Experimental and Theoretical Approaches to Actinide Chemistry (eBook)

John K. Gibson, Wibe A. De Jong (Herausgeber)

eBook Download: EPUB

2018
John Wiley & Sons (Verlag)
9781119115540 (ISBN)

Infos

A review of contemporary actinide research that focuses on new advances in experiment and theory, and the interplay between these two realms

Experimental and Theoretical Approaches to Actinide Chemistry offers a comprehensive review of the key aspects of actinide research. Written by noted experts in the field, the text includes information on new advances in experiment and theory and reveals the interplay between these two realms. The authors offer a multidisciplinary and multimodal approach to the nature of actinide chemistry, and explore the interplay between multiple experiments and theory, as well as between basic and applied actinide chemistry.

The text covers the basic science used in contemporary studies of the actinide systems, from basic synthesis to state-of-the-art spectroscopic and computational techniques. The authors provide contemporary overviews of each topic area presented and describe the current and anticipated experimental approaches for the field, as well as the current and future computational chemistry and materials techniques. In addition, the authors explore the combination of experiment and theory. This important resource:

• Provides an essential resource the reviews the key aspects of contemporary actinide research
• Includes information on new advances in experiment and theory, and the interplay between the two
• Covers the basic science used in contemporary studies of the actinide systems, from basic synthesis to state-of-the-art spectroscopic and computational techniques
• Focuses on the interplay between multiple experiments and theory, as well as between basic and applied actinide chemistry

Written for academics, students, professionals and researchers, this vital text contains a thorough review of the key aspects of actinide research and explores the most recent advances in experiment and theory.

Edited by

John K. Gibson, is Senior Scientist, Lawrence Berkeley National Laboratory, USA. He is experienced in fundamental actinide chemistry research, ranging from solid state synthesis of transuranium compounds to actinide chemistry in the gas phase.

Wibe A. de Jong, is Senior Scientist, Lawrence Berkeley National Laboratory, USA. He has years of experience in advancing fundamental actinide chemistry research, and in developing and using a variety of computational chemistry approaches.

Textauszug

1
Probing Actinide Bonds in the Gas Phase: Theory and Spectroscopy

Michael C. Heaven1 and Kirk A. Peterson2

1 Department of Chemistry, Emory University, Atlanta, Georgia, United States

2 Department of Chemistry, Washington State University, Pullman, Washington, United States

1.1 Introduction

Theoretical studies of actinide compounds have two primary goals. The first is to understand the chemical bonding within these materials and their physical properties. A sub‐focus within this task is the elucidation of the roles played by the 5f electrons. The second goal is to understand the reactivities of actinide compounds. The long‐term objective is to develop reliable computational methods for exploring and predicting actinide chemistry. This is highly desirable owing to the practical difficulties in handling the radioactive and short‐lived elements within this family.

Actinides pose a severe challenge for computational quantum chemistry models due to the large numbers of electrons and the presence of strong relativistic effects [1–4]. Although small molecules (di‐ and tri‐ atomics that contain just one metal atom) can be explored using rigorous theoretical models, the computational cost of this approach is currently too high for most problems of practical interest (e.g., actinide ions interacting with chelating ligands in solution). Consequently, approximate methods are applied. Ab initio calculations can be accelerated by using a single effective core potential to represent the deeply bound electrons of the metal atom [5, 6]. The relativistic effects are folded into this core potential, and the number of electrons explicitly considered by the calculations is greatly reduced. Semi‐empirical density functional theory (DFT) methods offer even better computational efficiency. It is, of course, essential that these approximate methods be tested against both accurate experimental data and the results of rigorous “benchmark” calculations.

There are clear advantages for using data obtained from gas‐phase measurements for the comparisons with theory. The ideal situation is to evaluate predictions for the bare molecule against experimental data that are untainted by interactions with solvent molecules or a host lattice. Gas‐phase spectroscopy can provide accurate determinations of rotational constants, dipole moments, vibrational frequencies, electronic excitation energies, ionization energies, and electron affinities [7–11]. Information concerning the geometric structure and the electronic state symmetries can be derived from the rotational energy level patterns, which can only be observed for the unperturbed molecule. The reactivities of actinide‐containing species can also be investigated in the gas phase, under conditions that facilitate theoretical comparisons [8, 12–15]. The majority of this work relies on mass spectrometry for the selection of the reactants and identification of the products. In addition to revealing reaction pathways, mass spectrometric experiments provide critical thermodynamic data, such as bond dissociation and ionization energies.

Over the past 30 years, there has been steady progress in the development of relativistic quantum chemistry methods, combined with a dramatic increase in the speed and capacity of computing platforms. On the experimental side, the application of laser‐based spectroscopy, guided ion beam, and ion‐trap mass spectrometry has significantly advanced our ability to explore the structure, bonding, and reactivity of actinide species. In this chapter, we present an overview of the theoretical and experimental techniques that are currently being used to gain a deeper understanding of actinides through the studies of small molecules in the gas phase. While some background material is presented, the primary focus is on the techniques that are currently being applied and developed. To limit the scope, the experimental section is strictly devoted to gas‐phase measurements. There is a large body of excellent spectroscopic work that has been carried out for actinide species trapped in cryogenic rare‐gas matrices. The data from these measurements are also of great value for tests of theoretical predictions, as the rare‐gas solid is usually a minimally perturbing host. For a review of the matrix work, see Reference [8]

1.2 Techniques for Obtaining Actinide‐Containing Molecules in the Gas Phase

The earliest spectroscopic studies of actinide‐containing molecules in the gas phase were carried out using compounds that possessed appreciable vapor pressures at room temperature. Hence, the hexafluorides UF6, NpF6 and PuF6 were studied by conventional spectroscopic means [8], with suitable precautions for handling radionuclides. The tetrahalides have lower room temperature vapor pressures, but workable number densities have been obtained using moderate heating of the samples [8]. Studies of thorium oxide (ThO) emission spectra were carried out using ThI4 as the source of the metal [16]. This reagent was subjected to a 2.45 GHz microwave discharge that was sustained in approximately 0.1 Torr of Ne buffer gas. The oxide was readily formed by the reaction of the discharge‐generated Th atoms with the walls of the quartz tube that contained the ThI4/Ne mixture. Molecular ions can also be generated at workable number densities using discharges with volatile compounds. An example is provided by the recent study of ThF+ reported by Gresh et al. [17]. ThF4 was used as the starting reagent, and the vapor pressure was increased by heating the quartz discharge tube to 1193 K. The tube was filled with approximately 5 Torr of He, and the mixture was excited by an AC discharge operated at a frequency of 10 kHz.

More commonly, actinide species are refractory solids that require somewhat extreme conditions for vaporization. Tube furnaces [18, 19], Knudsen ovens [20, 21], discharge sputtering [14], discharges [16, 17, 22], and laser ablation techniques [8, 23–25] have been successfully applied. High‐temperature vaporization is exemplified by studies of the electronic spectrum of uranium oxide (UO), carried out using a Knudsen oven to vaporize U‐metal samples that had been pre‐oxidized by exposure to air [21]. The crucible was heated to temperatures in the range of 2400–2600 K. This was high enough that thermally excited electronic transitions could be observed using conventional emission spectroscopy. The UO vapor pressure generated by the Knudsen oven was sufficient for the recording of absorption and laser‐induced fluorescence (LIF) spectra. Resistively heated tube furnaces have also been used for studies of gas‐phase UO. The advantage of this approach is that it provides a longer optical path length, and thereby yields spectra with greater signal‐to‐noise ratios. Extensive high‐resolution electronic spectra for UO were recorded by Kaledin et al. [18] using a tube furnace that was heated to 2400 K. More recently, Holt et al. [19] have used a tube furnace to record microwave absorption spectra for UO in the 500–650 GHz range. Transitions between the rotational levels of multiple low‐lying electronic states (including the ground state) were observed.

Many of the early mass spectrometric studies of gas‐phase actinide molecules were carried out using thermal vaporization in combination with electron impact ionization techniques [26–28]. Surface ionization of uranyl acetate has been used to generate UO+ and UO2+ [29]. In the thorium ion experiments of Armentrout et al. [14, 30], thorium powder was mounted in a cathode held at −2.5 kV. This electrode produced a discharge in a flow of Ar, and the resulting Ar+ ions generated Th+ by sputtering. Molecular ions, such as ThCH4+, were formed in a downstream flow tube. These techniques are suitable for measurements that rely on charged particle detection, but the number densities are usually too small for conventional spectroscopic observations (e.g., absorption or fluorescence detection).

For spectroscopic techniques that are compatible with pulsed generation of the target species, laser vaporization has proved to be a particularly versatile method. The products include cations, anions, and molecules in exotic oxidation states. Duncan [23] has published a very informative review of this technique. Typically, a pulsed laser (Nd/YAG or excimer with 5–10 ns pulse duration) is focused onto the surface of a target to produce a vapor plume. The products are cooled and transported using a flow of an inert carrier gas (e.g., He or Ar). If the plume does not contain the species of interest, the carrier gas can be seeded with a reagent that will produce the desired product. For example, gas‐phase UF has been produced by vaporizing U metal into a flowing mixture of He and SF6 [31].

Charged species are produced by the vaporization process, even in the absence of an externally applied electric field. Cations are formed by thermal processes and photoionization. Some of the liberated electrons attach to neutral molecules, providing a viable flux of negative ions under suitable conditions. External fields can be applied if the initial ion yield is not high enough for the detection technique.

A particularly valuable advantage of the laser vaporization technique is that it can be readily combined with a supersonic jet expansion nozzle. Figure 1.1 (reproduced from Reference [23]) shows this combination for a pulsed solenoid gas...

Produkt-Details

• Erscheint lt. Verlag: 8.1.2018
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Anorganische Chemie
• Themenwelt: Naturwissenschaften ► Chemie ► Physikalische Chemie
• Schlagworte: actinide • Actinide Bioinorganic Chemistry • Actinoide • Anorganische Chemie • Chemie • Chemistry • Computational Chemistry & Molecular Modeling • Computational Chemistry u. Molecular Modeling • Energie • Energy • experimental approaches to create new organometallic species that contain actinides in novel oxidation states • experimental techniques for creating gas-phase actinide ions and molecular species • Extended Actinide Bearing Solids • Formation of Actinide Nanostructures • guide to Approaches to Actinide Chemistry • Inorganic Chemistry • Introduction to Approaches to Actinide Chemistry • Kernenergie • New Spectroscopic Methods for Actinides • nuclear energy • Organometallic Actinides with Novel Oxidation States • Probing Actinides in the Gas-Phase • resource for Approaches to Actinide Chemistry • Speciation of Actinides in Solution • Transport of Actinides in the Environment: Separations and Coordination of Actinides
• ISBN-13: 9781119115540 / 9781119115540