The effect of fluctuations of a classical exterior medium on tunneling nonradiative processes in molecules

We examine nonradiative transitions in molecules with allowance for the effect of a classical polar exterior medium on tunneling charge transport. The approach allows for the vibrational frequencies of a molecule in the electron transition. In the case of slow fluctuations, the theory predicts a low-temperature (non-Arrhenius) increase in the tunneling nonradiative transition rate, and the results agree qualitatively with the experimental data. When the fluctuations of the exterior medium are rapid, at certain values of the molecular parameters the tunneling decay rate is found to decrease with increasing temperature because the conditions needed for resonant tunneling are violated. Zh. éksp. Teor. Fiz. 114, 1944–1953 (December 1998)     

Underdoped manganites: Canted antiferromagnetic ordering or two-phase ferro-antiferromagnetic state?

We calculate the energy of charge-carrier-induced canted ordering in conducting layered antiferromagnetic systems with double exchange. The quantum approach to the d-spins is used. In the jellium model the energy of the canted state is lower than the energies of both collinear ferro-and antiferromagnetic states over a certain range of charge carrier densities, beginning with arbitrarily small densities. Nevertheless, the canted state cannot be realized, because it is unstable against charge-carrier density fluctuations. The two-phase ferro-antiferromagnetic state can play the role of an alternative to canting. The case of an intermediate electronic-impurity phase separation is investigated. Zh. éksp. Teor. Fiz. 114, 2225–2237 (December 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

A shape model for the twisting power of chiral solutes in nematics

The twisting power of chiral probes in nematics is interpreted in terms of a shape model, in which the surface elements of the solute molecules tend to align with the local director. The theoretical treatment is based on a previous approach, suitable for relating order parameters in nematics to molecular shape, and leads to the definition of a molecular pseudo-tensor whose orientational average determines the pitch and handedness of the helical macrostructures. Results of numerical calculations performed for distorted biphenyl and binaphthyl molecules are in agreement with the experimental results.     

Orientational ordering of liquid crystals containing a difluoro-substituted phenyl ring

The orientational ordering of several liquid crystals containing a difluorosubstituted phenyl ring has been studied through the use of C-13 NMR. The fluorinated phenyl ring of these liquid crystals have C_s symmetry, so three order parameters are required to completely describe the ordering of this ring. All three of these order parameters have been calculated from carbon-fluorine dipolar coupling constants obtained from the carbon-fluorine splittings in the C-13 NMR spectra. Because of the complexity of the fluorine-coupled spectra, variable angle spinning (VAS) was used to resolve the carbon-fluorine splittings. In order to study the orientational ordering over wide ranges of temperature, we have developed an empirical correlation between the order parameter and the value of a carbon-fluorine dipolar coupling constant. This enabled us to study the change in the order parameter with temperature. The results of applying this method to several structurally similar liquid crystals containing the same type of difluorinated phenyl ring are presented. A comparison is made to a similar mono-fluorinated liquid crystal.     

Influence of humic substances on sorption and methylation processes of inorganic- and organo-tin species

The influence of humic substances on sorption and methylation processes for inorganic- and organotin species is presented. Four sediment samples from different locations of the Rivers Elbe, Mulde and Spittelwasser, Germany, with different organotin and humic contents were selected to extract the humic and fulvic acids. The various fractions—the original sediment, the humic acid, the fulvic acid and the residual sediment—were analysed for their organotin content. The individual buyltin species show quite different distribution patterns. Monobutyltin is found mostly associated with humic acids. Dibutyltin shows a nonunique behaviour. At low total organotin content, dibutyltin is found bonded to humic and fulvic acids, whereas at high organotin content dibutyltin is distributed more with the residual sediment. Most of the tributyltin remains in the sediment unextracted; only small quantities of it are in the fulvic acid fraction. Tetrabutyltin is only in the humic acid fraction when it binds to humic matter; it mostly remains in the sediment. General observations indicate that ionic butyltin species bind to fulvic acids whereas the non-polar tetrabutyltin is not found in the fulvic acid fractions in any of the samples. The appearance of monomethyl- and dimethyl-tin species in the humic and fulvic acid fractions after the alkaline extraction was surprising. There is a correlation between the humic content of the sample and the formation of methyltin species. Evidence is provided by experiments that humic substances act as methylation agents.     

Advanced in the synthesis of heterocyclic compounds from aliphatic nitro derivatives. A review

The literature on the synthesis of heterocyclic compounds from aliphatic nitro derivatives in the past decade has been summarized.Moscow State Textile Academy, 117918, Moscow. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1299–1336, October, 1994. Original article submitted October 6, 1994.     

Lattices of processes for graphs

The structure of the lattice of processes of a finite digraph with entries is investigated. Necessary and sufficient conditions for the isomorphism of a given lattice to the lattice of processes of a certain digraph are obtained. Bibliography: 5 titles. Translated by K. V. Shakhbazyan. Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 202, 1992, pp. 135–153.