Molecular electronics: Current state and future trends

The paper reviews the current state of molecular electronics and considers the most interesting trends in its development. It is noted that microscopic dimensions in quantum processes are not the only advantage of molecular technology. It is also important that chemical synthesis of molecular systems ensures the identity of assembled functional elements, thereby providing reliability and efficiency of quantum processes in molecular electronic devices. We propose using the quantum effect of the Peierls instability, which is readily realized in one-dimensional molecular systems. Advantages of this approach are discussed.Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 6, pp. 75–85, November–December, 1993.Translated by O. Kharlamova     

Phase Transitions in Channel Clathrates of Thiourea with Hexachloroethane and Tetrachloroethane

An explanation is proposed for the nature of phase transitions in channel clathrates of thiourea with hexachloroethane and tetrachloroethane. Analysis of experimental temperature dependences of specific heat, entropy, and NQR spectra indicates the existence of first and secondorder phase transitions. In the thiourea + hexachloroethane compound, the secondorder phase transition (94.60 K, entropy gain Rln2) implies a transformation of two equiprobable states (lowtemperature phase) into a single new state (hightemperature phase). The firstorder phase transition corresponds to clathrate channel distortion. In the thiourea + tetrachloroethane compound, the secondorder phase transition (224K, entropy change Rln 6) occurs with transformation of six equiprobable states (lowtemperature phase) into a single new state (hightemperature phase). The firstorder phase transition corresponds to clathrate channel distortion. The next secondorder phase transition observed in this compound at 248 K leads to disappearance of the four possible orientations of guest molecules in the clathrate channel of hexagonal section and to a transition of guest molecules to chaotic orientation (entropy increment equals Rln 4).     

Vibrational energy transfer in cyclopropane (C3H6)

Infrared laser-induced fluorescence measurements of vibrational relaxation in cyclopropane are presented. Following laser excitation of the CH-stretch vibrations υ₆ and υ₈ time-dependent fluorescence signals from υ₁₀ and υ₇/υ₁₁ were recorded. Activation and deactivation rate constants for C₃H₆C₃H₆ collisions were found for υ₁₀ and υ₇/υ₁₁. A simplified model for the vibrational relaxation of cyclopropane is discussed.     

Emulsifier-free emulsion polymerization with cationic comonomer

An earlier article¹ described the emulsion polymerization of styrene and various anionic comonomers, together with an anionic initiator, to give uniform latices at ca. 35% solids content. This article extends the work to cationic systems. Cationic comonomers 1,2-dimethyl 5-vinylpyridinium methylsulfate and 1-ethyl 2-methyl 5-vinylpyridinium bromide were synthesized and used with azobis(isobutyramidine hydrocholoride) initiator in the emulsifier- free emulsion polymerization of styrene. Recipes and results were generally comparable to those of the anionic systems, excepts for the dependence of particle diameter on comonomers concentration. Here the initial decrease was followed by an increase in particle diameter at higher comonomer content. The surface charge increased sharply with comonomer content.     

Concerning the theory of chirality functions : Part I. Construction and discussion of chirality functions by means of [2.2] metacyclophanes

By means of [2.2]metacyclophane (skeleton symmetry C_2h) the construction of qualitatively-complete chirality functions via the first and second “Naherungsansatze” is demonstrated. In this context, a detailed comment concerning the algebraic theory of chirality functions is given. As the first term of the chirality function (“abridged Ansatz”), both methods furnish a simple superposition rule (“quadrant rule”). The second component as deduced via the method of polynomials is of third degree in ligand specific parameters. A particular property of the second approximation method is derived. The consequences with respect to observables of the chirality phenomenon, in the case of the approximations being valid, are discussed.     

Cationic crosslinking of solid dgeba resins with ytterbium(III) trifluoromethanesulfonate as initiator

Solid bisphenol-A epoxy resin of medium molecular mass was cured using a Lewis acid initiator (ytterbium(III) trifluoromethanesulfonate) in three different proportions (0.5, 1 and 2 phr). A kinetic study was performed in a differential scanning calorimeter. The complete kinetic triplet was determined (activation energy, pre-exponential factor, and integral function of the degree of conversion) for each system. A kinetic analysis was performed with an integral isoconversional procedure (free model), and the kinetic model was determined both with the Coats-Redfern method (the obtained isoconversional value being accepted as the effective activation energy) and through the compensation effect. All the systems followed the same isothermal curing model simulated from non-isothermal ones. The growth-of-nuclei Avrami kinetic model A_3/2 has been proposed as the polymerization kinetic model. The addition of initiator accelerated the reaction especially when 2 phr was added. 0.5 and 1 phr showed very few kinetic differences between them.     

Zum rotameren Verhalten des Phenyl- und Phenyltricarbonylchrom-liganden in 2-substituierten 1,3-Dioxanen: Ein Vergleich

Dipole moment measurements, NMR-spectroscopical methods and dipole-dipole interaction calculations applied to 2-benchrotrenyl-1,3-dioxanes and a comparison with o-substituted 2-phenyl derivatives furnish evidence, that in both cases the rotameric state is mainly determined by the steric demand of the substituent being attached to phenyl: the declination angle () of the Cr(CO)₃-group from the bisectional cisoid relation (with respect to H on C-2) is identical with that for the ortho substituent and amounts to about 60 degs. The influence of the Cr(CO)₃-fragment on the conformational behaviour of the benzene moiety in geminal 2,2-disubstituted derivatives is consistent with these results: the assumption of an axial and gonal conformation for the phenyl ring, which had recently been established for 2-alkyl-2-phenyl-1,3-dioxanes, is no longer valid for the 2-alkyl-2-phenyltricarbonylchromium compounds.

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Mit 4 Abbildungen

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40. Mitt. über Stereochemie von Metallocenen; 39. Mitt. siehe Lit.

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Herrn Prof. Dr.H. Nowotny mit besten Wünschen zum Geburtstag gewidmet.