Low-temperature elastic constants and piezoelectric coefficients of langasite (La3Ga5SiO14)

A complete set of elastic constants C _ij and piezoelectric coefficients e _ij of a La₃Ga₅SiO₁₄ (langasite) single crystal was determined from 5.5 to 275.5?K by resonant ultrasound spectroscopy. Unlike a conventional crystal, the elastic constants C _ij of the langasite crystal showed three types of temperature dependence: (i) monotonic elastic stiffening upon cooling (C ₄₄ and C ₁₄); (ii) monotonic elastic softening (C ₆₆); and (iii) a stiffening-to-softening transition below 150?K (C ₁₁, C ₁₂, C ₁₃ and C ₃₃). In addition, a strong correlation between C ₆₆ and the piezoelectric coefficient e ₁₁ was confirmed. Group theoretical lattice dynamics analysis revealed that the novel phenomena of elastic softening and strong correlation are explained on the basis of two types of optical mode internal displacements which have the totally symmetric A ₁ and doubly degenerated E symmetries in the point group D ₃.     

Polymerization of N,N-Dialkylacrylamides with Anionic Initiators Modified by Diethylzinc

Abstract

N,N-Dimethyl-, diethyl-, and dipropylacrylamides were polymerized with 1,1-bis(4′-trimethylsilylphenyl)-3-methylpentyllithium (I) in the presence and absence of diethylzinc in THF. Although the polymers produced with I in the absence of diethylzinc have rather broad molecular weight distributions, the addition of diethylzinc to the polymerization systems causes narrow molecular weight distributions of the polymers. The addition of diethylzinc also affect the stereospecificities of the polymers obtained. The poly(N,N-diethylacrylamide) produced with I/diethylzinc (molar ratio of 1/3-15) is highly syndiotactic, while the one obtained with I is isotactic. The configuration of the poly(N,N-dimethylacrylamide) is changed from isotactic to syndio and heterotactic rich by the addition of diethylzinc to the polymerization mixture. Little effect of diethylzinc is observed on the stereospecificity of the polymerization of N,N-dipropylacrylamide. The stoichiometric additive effect of Et₂Zn toward the initiator in the polymerization of DEAA suggests that the coordination of Et₂Zn aggregates with the propagating carbanionic species narrows the molecular weight distribution and controls the tacticity of the polymer.     

Functions of Acyclic Poly(ethylene Oxide) Homologs: Acceleration of Nucleophilic Reactions and Selective Ion Transport through Membranes

The acceleration effect of poly(ethylene oxide) on nucleophilic reactions was investigated. The enhancement of the reaction rate was interpreted by the cooperative solvation of alkali metal ions with ethereal oxygens of PEO resulting in active nucleophilic anions. In relation to the complex formation of alkali metal ions with PEO, the oligo(ethylene oxide) derivatives were prepared as the synthetic ionophores, which were able to transport alkali metal ions selectively through a liquid membrane against the alkali metal ion concentration.     

An Attempt at the Preparation of Macrocyclic Polymer

Syntheses of cyclic oligomer and polymer were attempted with magnesiacyclohexane as the initiator in HMPA. The oligomer of α-methylstyrene, initiated by magnesiacyclohexane, was treated with dimethyldichlorosilane in order to obtain a stable cyclic oligomer. The products were investigated by GPC, IR, NMR, and elemental analysis. It was found that magnesia-cyclohexane could not be activated enough by HMPA, and that the magnesium-carbon bond was not stable in HMPA, and that the chain transfer reaction might occur repeatedly during the polymerization. Consequently, the yield of cyclic oligomer was so low that cyclic compounds could not be discriminated from the linear oligomer.     

Infrared femtosecond laser pulse induced permanent reduction of Eu3+ to Eu2+ in a fluorozirconate glass

We report what is believed to be the first observation of permanent photoreduction of Eu(3+) to Eu(2+) in transparent and colorless Eu(3+) -doped fluorozirconate glass at room temperature, using an infrared femtosecond laser. Difference absorption and electron-spin-resonance spectra of the glass before and after laser irradiation showed that a portion of the Eu(3+) ions in the focused part of the laser inside the glass were reduced to Eu(2+) ions after laser irradiation. It is suggested that Eu(3+) ions act as electron-trapping centers, whereas active sites in the glass matrix act as hole-trapping centers, leading to the formation of Eu(2+) ions. The observed phenomenon is inferred to be useful in the fabrication of optical memory devices with high storage density and waveguide-type micro-optical devices.     

Characterization of BCN films synthesized by radiofrequency plasma enhanced chemical vapor deposition

Boron carbonitride (BCN) films have been synthesized on Si(1 0 0) substrate by radio frequency plasma enhanced chemical vapor deposition using tris-(dimethylamino)borane (TDMAB) as a precursor. The deposition was performed at the different RF powers of 400-800 W, at the working pressure of 2×10⁻¹ Torr. The formation of the sp²-bonded BCN phase was confirmed by Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy measurements showed that B atoms were bonded to C and N atoms to form the BCN atomic hybrid configurations with the chemical compositions of B₅₂C₁₂N₃₆ (sample 1; prepared at the RF power of 400 W), B₅₂C₁₀N₃₈ (sample 2; at 500 W) and B₄₆C₁₈N₃₆ (sample 3; at 800 W), respectively. Near-edge X-ray absorption fine structure (NEXAFS) measurements indicated that B atoms were bonded not only to N atoms but also to C atoms to form various configurations of sp²-BCN atomic hybrids. The polarization dependence of NEXAFS suggested that the predominant hybrid configuration of sp²-BCN films oriented in the direction perpendicular to the Si substrate.     

High-spin radical cations of a dendritic oligoarylamine

A new dendritic oligoarylamine, N,N,N',N',N",N"-hexakis[4-(di-4-anisylamino)phenyl]- 1,3,5-benzenetriamine (BTA) 2, which contains a 1,3,5-benzenetriamine molecular unit as an potential precursor of a high-spin molecule and three oligoarylamine moieties as spin-carrying units surrounding the core BTA, has been prepared by the sequential palladium-catalyzed amination reactions. The redox property has been investigated by cyclic voltammetry, and the highly charged states up to the hexacation are accessible to 2. The polycationic high-spin species have been generated by stepwise chemical oxidation, and the electronic structures have been examined in detail by the continuous wave (CW) and pulsed ESR spectroscopy in comparison with the previously studied 1. The pulsed ESR technique enabled us to determine the definite spin multiplicity of the generated polycationic species of 2. It was confirmed that the dominant oxidized species observed by the two- and three-electron oxidations were assigned to the spin triplet 2(2+) and the spin quartet 2(3+), respectively. Moreover, these high-spin polycationic species turned out to be far more stable as compared to 1, and the isolation of 2(3+) as the SbCl(6)(-) salt has been accomplished. The temperature dependence of the magnetic susceptibility for the 2(3+)(SbCl(6)(-))(3) salt revealed that the intramolecular ferromagnetic interaction exists in 2(3+), and moreover, the trication 2(3+) was found to be deformed in the solid state.     

Characterization of atmospheric tritiated water concentration in the vicinity of the fukushima daiichi nuclear power plant

Journal of Radioanalytical and Nuclear Chemistry - This paper discusses the spatiotemporal variation in atmospheric tritiated water (HTO) concentration near the Fukushima Daiichi Nuclear Power...     

Contracted polarization functions for B to Ar

Using optimal exponents for B through Ne given by Dunning and those for Al through Ar by Woon and Dunning, d-type contracted polarization functions (2d/1d), (3d/1d), and (3d/2d) are generated from natural orbitals of atomic single and double excitation configuration interaction (SDCI) calculations, where the numbers before and after the slash are those of the primitive and contracted Gaussian type functions. The resulting contracted functions are tested on N₂ and P₂ molecules by self-consistent field and SDCI calculations, which clarify characteristics of the present polarization functions. Received: 5 June 1997 / Accepted: 20 August 1997     

A Light-Harvesting/Charge-Separation Model with Energy Gradient Made of Assemblies of meta-Pyridyl Zinc Porphyrins

Self-assembly of porphyrins is a fascinating topic, not only for mimicking chlorophyll assemblies in photosynthetic organisms, but also for the potential of creating molecular-level devices. Herein, zinc porphyrin derivatives bearing a meta-pyridyl group at the meso position were prepared and their assemblies studied in chloroform. Among the porphyrins studied, one with a carbamoylpyridyl moiety gave a distinct ¹H NMR spectrum in CDCl₃, which allowed the supramolecular structure in solution to be probed in detail. Ring-current-induced chemical-shift changes in the ¹H NMR spectrum, together with vapor-pressure osmometry and diffusion-ordered NMR spectroscopy, among other evidence, suggested that the porphyrin molecules form a trimer with a triangular cone structure. Incorporation of a directly linked porphyrin–ferrocene dyad with the same assembling properties in the assemblies led to a rare example of a light-harvesting/charge-separation system in which an energy gradient is incorporated and reductive quenching occurs.