Epithermal neutron activation analysis of short-lived nuclides in geological material

The cadmium ratios of 52 short-lived nuclides have been measured. Epithermal neutron irradiation reduces the activities of²⁰F,²⁷Mg,²⁸Al,³⁸Cl,⁴⁹Ca,^46mSc,⁵¹Ti,⁵⁶Mn and⁶⁶Cu by factors of 20–30. The calculated improvements in detection limits for Ga, Br, Rb, Y, Mo, Rh, Pd, Ag, In, Sn, Sb, I, Ba, Nd, Sm, Gd, Dy, Er, Yb, Hf, W, Re, Pt, Au, Th and U are in the range 1–6. Hafnium was measured in USGS rocks: AGV-1 (4.9 μg g⁻¹), G-2 (7.5 μg g⁻¹) and GSP-1 (14.7 μg g⁻¹) and IAEA standards: SOIL-5 (6.3 μg g⁻¹ and SL-1 (4.6 μg g⁻¹). CCRMP reference concentrates PTC and PTM were analysed for rhodium (1.1 and 0.75 μg g⁻¹, respectively) and silver (69 and 5.8 μg g⁻¹, respectively).     

Ab initio studies of the ground-state potential energy surface of formamide

Ab initio calculations employing a standard double-zeta basis set augmented with various polarization functions have been used to investigate the lowest energy region of the ground-state potential energy surface of the formamide molecule. Hartree-Fock calculations with only d polarization functions on the nonhydrogen atoms located two stable minima, that with geometry distorted from planarity having slightly lower energy; only one stable minimum with planar structure is found when p polarization functions on the hydrogens are included. In contrast optimizations, which account approximately for the correlation energy using second-order Møller-Plesset perturbation theory consistently favor a single slightly nonplanar minimum energy geometry.     

Self-consistent electronic band structures of extended polydiacetylene chains with realistic model side groups

Self-consistent semi-empirical band structure calculations for isolated extended polydiacetylene chains with a variety of realistic model side groups have been performed. For side groups with a CH₂ group next to the chain backbone the predicted band gap Δ ≈ 0.5 eV is substantially independent of the detailed structure of the side group and much smaller than the optical absorption thresholds experimentally observed. These results support the hypothesis that the low energy electronic excitations of these systems are better described by an excitonic, rather than an electronic, band model.     

Fine-powder Al2O3 and SiO2 for preparation of multielement standards for rare-earth element analysis

The present paper based on experimental results contains discussions and suggestions on the possible use of fine-powder Al₂O₃ and SiO₂ with their original content of microimpurities of up to 40 elements, as multielement standards for neutron activation analysis. For example, activation analysis of As, Au, Ba, Cr, Cs, Fe, Ga, K, Ni, Sb, Sc, Se, Sr, Ta, Th, Ti, U, W, Zn, Zr and the REE La, Ce, Nd, Sm, Eu, Tb, Tm, Yb contained in SiO₂ powder off MERCK reagents showed their concentrations to be 0.1 to 5% of those in IAEA standard SL-1. In Al₂O₃ this level is even lower, approximately 10 times and more for the majority of the above-mentioned elements. As Al₂O₃ and SiO₂ are good sorbents for the majority of elements, additional introduction of some elements may allow more methods of analysis. The homogeneity of Al₂O₃ and SiO₂ samples both in the original state and after introduction of some elements was determined by neutron activation analysis, and the SD did not exceed 1% for an Al₂O₃ sample weight of 0.1 g, and 2% for SiO₂.     

Electronic band structure of model polydiacetylene chains

Calculations are reported of the electronic band structure of model polydiacetylene extended chains, with the large side groups represented by H or CCH, using the extended Hückel linear combination of atomic orbitals approximation. Predicted band gap widths are in good agreement with the results of an earlierπ-band calculation, and considerably smaller than experimental values.     

Radiochemical fire-assay for determination of the platinum group elements

The radioactive study of Ni–NiS bead fire-assay followed by neutron activation analysis showed nearly quantitative collection of the platinum group elements and gold in the bead. However, there was clearly some loss of gold and ruthenium at the dissolution and filtration stage. A preliminary study of the radiochemical fire-assay method indicates that the technique could overcome the problem of losses. Chemical carrier behaves in the same way as platinum metal concentrate during the analytical procedure and could be used to evaluate yields during each determination. When applied to rock samples, the radiochemical fire-assay gives good sensitivity and could be applied to the determination of nanogram concentrations of the platinum group elements without the problem of high blank values from reagents.     

Carbonates, thiocarbonates, and the corresponding monoalkyl derivatives. 1. Their preparation and isotropic (13)C NMR chemical shifts

Three series of potassium carbonate and thiocarbonate salts were synthesized, and the corresponding (13)C isotropic solid-state NMR and the aqueous solution (13)C and (1)H NMR data were collected. The series of compounds that were studied consists of (1) the parent compounds, i.e., potassium carbonate, K(2)CO(3), potassium hydrogen carbonate, KHCO(3), potassium monothiocarbonate, K(2)CO(2)S, potassium dithiocarbonate, K(2)COS(2), and potassium trithiocarbonate, K(2)CS(3), (2) the oxygen monoalkyl substituted derivatives of the parent compounds (OR series), i.e., three potassium O-alkylcarbonates, KO(2)COR, three potassium O-alkylmonothiocarbonates, KOSCOR, and three potassium O-alkyldithiocarbonates, KS(2)COR, all with R = CH(3), CH(2)CH(3), CH(CH(3))(2), and (3) the sulfur monoalkyl substituted derivatives of the parent compounds (SR series), i.e., two potassium S-alkylmonothiocarbonates, KO(2)CSR; two potassium S-alkyldithiocarbonates, KOSCSR, and two potassium S-alkyltrithiocarbonates, KS(2)CSR, all with R = CH(3) or CH(2)CH(3). The preparation and proper characterization of KO(2)CSR and KOSCSR with R = CH(3) and CH(2)CH(3) along with new IR and X-ray powder diffraction data for several other compounds in the series are reported for the first time in this study. Solution NMR data for KO(2)CSR (R = CH(3), CH(2)CH(3)) and KOSCSR (R = CH(3)) and solid-state NMR data for K(2)CO(2)S and K(2)COS(2) could not be obtained because they are unstable under the corresponding measurement conditions. The isotropic chemical shift values of the central carbon atoms obtained from solid-state MAS (magic angle spinning) NMR experiments deviate at most by 3 ppm from the corresponding solution values. Two major trends in the (13)C chemical shift values of the central carbon atoms were found. First, if an oxygen atom in a parent compound or in an alkyl-substituted derivative is replaced by a sulfur atom, a significantly higher chemical shift value is observed. This trend is discussed in terms of the paramagnetic contribution to the chemical shielding constant. Second, the size of the alkyl group in the monoalkyl derivatives has a very small effect on the chemical shift values of the central carbon atoms. This observation is explained using the concept of varying inductive effects produced by alkyl groups. The trends observed for the (13)C and (1)H chemical shift values of the alkyl groups follow common concepts on the structure dependency of chemical shifts.     

Molecular states of atoms. I. Orbital energy parameters

Atomic valence state energies are analyzed to obtain values of orbital energy parameters that may be used in semiempirical molecular orbital calculations. Difficulty in defining the interaction between orbitals with non-integer electron populations is systematically avoided by distinguishing between a valence state and a molecular state of an atom, only the latter state having non-integer spin paired orbital occupancy. Application of the virial theorem to the molecular state enables a value for the orbital kinetic energy to be obtained from the valence state orbital energy parameters once an arbitrary configuration is defined as reference. The orbitals then are eigenfunctions of the atomic Fock operator for that reference molecular state and, with their energy parameters, may be employed as a fixed basis set for molecular orbital calculations.     

Geometrical properties of some Euler and circular cubics. Part 2

This paper is a continuation of Part 1, previously published under the same title. The reader is asked to refer back to Part 1 for the Glossary and references.     

On a conjecture of Brandler

Quadratic residuacity properties of real quadratic units are studied.