65th birthday     

A contribution to the ecology and enology of platinum

 Since the introduction of the catalytic cleaning of vehicle exhaust gases, platinum is discussed as environmentally relevant element. Results of such investigations are compared with “natural” platinum background concentrations, which were determined in selected environmental and biological samples. Wine serves as an example for the following the path of platinum, beginning with the uptake during the plant growth, continuing with the fermentation process of grape juice and resulting in the final product. Received: 30 August 1996/Accepted: 13 October 1996     

A two frequency method for the interpretation of multiple NQR spectra

A new method for the interpretation of NQR spectra is described. The indentification of multiple lines belonging to different transitions \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{1}{2} \to \frac{3}{2}{\rm and}\frac{3}{2} \to \frac{5}{2} $\end{document}is carried out by using the ‘seizure’ of the corresponding low or high frequency transitions in a two-frequency pulse experiment.     

A contribution to the study of the substances of cotton hulls

Chemistry of Natural Compounds -     

A contribution to the modelling of an analytical column process

Summary Two attempts to set a mathematical model of an ion-exclusion column process (sulphonic acid ionexchange resin in the H⁺-form; HCl in the eluent) are described. The outcome of an experimental approach using curve fitting procedures has a similarity with the outcome of an approach departing from physical reasoning.On leave from Lisbon University, Portugal     

A contribution to the theory of D.C. polarographic current-voltage curves: Electrode reactions with three parallel charge transfer processes

Equations of polarographic current-voltage curves are derived for electrode reactions with three parallel charge transfer processes coupled by two chemical reactions on the basis of the staeady-state assumption in the reaction layer. If the rate constants of the charge transfer processes are sufficiently different from one another, the current-voltage curve consists of three separated waves and the equation for each wave has the same form as for a simple electrode reaction with one pathway.