Study of ferrofluids by Mössbauer spectroscopy

The parameters of the log‐normal size distribution of a MnFe₂O₄ ferrofluid powder sample have been determined by X‐ray diffraction. The mean blocking temperature was determined from the maximum of _Xi . Mössbauer spectra at 4.2–300 K are interpreted by a new simple theory of superparamagnetism and taking a reduction of the internal magnetic field for small particles, a size dependence of the anisotropy constant, the size distribution and collective excitations into account.     

Conditions for equality of decomposable symmetric tensors

Let x₁,…,x_m be linearly independent vectors. We give a necessary and sufficient condition for $\text{x}{}_1\text{1…1x}{}_{\mathrm{m}}\text{=y}{}_1\text{1…1y}{}_{\mathrm{m}}$ to hold. Some consequences of this result are also presented.     

Semi microscopic description of84Kr

The experimental information on the⁸⁴Kr nuclei is compared with the model calculation in which two neutron holes are coupled to the vibrational field. Based on the lower-order terms of a perturbative expansion of theE2 andM1 transition matrix elements, a simple rule is obtained for the sign and the magnitude of theδ (E2/M1) ratios for the transitions between the second and first 2⁺ states in some vibrational nuclei. [Nuclear structure⁸⁴Kr, calculated levelsJ, π and δ(E2/M1), Cluster-phonon model. Pairing interaction].     

Steric and electronic contributions to conformational effects on chemical shifts of acyclic alcohols

Our calculations on bi- and polycyclic alcohols reveal that the Mulliken charge distribution and chemical shift patterns due to hyperconjugation of lone pairs on oxygen with neighboring groups break down or are attenuated for certain spatial relationships of the hydroxyl group. Since in strained ring systems other effects on these parameters may be present, we applied a similar analysis to acyclic alcohols. Calculations at the B3LYP/6-31G^* level on conformers of methanol, ethanol, 1- and 2-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 2-butanol, 2-methyl-2-butanol, 1- 2- and 3-pentanol and 2-methyl-3-pentanol, where hyperconjugation may be present, reveal steric effects as modifiers of hyperconjugative patterns affecting carbon-13 chemical shifts in such alcohols. Contrary to what is observed in bi- and policyclic systems, where electrostatic effects interfere with effects due to hyperconjugation, these steric effects may be the main cause for the attenuation of deshielding of nuclei that are subject to hyperconjugation. Electrostatic effects are also present but they do not interfere with hyperconjugation by lone pairs. Conformational effects fall off sharply after the third carbon in the chain.     

Batch-injection stripping voltammetry (tube-less flow-injection analysis) of trace metals with on-line sample pretreatment

The most essential limitation of batch-injection analysis (BIA) methodology compared to other flow methods (CFA, FIA, SIA) is the lack of possibility of on-line sample processing in the measuring system. Some procedures of on-line sample pretreatment in BIA are possible by changing the plastic tip of the automatic micropipette used for sample injection into a flow-through reactor, e.g. by packing it with a bed of a solid sorbent. This concept is employed in the voltammetric stripping determinations of trace metals using a bed of commercial chelating resin Chelex-100. It was found that, besides the electrochemical preconcentration of analytes in the form of amalgams on the surface of mercury thin film electrodes, an approximately 10-fold additional preconcentration can be achieved on the packed sorbent bed by using different volumes of aspirated sample solution and eluent. This procedure allows also efficient elimination of some matrix effects.     

Two-dimensional arrays in the analysis of trends in series of molecules: strongly subspectral molecular graphs, formula periodic tables, and number of resonance structures

Two-dimensional arrays consisting of strongly subspectral molecular graphs and formula periodic tables for polycyclic aromatic hydrocarbons are briefly reviewed. New two-dimensional arrays for free-radical benzenoid hydrocarbons are presented with general analytical expressions for counting their number of resonance structures (SC). The structural origin of the coefficients to these analytical expressions is discussed.     

The recognition of polyrex hydrocarbon radicals

Rapid topological tests for discerning radical benzenoids from nonradical ones are reviewed. These tests include HückelB matrix, Gordon-Davison, Hall-Tutte, path, and excised internal structure inspection methods. The potential magnetic properties of type 2 diradicals are pointed out.Received by the publisher 5 July 1988     

Polymerization of ethylene using a high-activity Ziegler–Natta catalyst. II. Molecular weight regulation

This article describes studies on the variables that regulate the molecular weight in ethylene polymerization using a highly active Ziegler–Natta catalyst with hydrogen for molecular weight control. The dependence of the degree of polymerization on the concentration of catalyst, cocatalyst, monomer, partial pressure of hydrogen, and temperature has been established. The rate constant for chain transfer with cocatalyst has been evaluated. © 1993 John Wiley & Sons, Inc.