Models of slow rotational mobility of paramagnetic probes in glassy media

Experimental ESR spectra of the 2,2,6,6-tetramethyl-4-oxopiperidinoxyl (TEMPON) radical probe in the glycerol, polystyrene, and polyvinylbutyral matrices measured in the temperature range 77–373 K were quantitatively compared with the ESR spectra calculated using the known theoretical models of rotational mobility. It was shown that simulation of ESR spectra by the nonlinear least-squares method is an efficient procedure for discriminating between theoretical models. The temperature ranges were determined in which it is possible to achieve quantitative agreement between experimental and theoretical spectra as well as the ranges in which theoretical models are insufficient to quantitatively describe the experimental results. It was established that the widely used model of Brownian diffusion in isotropic medium is inadequate to describe the ESR spectra in the case of slow motions of small probe molecules. It was found that specific interactions (formation of weak complexes) between the probe molecules and the molecules of the medium results in strongly anisotropic molecular rotational motions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1065–1073, June, 2000.     

Vacuum moulding of a superplastic in two dimensions

A mathematical model is proposed for the process of vacuum superplasticforming. The model exploits the fact that in most industrialapplications the sheet aspect ratio (thickness/sheet width)is small. After an initial consideration of some of the moregeneral properties and the literature of superplastic materials,the elastic/plastic deformation of an internally-inflated thin-walledcylinder is examined. Plates of arbitrary geometry are thenconsidered. A quasisteady model in which the sheet moves througha sequence of steady states is developed. Some simplified closed-formsolutions are examined, but for general cases a system of nonlinearpartial differential equations must be solved numerically. Anefficient and accurate semi-explicit numerical scheme is proposedand a simplified stability analysis is presented; the methodis then used to compute properties of superplastic vacuum mouldedsheets in a number of practically motivated cases.     

EXAFS structural studies of electrodeposited Co and Ni hexacyanoferrate films

XAS (EXAFS and XANES), XPS and IR spectroscopies were used to extract redox compositional and structural information on films of electrodeposited Co and Ni hexacyanoferrates whose redox state was manipulated electrochemically. The X-ray methods provided direct information on the metal species and IR provided indirect information via the behaviour of the ligand vibration. XPS responses showed that the electrochemical response of Co hexacyanoferrate is attributable to Co (except for a small amount of Fe^II oxidation at very positive potentials), and of Ni hexacyanoferrate to Fe; XANES edge shifts confirm these deductions. Local structure around the metal atoms was extracted from EXAFS data in terms of M′–N, M′–C and M′–Fe (M′ = Co or Ni) distances and the associated Debye-Waller factors as functions of film charge state. For Co hexacyanoferrate, the redox variation of static disorder was consistent with a molecular model involving discrete Co^II and Co^III sites, whose populations respond to potential, but not with a solid-state model.     

The true potential energy curves for different states of SiO and SiS molecules

The true potential energy curves forX ¹ Σ ⁺, a³ π _r,A ¹ π,e ³ Σ ⁻,E ¹ Σ ⁺,c ³ π _i states of astrophysically important molecule SiO, whose spectra were observed in the sunspots, have been constructed using the method of Lakshman and Rao. Dissociation energy and ionization potential have been estimated as 65,350 cm⁻¹ and 92,854 cm⁻¹ respectively and found to be in good agreement with the values cited in literature. The true potential energy curves forX ¹ Σ ⁺,D ¹ π andE ¹ Σ ⁺ states of SiS molecule have been constructed by the same method and its dissociation energy in the ground state has been estimated using Hulburt-Hirschfelder potential function as 54,765 cm⁻¹ in good agreement with 53,250.9 cm⁻¹ given by Herzberg.     

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.     

An EXAFS study of some gold and palladium cluster compounds

Gold L₃-edge EXAFS measurements at 80 K on Au₅₅(PPh₃)₁₂Cl₆ confirm that the Au-Au distances in this amorphous metal cluster compound are significantly shorter than in bulk gold. The nearest-neighbour Au-Au distances are all equal within experimental uncertainty. Outer-shell Au-Au distances have also been resolved. The results are consistent with the cuboctahedral structure originally proposed for this cluster, but not the polyicosahedral one recently suggested. Very similar results have been obtained from the sulphonated water-soluble derivative Au₅₅(PPh₂C₆H₄SO₃Na)₁₂Cl₆. In contrast, EXAFS of Au₁₁{PPh₂(p-ClC₆H₄)}₇I₃ has clearly resolved the two nearest-neighbour Au-Au distances associated with its icosahedral structure.Palladium K-edge EXAFS has been used to study the cluster Pd₅₆₁(phen)₃₆O₂₀₀. The Pd-Pd distance is nearly equal to that in bulk palladium. The results show a cubic close-packed cluster structure for this material, in contrast to the icosahedral structure reported for Pd₅₆₁(phen)₆₀(OAc)₁₈₀.     

Long-wave light absorption and photochemical decomposition of peroxide radicals

Photochemical decomposition of alkylperoxide radicals in glassy matrices at 77 K was experimentally studied. Irradiation with light up to 405 nm leads to the photodecomposition of peroxide radicals. The quantum yield of the reaction was estimated to be 10^–2. A light-induced angular dependence of the ESR spectra of peroxide radicals resulting from photoselection was detected. The photoselection found proves that the photodecomposition is induced by the absorption of light in the inherent absorption band of RO₂.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1074–1077, June, 1995.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 95-03-08505) and the International Science Foundation (Grants NBU 000 and NBU 300).