Kinetics and mechanism of the autoxidation of pentaerythrityl tetraheptanoate at 180–220°C

A kinetic and mechanistic study of the autoxidation of liquid pentaerythrityl tetraheptanoate (PETH) at 180–220°C has been carried out utilizing a stirred-flow reactor. The results are consistent with the occurrence of a chain reaction scheme similar to that proposed for n-hexadecane autoxidation, namely, the formation of monohydroperoxides by the intermolecular abstraction reaction (3), the formation of α,γ- and α,δ-dihydroperoxides and α,γ- and α,δ-hydroperoxyketones by intramolecular peroxy radical abstraction reactions (4) and (4*), the bimolecular termination of peroxy radicals, reaction (6), and the rapid conversion of α,γ-hydroperoxyketones to the corresponding cleavage acids and methyl ketones, reaction (7). Comparisons of various rate parameters for the n-hexadecane and PETH systems reveal that the values of k₇ and (k₃/H atom)/(2 k₆)^1/2 are within experimental uncertainties identical for the two systems at 180°C. The proposed reaction scheme includes the concurrent formation of hydroxy radicals and hydroperoxyketone species. The results of kinetic analysis and the experimentally observed isomer distributions of primary and secondary monohydroperoxide products at high and low oxygen pressures suggest that ≈60% of the hydrogen abstractions from PETH at high oxygen pressures occur by hydroxy radicals.     

Synthesis of pyrochlore tungsten trioxide thin film and electrochemical lithium intercalation

The pyrochlore tungstate thin film has been prepared by an autoclave hydrothermal method at a temperature of 200 °C. The film was characterized by X-ray diffractometry, X-ray photoelectron spectroscopy and scanning electron micrograph measurements, showing that pure pyrochlore phase of sodium tungstate containing a small amount of water was formed by heating the film at a temperature of 350 °C. The cyclic voltammetric and galvanostic measurements revealed that a reversible electrochemical lithium intercalation into the crystal lattice of pyrochlore tungstate film takes place with charge-discharge cycling. Furthermore, the thermodynamics and kinetics of electrochemical lithium intercalation into the pyrochlore sodium tungstate film were also studied.     

Low-temperature elastic moduli and internal dilational and shear friction of polymethyl methacrylate

Longitudinal and transverse wave velocity, five kinds of elastic parameters (young, shear and bulk moduli, Lamér; parameter. Poisson's ratio) and dilational and shear internal friction of polymethyl methacrylate were simultaneously measured over a temperature range 68–400K, using the ultrasonic pulse method. All elastic moduli decrease with increasing temperature, suggesting lowering of molecular bonding strength. From dilational and shear internal frictions, the 139, 382, and 385K peaks are interpreted as rotation of x-methyl estel groups, micro Brownian motion of main chains and stress relaxation, and abrupt increases from 242 and 395K are regarded as free motion of side chains and glass transition, respectively. ©1995 John Wiley & Sons, Inc.     

Effect of the 16O → 18O isotopic substitution and a magnetic field on the spin dynamics in the (La0.25Pr0.75)0.7Ca0.3MnO3 manganite according to the 139La NMR data

¹³⁹La nuclear magnetic resonance spectra have been recorded and the spin-spin relaxation times in the paramagnetic region in external magnetic fields of 5 and 9.4 T have been measured for two types of manganites: initial (La_0.25Pr_0.75)_0.7Ca_0.3Mn¹⁶O₃ and enriched with the ¹⁸O isotope ((La_0.25Pr_0.75)_0.7Ca_0.3Mn¹⁸O₃). The manganite enriched with the heavier oxygen isotope exhibits disappearance of a signal in the charge-ordering region (T < T _co) in an external field of 5 T. This effect is related to the anomalous increase in the spin-spin relaxation rate. With an increase in the external magnetic field to 9.4 T, the difference in the behavior of the relaxation rate for the two samples in the charge-ordering region remains pronounced, although the magnitude of this effect becomes much smaller. The observed giant isotope effect is a bright evidence of the important role of oxygen motion in the formation of long-range magnetic order in the manganites under consideration. Original Russian Text ? K.N. Mikhalev, I.E. Litvinov, Z.N. Nigamat’yanova, A.Yu. Yakubovskii, A.R. Kaul’, O.Yu. Gorbenko, K. Kumagai, Y. Furukawa, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 5, pp. 726–729.     

Observation of a topological and parity-dependent phase of m = 0 spin states

A Ramsey interrogation scheme was used to measure the phase shift of laser-cooled 87Rb clock-transition pseudospins arising as a result of a reversal of a bias-magnetic field, i.e., B--> -B, during the interrogation. While no phase shift occurred when the reversal was sudden, the Ramsey fringes were shifted by a factor of pi when the reversal was adiabatic. We thus verified the prediction that the spin states |F,m=0 acquire a purely topological and parity-dependent phase factor of (-1)F as a result of B--> -B.     

Pressure-induced intermetallic valence transition in BiNiO3

The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi(3+)(0.5)Bi(5+)(0.5)Ni(2+)O3 with ordering of Bi(3+) and Bi(5+)charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi(3+)Ni(3+)O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.     

Adsorbed states and scanning tunneling microscopy induced migration of acetylene on Cu(110)

The authors have studied adsorption of acetylene on Cu(110) by means of low-temperature scanning tunneling microscopy. Adsorbed molecules preferentially aggregate at 40 K to yield dimer, trimer, and larger islands on the surface. Isolated species (monomer) adsorbs on the fourfold hollow site with approximately sp3 rehybridization as characterized by inelastic electron tunneling spectroscopy. Tunneling electron induces an acetylene molecule to migrate along the trough of Cu(110). The migration proceeds in two steps: the molecule first hops to the adjacent long-bridge site and then to the next fourfold site. The voltage and current dependencies of the hopping probability show that the migration is induced by inelastic electron tunneling that causes vibrational excitation of mainly C-H stretch mode.     

Metabolic analysis of the cinnamate/monolignol pathway in Carthamus tinctorius seeds by a stable-isotope-dilution method

The present study established a system for comprehensive metabolic analysis of the cinnamate/monolignol and lignan pathways by the use of a stable-isotope-dilution method. The system was successfully applied to characterization of the pathways in Carthamus tinctorius cv. Round-leaved White maturing seeds in combination with administration of stable-isotope-labelled precursors. Experimental results obtained using this technique strongly suggested the intermediacy of ferulic acid in lignan biosynthesis in the plant.     

Pressure-induced structural, magnetic, and transport transitions in the two-legged ladder Sr3Fe2O5

The layered compound SrFeO(2) with an FeO(4) square-planar motif exhibits an unprecedented pressure-induced spin state transition (S = 2 to 1), together with an insulator-to-metal (I-M) and an antiferromagnetic-to-ferromagnetic (AFM-FM) transition. In this work, we have studied the pressure effect on the structural, magnetic, and transport properties of the structurally related two-legged spin ladder Sr(3)Fe(2)O(5). When pressure was applied, this material first exhibited a structural transition from Immm to Ammm at P(s) = 30 ± 2 GPa. This transition involves a phase shift of the ladder blocks from (1/2,1/2,1/2) to (0,1/2,1/2), by which a rock-salt type SrO block with a 7-fold coordination around Sr changes into a CsCl-type block with 8-fold coordination, allowing a significant reduction of volume. However, the S = 2 antiferromagnetic state stays the same. Next, a spin state transition from S = 2 to S = 1, along with an AFM-FM transition, was observed at P(c) = 34 ± 2 GPa, similar to that of SrFeO(2). A sign of an I-M transition was also observed at pressure around P(c). These results suggest a generality of the spin state transition in square planar coordinated S = 2 irons of n-legged ladder series Sr(n+1)Fe(n)O(2n+1) (n = 1, 2, 3, ...). It appears that the structural transition independently occurs without respect to other transitions. The necessary conditions for a structural transition of this type and possible candidate materials are discussed.