Search NASA⌕ Search

Engineering topics

Johnstone, Erik V.

Publications and source records attributed to Johnstone, Erik V..

4 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Technetium: The First Radioelement on the Periodic Table

The radioactive nature of technetium is discussed using a combination of introductory nuclear physics concepts and empirical trends observed in the chart of the nuclides and the periodic table of the elements. Trends such as the enhanced stability of nucleon pairs, magic numbers, and Mattauch's rule are described. Here, the concepts of nuclear binding energies and the nuclear shell model are introduced and used to explain the relative stability of radionuclides and, in particular, the isotopes of technetium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Molecular and electronic structure of Re 2 Br 4 (PMe 3 ) 4

The dinuclear rhenium(II) complex Re 2 Br 4 (PMe 3 ) 4 was prepared from the reduction of [Re 2 Br 8 ] 2– with ( n -Bu 4 N)BH 4 in the presence of PMe 3 in propanol. The complex was characterized by single-crystal X-ray diffraction (SCXRD) and UV–visible spectroscopy. It crystallizes in the monoclinic C2/c space group and is isostructural with its molybdenum and technetium analogues. The Re–Re distance (2.2521(3) Å) is slightly longer than the one in Re 2 Cl 4 (PMe 3 ) 4 (2.247(1) Å). The molecular and electronic structure of Re 2 X 4 (PMe 3 ) 4 (X = Cl, Br) were studied by multiconfigurational quantum chemical methods. The computed ground-state geometry is in excellent agreement with the experimental structure determined by SCXRD. The calculated total bond order (2.75) is consistent with the presence of an electron-rich triple bond and is similar to the one found for Re 2 Cl 4 (PMe 3 ) 4 . The electronic absorption spectrum of Re 2 Br 4 (PMe 3 ) 4 was recorded in benzene and shows a series of low-intensity bands in the range 10 000–26 000 cm –1 . The absorption bands were assigned based on calculations of the excitation energies with the multireference wave functions followed by second-order perturbation theory using the CASSCF/CASPT2 method. As a result, calculations predict that the lowest energy band corresponds to the δ* → σ* transition, while the next higher energy bands were attributed to the δ* → π*, δ → σ*, and δ → π* transitions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

β-Technetium Dichloride: Solid-State Modulated Structure, Electronic Structure, and Physical Properties

A second polymorph of technetium dichloride, β-TcCl 2 , has been synthesized from the reaction of Tc metal and chlorine in a sealed tube at 450 °C. The crystallographic structure and physical properties of β-TcCl 2 have been investigated. The structure of β-TcCl 2 consists of infinite chains of face sharing [Tc 2 Cl 8 ] units; within a chain, the T c ≡T c vectors of two adjacent [T c 2 Cl 8 ] units are ordered in the long-range where perpendicular and/or parallel arrangement of Tc≡Tc vectors yields a modulated structure. Resistivity and Seebeck measurements performed on a β-T c Cl 2 single crystal indicate the compound to be a p-type semiconductor while a magnetic susceptibility measurement shows technetium dichloride to be diamagnetic. A band gap of 0.12(2) eV was determined by reflectance spectroscopy measurements. Theoretical calculations at the density functional level were utilized for the investigation of other possible stable forms of T c Cl 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗