Condensed aromatics—XXIV. Cyclo[d.e.d.e.e.d.e.d.e.e]decakisbenzene

The macrocyclic condensed aromatic hydrocarbon C₄₀H₂₀ cyclo[d.e.d.e.e.d.e.d.e.e] decakisbenzene is treated. It belongs to the class of cycloarenes. The simple Hückel molecular orbital analysis has been performed and employed in the construction of a harmonic force field. The set of calculated vibrational frequencies is reported.     

One-Dimensional Covalent Organic Frameworks for the 2e− Oxygen Reduction Reaction

Two-dimensional covalent organic frameworks (2D COFs) are often employed for electrocatalytic systems because of their structural diversity. However, the efficiency of atom utilization is still in need of improvement, because the catalytic centers are located in the basal layers and it is difficult for the electrolytes to access them. Herein, we demonstrate the use of 1D COFs for the 2e⁻ oxygen reduction reaction (ORR). The use of different four-connectivity blocks resulted in the prepared 1D COFs displaying good crystallinity, high surface areas, and excellent chemical stability. The more exposed catalytic sites resulted in the 1D COFs showing large electrochemically active surface areas, 4.8-fold of that of a control 2D COF, and thus enabled catalysis of the ORR with a higher H₂O₂ selectivity of 85.8 % and activity, with a TOF value of 0.051 s⁻¹ at 0.2 V, than a 2D COF (72.9 % and 0.032 s⁻¹). This work paves the way for the development of COFs with low dimensions for electrocatalysis.     

Radiation stability tests of the SMELS materials in the LVR-15 experimental reactor at Řež

Radiation stability of a set of three synthetic multielement standards (SMELS) designed for validation of k ₀-standardization in neutron activation analysis was tested by monitoring mass and element content changes following irradiation at a high neutron fluence rate (up to 9^.10¹³ n^.cm^-2.s^-1) for stepwise increased periods of time. No mass changes were detected and for all the 26 elements investigated only a chlorine content loss was observed. Thus, radiation stability of the SMELS materials for the given purpose and their suitability for use also in high-power reactors was demonstrated.     

Introduction to the dynamic theory of the (H+, e−) couple insertion in γ-MnO2

This paper presents a phenomenological theory of coupled H⁺ and e⁻ diffusion in γ/ɛ-MnO₂. A set of five dynamic regimes governs the (H⁺, e⁻) insertion during the entire reduction (0.08 < r < 1) of the dioxide. It is imposed by a global balance of several physical quantities which depend on the reduction level r. The principal potential factor for the diffusion in a given direction is the difference between the components, along this direction, of the self velocities V _e− and V _H+. The longitudinal component E _ld of the internal electric field vector E _d plays, with respect to electrons and protons, the velocity regulator role. The transversal component E _td allows some homogenisation of (H⁺,e⁻) concentration during the first half of the reaction and trapping of electrons in the second half. Towards r≈ 0.50, V _e− and V _H+ are equal, the internal electric field reverses its orientation after taking a null value and the first Mn³⁺ ions appear. This introduction to the dynamic theory of γ/ɛ-MnO₂ is a new finding and can interpret most of its physical/chemical properties, especially the behaviour change around r≈ 0.50. Received: 11 November 1998 / Accepted: 25 March 1999     

Condensed aromatics—XXIII. Cyclo[d.e.d.e.d.e.d.e.d.e.d.e]dodecakisbenzene (kekulene)

Some topological properties of coronoids are given. These systems have chemical counterparts in cycloarenes. For one of these molecules, viz. C₄₈H₂₄ kekulene, the simple Hückel molecular orbital analysis has been performed and employed in the construction of a harmonic force field. Finally the set of calculated vibrational frequencies is reported.     

（e, 2e） Investigations of the Simultaneous Ionization and Excitation of Helium to He＋（n=2）

Three-Coulomb-wave method is employed to treat the process of （e, 2e） simultaneous ion- ization and excitation to the n=2 state of helium, with radial and angular correlated wave-function of He target. The triple differential cross sections are calculated and analyzed in very asymmetric coplanar geometry at incident energies of 5.50, 1.50 and 0.57 keV. Results are compared with the absolute measurements and the theoretical first and second Born approximation. The present triply differential cross section （TDCS） is found to be in good agreement with experimental data qualitatively. The distinguishing feature noted in TDCS structure is the presence of intense recoil peak that for certain parameters is even larger than the binary peak, an unusual feature for the single-ionization process at high and intermediate energies.     

About the Secondary Electron Emission Yield, δ, from e−-Irradiated Insulators

 To account for the published yield curves, δ = f ( E ₀), of some insulators such as diamond, an elementary theory of secondary electron emission (s.e.e) is used. This theory permits the estimation of the effective attenuation lengths of the emitted electrons during their transport and to explain the significant differences between the s.e.e. from metals and that from insulators. Some practical consequences related for instance to the lack of topographic contrast in LVSEM and to charging mechanisms in electron beam techniques are then deduced.     

简法制备光学活性化合物(100% e.e.)

近年来,有机合成化学家对制备高纯度光学活性化合物极感兴趣。1984年,美国Purdue大学Brown教授等曾报道获得高纯度光学活性化合物的简单方法。该方法是以手性试剂单异松蒎烯基硼烷(mono-isopinocampheylborane,IpcBH_2)1,与二取代、三取代烯烃2发生不对称诱导的硼氢化反应,此时可引入一个或二个手性中心。由于主、副产物结晶性质有明     

An ab initio molecular dynamics study on the dissociative recombination reaction of HD2O+ + e−

An ab initio molecular dynamics simulations have been carried out for the dissociative recombination reaction of the deuterium-substituted hydronium cation, HD₂O⁺ + e ⁻, at the state-averaged multiconfigurational self-consistent field level. In the present simulations, five electronic states of HD₂O were included explicitly, and nonadiabatic transitions among adiabatic electronic states were taken into account by the Tully’s fewest switches algorithm. It is shown that the dominant products, OD + D + H, were generated in 63% of trajectories, while the products, OH + 2D, were generated in only 11% of trajectories, indicating that the release of a light fragment H is favored over the release of a heavy fragment D. This result is in conformity with the observation that there is a larger amount of deuterium substituted species than the non-substituted species in the interstellar space. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Evaluating membrane cell e.m.f.ś from different schems of integration of the relevant nernstian equations

Computing the e.m.f. E (and the related membrane potential E_M) of the general concentration cell: electrode J ¦ IJ (m₁) in S ¦ membrane I ¦ IJ (m₂) in S ¦ electrode J (where the membrane perm-selective to counterion I separates two solutions of molalities m₁ and m₂ of the same electrolyte IJ, one mole of which forms n_I moles of I of valency z_I and n_J moles of coion J of valency z_J, with n=n_I+n_J) requires integration between limits m₁ and m₂ of the differential equation: dE=[nRT/Fn_J|z_J|] [τ_Iz_I, - mτ_sM_s]dln(mγ)_IJ, where the transference numbers τ_I of I and τ_s of the solvent S through the membrane are customarily taken as constant. The key point is the integration of the term mdln(mγ), which in the current practice is oversimplified to give [(m₁ + m₂)/2] in (m₂γ₂/m₁γ₁), and may cause large overestimation of the solvent-transfer contribution, with unreliable results for E and E_M. The procedure recommended here is instead that of splitting

mdln(mγ) into the “ideal part”

mdlnm=m₂-m₁ and the “non-ideal part”

mdlnγ, the latter being safely integrable putting lnγ as an explicit function of m in terms of the extended Debye-Hückel equation. Alternative and equally reliable procedures are described.     

Stereochemistry of electrophilic attack at 34e⁻ dimetallic complexes: the case of diiron dithiolato carbonyls + MeS⁺

Experimental and computational experiments show that the electrophile MeS(+) attacks a single Fe center in Fe(2)(propanedithiolate)(CO)(4)(PMe(3))(2) followed by isomerization of this terminal thiolato complex to the corresponding μ-SMe derivative.     

Verification of k 0-NAA results at the LVR-15 reactor in Řež with the use of Au+Mo+Rb(+Zn) monitor set

Long-time experience in neutron flux monitoring on irradiation in the LVR-15 research reactor in ?e? proved that Au+Mn+Rb and Au+Mo+Rb(+Zn) monitor sets for short and long irradiation, respectively, are more suitable in our conditions than the most frequently used Au+Zr set. The advantages of the former monitor set have been described previously, in the present work we discuss the advantages of the latter monitor set for long irradiations in varying active core configurations of the LVR-15 reactor. The successful application of the Au+Mo+Rb(+Zn) monitor set has been verified by comparative determination of the neutron flux parameters α (epithermal flux distribution parameter), f (thermal-to-epithermal neutron flux ratio), and F _c,Au (comparator factor) using this and the Au+Zr monitor set, and by analyses of certified reference materials, namely NIST SRMs 1547 Peach Leaves, 2711 Montana Soil, and 1633b Trace Elements in Coal Fly Ash.     

Chronopotentiometric Study of the Reaction Am (III) + 3e → Am(o) in Molten Licl-KCl Eutectic

Abstract

A chronopotentiometric study of the reaction Am(III) + 3e → Am(O) in molten LiCl-KCL is reported. The diffusion coefficient ^DAm(III) increases from 3.5 to 26.0 10^?6 cm² sec^?1 when the temperature was raised from 400 to 650°C. The mechanism of the diffusion is discussed on the base of the hole theory for molten salts.