Theoretical studies of solvent effects on nuclear spin-spin coupling constants

A theoretical study is presented of the effects of solvent molecular motion on nuclear spin-spin coupling constants. Solvent molecules are treated as point dipoles arranged around the solute molecule in a cubic closest packed arrangement. Average dipole moment vectors are given by a rotational Boltzmann average and the resulting perturbation is included in the Fock matrices. Calculations of nuclear spin-spin coupling constants were performed by self-consistent perturbation theory in the INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. Calculated results are compared with the experimental data as well as the results of previous models.     

Routine screening and quantitation of urinary corticosteroids using bench-top gas chromatography-mass-selective detection.

A method was developed for the routine screening, confirmation and quantitation of corticosteroids in human urine using bench top capillary gas chromatography (GC)-mass-selective detection. The free and conjugated corticosteroid fractions were isolated by liquid-liquid partition. After evaporation to dryness under vacuum the corticosteroid residues were derivatized to form the methyloxime trimethylsilyl ether derivatives. Both GC retention data and characteristic spectral data based on authentic reference standards were used for the identification and quantitation of cortisol, cortisone, tetrahydrocortisol and tetrahydrocortisone in the ppb (ng/ml) concentration range. The method is simpler and more efficient than the other GC-mass spectrometric (MS) techniques. It is also more sensitive than the liquid chromatographic-MS method.     

Chemiluminescence of IF in the gas phase reaction of I2 with F2

Emission from both the state and the previously unreported $\text{A}{}^3\text{Π}{}_1$ state of IF has been observed in the gas phase reaction of I₂ with F₂ at low pressures. For the state the transition moment and vibrational populations were extracted from the spectra by a least-squares method whereby theoretical band shapes were fit to the experimental data. The effect of flow rates of reactants and Ar on the relative emission from the two electronic states, the effect of pressure on the state, and extinction of emission near 470 nm all favor the population of excited electronic states through a four-center reaction complex, rather than association of F and I atoms.It is argued that there is an avoided curve crossing between the lowest two ³Π_0⁺ states of IF, and that the ground state dissociation energy is 23 229 ± 100 cm^?1. The radiative lifetime of the state is estimated to be 10^?3 sec and to be much shorter than that of the $\text{A}{}^3\text{Π}{}_1$ state.     

Superconducting and normal state properties of Ag1−xSn1 + xSe2−y

The occurrence of superconductivity in a new ternary non-transition element compound with the cubic NaCl structure is reported. The composition of this compound is Ag_1?xSn_{1 + x}Se_2?y, with a maximum homogeneity range given by ?0.02 ?x?0.24 and 0 ? y ?0.1. The superconducting transition temperature (T_c) ranges from a minimum of 4.5 K for x = ?0.02, y = 0 to a maximum of 6.9 K for x = 0.24,y = 0. These T_c's are higher than previously observed in nontransition element NaCl structure compounds. Electrical resistivity data show a positive temperature coefficient, and room temperature magnetic susceptibility measurements reveal a rapidly increasing density of states at the Fermi level with increasing x. Two ionic models for calculating the carrier concentration of Ag_1?xSn_{1 + x}Se_2?y are discussed, and a third model based on the band picture is proposed. The composition dependence of the magnetic susceptibility was used to test the validity of each of the models.     

Superconductivity of neutron irradiated Mo3Os

The effect of high energy neutron irradiation (E>1 MeV) on the superconducting transition temperature, T_c, of the A-15 compound Mo₃Os is reported. T_c was found to decrease with increasing neutron dose, but at a rate considerably less than observed in other A-15 compounds composed of non-transition metals. The results lend support to the idea that the effect of ordering on T_c is smaller for A-15 compounds composed only of transition metals than those composed of transition and non-transition metals.     

High-spin states and two-quasiparticle structure in 172Hf

Five rotational bands in ¹⁷²Hf have been observed to high spin following the ¹⁶⁰Gd(¹⁶O, 4n)¹⁷²Hf reaction, using γ-γ coincidence techniques. The ground-state band is extended to spin 20⁺ without backbending, in contrast to the neighbouring even-even isotones. The most strongly populated side-band, consisting of separated odd-spin and even-spin sequences, appears to result from the partial decoupling of an i_{$\text{13}\text{2}$} neutron. Three other bands are identified with the following band-heads: an 8^? state at 2006 keV with 163 ns half-life; a 6⁺ state at 1685 keV with a half-life < 16 ns; and a level at 1857 keV with spin 4, 5, 6 or 7 and a half-life < 16 ns. The differences between the bands are discussed in terms of the influence of Coriolis forces and the two-quasiparticle level structure.     

Making Manufacturing Practices Tacit: A Case Study of Computer-Aided Production Management and Lean Production

The idea that operations in batch repetitive manufacturing can be managed using formal manipulations on representations of the manufacturing practice is at the heart of an approach called Computer-Aided Production Management. It is argued that the plausibility of this approach arises from certain widely held, but now controversial, views about the nature of purposeful human activity, which assume the need for a central representational model. Alternative theories of cognitive activity can be used to clarify the approach of a recently influential group of production management techniques, known as Lean Production, which attempt to use changes to the situation of production as their leverage point. It is claimed that implementation of these ideas depends on rendering desirable practices tacit, and this idea is illustrated using a case study.     

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

The magnetization of single-crystal HoNi₂B₂C has been measured as a function of applied field (H) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi₂B₂C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c-axis and with a much weaker temperature dependence for H applied parallel to the c-axis, indicating that the Ho⁺³ magnetic moments lie predominately in the tetragonal a−b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T_N=5.0 K. Remarkably, in 20 Oe applied field both superconductivity (T_c=8.0 K) and antiferromagnetism (T_N=5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H−T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H_c2 with a deep minimum at T_N=5 K. The high-field magnetization data for H applied perpendicular to the c-axis also reveal a cascade of three phase transitions for T < 5 K and H < 15 000 Oe, contributing to the rich H versus T phase diagram for HoNi₂B₂C at low temperatures.