试论化学教学中的"自我对话"教育

“自我对话”教育强调学习者的主体作用和能动作用,它符合新课程理念和建构主义学习理论。实施“自我对话”教育必须注意激发学生的“自我对话”意识,并通过学前“知我性对话”、学中“生成性对话”和学后“反思性对话”等形式加以落实。     

Transformed steroids 117. Synthesis of 5α-hydroxy-6-oxosteroids with a δ-lactone ring E

The paper is devoted to the synthesis of steroidal 5-hydroxy-6-oxolactones from ethyl esters of 5,6-dienic acids. By epoxidating the latter it has been shown that, in addition to the usual formation of 5,6-oxides, opening of the 16, 17-oxide ring initially formed takes place and this is accompanied by intramolecular cyclization to a 17,20-dihydroxy--lactone. The trans-opening of the 5,6-epoxide in the epoxy--lactone and subsequent oxidation with N-bromosuccinimide has led to a new representative of steroidal -lactones — the 23,16--lactone of the 3-acetate of 3, 5, 16, 17, 20-pentahydroxy-6-oxo-24-norcholan-23-oic acid.N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 208–213, March–April, 1980.     

Quantum-chemical study of allylic compounds with two bonded heavy atoms

It has been shown by the CNDO method that the bathochromic shift of the long-wave absorption band in the transition from allylstannane to compounds of the type C=C-C-Sn-X and C=C-Sn-X (where X is a heavy atom) is connected with the formation of a low-energy vacant *_S-X orbital, localized mainly in the region of the Sn-X chemical bond, and of an occupied _Sn-X orbital, the energy of which is somewhat higher than of the _C-Sn orbital. The dependence of the position of the long-wave absorbance region on conformation is related to the fact that, in planar and nonplanar conformers, the long-wave transitions are of a different type ( * and *, respectively); the bathochromic shift is determined to a large degree by the difference in the energies of the highest occupied MO ( - ) in the s-trans form. In the nonplanar conformers the heavy atom orbitals interact with the -orbital of the ethylene moiety through the bridge group; this leads to a significant delocalization of the HOMO and to a considerable change in its energy. On the other hand, their interaction with the *-orbital in compounds of the C=C-C-Sn-X type is very low and does not favor the delocalization of lower vacant MO. In vinyldistannane the *-orbital is noticeably delocalized, due to the interaction with the *_Sn-Sn orbital in planar and with the *_Sn-Sn orbital in nonplanar conformers.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 636–641, March, 1992.     

Investigations of 2,3′-biquinolyl. 9. Reaction of 1′,4′-dihydro-2,3′-biquinolyl with organolithium compounds

The 1,4-dihydro-2,3-biquinolyl reacts with organolithium compounds to form the mixture of 4-R-1,4-dihydro-2,3-biquinolyls and 2-R-1,2-dihydro-2,3-biquinolyls in the ratio analogous to the conversion in the series of 2,3-biquinolyl. The utilization of the complex, the organolithium compound-TMEDA, in this process leads, after treatment of the reaction mixture with water, exclusively to 2-R-1,2-dihydro-2,3-biquinolyls. Treatment with methyl iodide leads to 1-methyl-2-R-1,2-dihydro-2,3-biquinolyls.     

Synthesis of cyclic dendronized polymers via divergent "graft-from" and convergent click "graft-to" routes: preparation of modular toroidal macromolecules

Cyclic dendronized polymers represent a new class of polymers which exhibit a donut-like "toroidal" shape. Two previous unreported methods for preparing this architecture, the divergent "graft-from" and the convergent click "graft-to" approaches, are explored and the resulting products fully characterized. This route is particularly attractive because it enables production of exact linear and cyclic dendronized analogues, enabling direct comparison of their physical properties. In this preliminary work, the divergent "graft from" approach appears to lead to materials with broad PDI at high DP, whereas the "graft to" approach yields more well-defined dendronized cyclic polymers at larger DP. On the basis of reports to date, a combination of click cyclization followed by click "grafting to" provides the most versatile route for the synthesis of cyclic dendronized polymers.     

Surface tension as a function of temperature for some molten metals

Results are given for Sn, In, and Ge from the melting points up to 1700 C, for Pb up to 1400 C, for Tl up to 1100 C, for Bi up to 1300 C, and for Cd up to 600 C. In every case the surface tension is a linear function of temperature. Estimates are made of the critical temperatures of Cd and Ge. It is shown that deductions on the structure of the melts can be drawn from surface-tension data.     

FT-Raman Spectra of o-, m-, and p-Nitrophenol Included in Cyclodextrins

FT-Raman spectra of o-, m-, and p-nitrophenol included in -cyclodextrin (CD), -CD, hydroxypropyl (HP) -CD, andsulfated -CD were recorded. The phenyl (C=C) band of o- and p-nitrophenol in the CD inclusion complexes was shifted to higher wavenumber thanthat of pure o- and p-nitrophenol,whereas the phenyl (C=C) band of m-nitrophenol in the CD inclusion complexes was shifted to lower wavenumber than that of pure m-nitrophenol. The ring CH peak of o-nitrophenol in the CD complexes was shifted to higher wavenumber than that of pure o-nitrophenol, whereas the ring CH peak of m- and p-nitrophenol in the CD complexes was shifted to lower wavenumber than that of pure m- and p-nitrophenol.     

Dielectric properties of moist CaCO3 filled polyethylene composites

The capacitance and the dielectric loss tangents of CaCO₃ filled polyethylene composites were studied. Composite samples, prepared by polymerizing ethylene on the surfaces of fillers, pre-treated by polymerization catalysts, were compared to normal mechanical mixtures. Dielectric dispersion,, and loss, , proved to be sensitive to heating or vacuum treatment. Investigation of samples under conditions of different relative humidities showed that the dielectric dispersion is due to adsorbed water. Both and increased with decreasing frequency and the ratio of loss and dispersion was nearly constant. Dielectric data measured at different relative humidities could be represented by a single Cole-Cole plot. Samples soaked in water for different periods yielded qualitatively similar but quantitatively different Cole-Cole plots. Composite samples showed higher losses at similar humidities.Possible interpretations in terms of a molecular relaxation model, an interfacial relaxation model, including a charged double layer mechanism, percolation theory and the universal response theory were examined, but none was able to fully explain the observed phenomena.     

On the recognition of structural features: Some general problems illustrated by hamiltonian paths in polyhexes

Human and machine recognition skills are discussed, though not comprehensively reviewed, and some of the difficulties are illustrated by algorithms written to search for Hamiltonian paths in polyhexes. The most successful strategy for this is based upon the branching graph, a recently introduced graph-theoretical device which can aid the recognition of edges that arenot part of a Hamiltonian path. Another, more widely applicable approach that is interesting, although in this preliminary form only a little better than random methods, uses the metaphor of biological evolution, and tries to breed and grow paths subjected to natural selection.     

Invertebrate organisms as biological indicators of heavy metal pollution

Some species of invertebrate animals are known to be efficient accumulators of trace elements. Generally, metal accumulation by such organisms is based on efficient detoxification mechanisms, such as intracellular compartmentalization, or metal inactivation by binding to metallothioneins. Metal accumulators have often been used as accumulation indicators of environmental metal pollution. This means that, ideally, metal concentrations in the animal’s body reflect quantitatively or semiquantitatively environmental pollution levels. In reality, however, many factors, such as the animal’s weight and age, can disturb such quantitative relationships. These factors have, therefore, to be considered carefully before an invertebrate is utilized as accumulation indicator for metal pollution. Apart from accumulation, many invertebrates exposed to elevated metal concentrations respond to this stress by metal-induced synthesis of metallothioneins. Additionally, metallothionein in metal-loaded organisms can be present in different isoforms that are specifically synthesized in response to different metals. These facts make metallothionein a potential biomarker for metal stress in invertebrates. One possibility may be to assess parameters of metallothionein synthesis at the molecular or biochemical level. Moreover, metallothionein isoform patterns could provide information on different isoforms synthesized in response to different metals or chemicals. In any case, however, care must be taken to consider intrinsic physiological parameters, such as nutritional or developmental factors, which could also interfere with metallothionein synthesis.     

Comparison of microwave and lower-frequency discharges for plasma polymerization

A plasma sustained by surface waves (SW) has been used to study the deposition rate R of hydrocarbon and fluorocarbon plasma polymer films as a function of excitation frequency f=/2 in the range 12–400 MHz. The SW technique allows one to vary only f while keeping constant all other parameters known to influence R, for example, power density in the plasma P. A plot of R/P at a total pressure of 200 m Torr (27 Pa) displays two plateaus, that at f<30 MHz being about five times lower than that at f100 MHz. This is attributed to the fact that electron energy distribution functions differ fundamentally at radio- (f50 MHz) and microwave (f100 MHz) frequencies, for the gas pressure range considered.     

Dynamic shear modulus in the splitting region of poly(alkyl methacrylates)

A systematic study of the dynamic shear modulusG ^*=G+G in three poly(alkyl methacrylates) (PEMA, PnPMA, PnBMA) at frequencies between 0.001 and 500 rad/s is presented. As the splitting frequencies _s are low, aging effects can be observed in the splitting region. There is a systematic shift of the splitting frequency _s to lower values with increasing length of the alkyl side group. In PnBMA a separate shear appearance is observed about two frequency decades below the local mode . This is discussed in terms of the concept of minimal cooperativity. Aging effects are: Shift of the maximum loss frequency to lower values, peak sharpening of the relaxation, and intensity changes of and . These effects are discussed in terms of the sequential aging concept. Aging leads to a pronounced bending of the traces upwards from the equilibrium line in the Arrhenius diagram. These non-equilibrium phenomena are promoted by the small slope m=d(log )/dT of the trace in the splitting region.Dedicated to Prof. E. W. Fischer at the occasion of his 65th Birthday Lieber Herr Fischer, die Hallenser Polymerphysiker danken Ihnen aufrichtig für die warmherzige und effektive Förderung der Polymerwissenschaften im Raum Halle-Merseburg.     

Solvent effects on chemical processes. 4. Complex formation between naphthalene and theophylline in binary aqueous-organic solvents

The standard free energy change for complex formation is written as a sum of effects arising from solvent-solvent interactions (the general medium effect), solvent-solute interactions (the solvation effect), and solute-solute interactions (the intersolute effect). The general medium effect is given by gA(–_o), where g is a curvature correction factor to the solvent surface tension , A is the change in surface area as the two solvent cavities containing the substrate (naphthalene) and ligand (theophylline) collapse into a single cavity containing the complex, and _o is the value of surface tcnsion at which there is no net solvophobic interaction; is defined to be the value appropriate to the equilibrium mean solvation shell composition. The solvation effect is modeled by equilibrium stoichiometric formation of solvated species. All data are related to the fully aqueous system to give _MG^o, the solvent effect on the free energy change, as an explicit function of solvent composition. Solvent effects on bimolecular association are related to solvent effects on the solubilities of the substrate, ligand, and complex. Approximation methods for interpreting such systems are described and are applied to the naphthalene-theophylline complex. It is shown that complex destabilization in mixed aqueous-organic solvents (relative to the fully aqueous system) may receive contributions from both the general medium and the solvation effects, and that these contributions can be quantitatively estimated.     

Chitosan–Carrageenan Gels

Phase behavior of the systems during the formation of polyelectrolyte complexes obtained by mixing of aqueous solutions of chitosan and -, -, or -carrageenan was studied. The gelation was shown to occur throughout the whole bulk solution at chitosan and -carrageenan concentrations higher than 0.1 and 0.3 wt %, respectively. At lower polysaccharide concentrations, the polyelectrolyte complexes precipitated. The study was performed at the polysaccharide concentrations not higher than 1 wt %. The gel systems were investigated by the dynamic rheology method. The mechanical characteristics of -carrageenan-containing gels were found to be mainly governed by the chitosan content; the viscosity of these gels was independent of temperature. Gels obtained with - and -carrageenans were sensitive to temperature because of the helix–coil conformational transitions in their molecules. The mechanical strength and stiffness of gels increase in the ––-carrageenan series. This effect was explained by the formation of additional crosslinks by double helixes of - and -carrageenan molecules.     

Synthesis of tetracarbocyanines by the scission of the pyrylium ring of benzopyrylocyanines

It has been shown that the reaction of monomethinecyanines containing 4-benzopyrylium residues with quaternary salts of derivatives of 2-[(1, 5, 5-trimethylcyclohex-1-en-3-ylidene)-methyl]benzothiazole leads to the scission of the pyrylium ring and the addition of the quaternary salt to the position of scission. The reaction gives tetracarbocyanines containing an o-hydroxyphenyl or an o-acetoxyphenyl substituent in the polymethine chain in addition to a hydrocarbon ring.     

Elimination of bremsstrahlung in activation analysis by the deflection of β-particles in a magnetic field

The structureless background at X-ray and low energy -ray region, resulting from bremsstrahlung due to the stopping of -radiations, causes serious problems in nuclear spectroscopy. In this study, in order to reduce the background and therefore to increase the sensitivity of the nuclear analytical technique and the number of elements observable, a methodology is developed to deflect the -particles by a magnetic field. The experimental setup consists of a permanent magnet /1 kG/, Ge/Li/ solid-state detector, and multichannel analyzer.     

Changes in the band structure of a simple polymer on lowering the translational symmetry

By combining the one-dimensional crystal orbitals of a simple polymer under the action of a perturbation which extends the repeating segment from one site to n sites a perturbation method allows the band structure of the new chain to be calculated. As well as estimating the band gaps to a fairly good agreement with the results of the direct method the present technique shows how specific gaps created by the symmetry-lowering will respond to the characteristics of the perturbations imposed.     

Determination of Total Selenium Content in Cereals and Bakery Products by Flow Injection Hydride Generation Graphite Furnace Atomic Absorption Spectrometry Applying in-situ Trapping on Iridium-Treated Graphite Platforms

A flow injection hydride generation graphite furnace atomic absorption spectrometric (FI-HG-GFAAS) method was applied to the determination of Se in Se-doped and undoped cereals and bakery products. For the purpose of doping, the soils used for the cultivation of the cereals were dosed with Se-doped foliar fertilizers. The samples were dissolved in a mixture of HNO₃ and H₂O₂ solutions using microwave-assisted digestion. The decomposition of H₂Se generated from the sample solutions and the trapping of elemental Se were performed at a temperature of 300°C on an Ir-pretreated integrated graphite platform of a transversally heated graphite atomizer (THGA). For release of the trapped Se within a fairly short atomization time (5s), an atomization temperature of 2200°C was observed to be optimal. The overall efficiency of hydride generation, transport and trapping was 86%.The upper limit of the linear dynamic range of calibration was 10µgL^–1, which corresponds to 0.5µgg^–1 for solid samples. Recovery of the samples spiked with Se^VI solutions was found to be 93±6% on average. The relative standard deviation of the determinations was less than 8%. The limit of detection was found to be 0.06µgL^–1, corresponding to 3ngg^–1 for solid samples. The accuracy of the method was verified with the use of IAEA-155 (whey powder) certified reference material. End-capped THGA tubes resulted in an extension of the linear calibration range compared to that of standard THGAs.The Se content in bakery products made of undoped cereals ranged from 7.7 to 68ngg^–1 (wet weight) in 18 samples, whereas the Se content of the corresponding cereals was found to be below 100ngg^–1 (wet weight). The Se level of cereals grown on soils treated with Se-doped fertilizers ranged from 128 to 1046ngg^–1 (wet weight), and it depended linearly on the Se concentration of the corresponding foliar fertilizer.     

Cucurbituril and α- and β-Cyclodextrins as Ligands for the Complexation of Nonionic Surfactants and Polyethyleneglycols in Aqueous Solutions

The complexation of nonionicsurfactants and polyethylene glycols by the ligandscucurbituril, - and -cyclodextrin wasstudied in aqueous solution. All the examined guestmolecules form complexes with these ligands.Calorimetric titrations were performed to determinedirectly the stability constants and reactionenthalpies. The presence of an aromatic part innonionic surfactants leads to more stable complexeswith -cyclodextrin than with the other ligands.If the surfactants contain no benzene group, theinteractions with -cyclodextrin are strongercompared to other ligands. The chain length of thepolyethylene glycols has only an influence upon thevalues of the reaction enthalpy in the case of-cyclodextrin.