Computational studies of the relation between bond strength and QTAIM properties in molecular

187 Pages·2016·3.89 MB·English

Computational studies of the
relation between bond strength
and QTAIM properties in
molecular actinide compounds
Qian-Rui Huang
A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
of
University College London.
Department of Chemistry
University College London
October 10, 2016
2
I, Qian-Rui Huang, confirm that the work presented in this thesis is my
own. Where information has been derived from other sources, I confirm that
this has been indicated in the work.
3

Abstract
Abstract
In the development of the reprocessing of spent nuclear fuels, liquid extraction
with ligands designed to selectively chelate minor actinides (MAs) in the pres-
ence of other cations is required. Extractants based on nitrogen donor ligands
of the 2,6-bis(triazinyl)-pyridine (BTP) family can show high separation fac-
tors for the MAs from lanthanide fission products such as europium, and are
also both radiation and low pH tolerant.
In this PhD thesis, density functional theory (DFT) and the quantum
theory of atom-in-molecules (QTAIM) are used to investigate the nature of
actinide-nitrogen bonding in order to provide enhanced understanding of the
selectivity of BTP and related ligands for the MAs. Several different DFT
methods are initially benchmarked by calculating ionisation energies and bond
dissociation energies of actinide oxides (for which high-level ab initio data are
available in the literature). Subsequently, a series of calculations have been
performed on some simple actinide complexes with one or three nitrogen-based
ligands. QTAIMmetricsareusedtodescribetherelativerolesofcovalencyand
ionicity in the An-N bonding, and strong correlation is found between bond
strength and partial charge difference of the actinide cations on compound
formation.
De Sahb et al.[1] have proposed that the chemical properties of BTP and
other polyazine-based ligands should reflect the contribution of their compo-
nentsingleazinedonorgroups; toprobethis, aninvestigationoftheinteraction
energies of actinide-bisazines, lanthanide-bisazine and complexes of the azine
components has been carried out. Strong correlation between the interaction
5
Abstract
energies of M-bisazines and the azine ligand components has been found. The
interaction energy of a bisazine constructed from two of the same azine groups
is shown to be a better indicator to the binding strength of bisazine ligands
than the interaction energy of individual azine groups.
6
Acknowledgements
Acknowledgements
First and foremost, I would like to thank my supervisor, Prof. Nik Kaltsoyan-
nis, for encouraging my research and for helping me finish this PhD project. I
am extremely grateful for all the advice you gave, and for your patience over
the past four years.
I would also like to thank Dr. Andrew Kerridge for being my secondary
supervisor. Thank you for all the help and the useful suggestions.
Special thanks to Dr. Hazel Cox and Dr. Martijn Zwijnenburg for serving
as my viva examiners. Thank you for the enjoyable conversation during the
viva exam, and for all your careful and brilliant comments and suggestions.
I would like to thank everyone who help me greatly in these four years
although it would be impossible to name you all here. As a famous chinese
writer said, “Since there are too many people that we feel grateful to, lets thank
heaven then.” Thank you everyone.
I will always be grateful to University College London for the Overseas
Research Scholarship and for the computing resources.
Last but not least, I would like to thank my family for their unconditional
support in everything that I do.
7

Contents
1 Introduction 27
1.1 The reprocessing of nuclear waste . . . . . . . . . . . . . . . 27
1.2 Similarity and differences between lanthanides and actinides 29
1.3 The SANEX process and BTP-like ligands . . . . . . . . . . 30
2 Theoretical Background 35
2.1 Density functional theory . . . . . . . . . . . . . . . . . . . . 35
2.2 Exchange-correlation functionals . . . . . . . . . . . . . . . . 38
2.3 Self-consistent field procedure . . . . . . . . . . . . . . . . . 40
2.3.1 Damping . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.3.2 Direct Inversion in the Iterative Subspace (DIIS) . . 42
2.3.3 Quadratically Convergent SCF (QCSCF) . . . . . . . 44
2.4 Wavefunction stability . . . . . . . . . . . . . . . . . . . . . 46
2.4.1 Constraints in Hartree-Fock theory . . . . . . . . . . 46
2.4.2 Stability analysis of wavefunctions . . . . . . . . . . . 48
2.5 Geometry optimisation . . . . . . . . . . . . . . . . . . . . . 52
2.6 Relativistic effects . . . . . . . . . . . . . . . . . . . . . . . . 53
2.7 Basis sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.8 The Quantum Theory of Atoms in Molecules . . . . . . . . . 60
2.8.1 Critical points . . . . . . . . . . . . . . . . . . . . . . 60
2.8.2 Atom basin and bond path . . . . . . . . . . . . . . . 61
2.8.3 Bond properties and atomic properties . . . . . . . . 62
2.9 Atomic charge analysis schemes . . . . . . . . . . . . . . . . 64
2.9.1 Mulliken population analysis . . . . . . . . . . . . . . 65
CONTENTS
2.9.2 Hirshfeld population analysis . . . . . . . . . . . . . 66
2.9.3 Natural population analysis . . . . . . . . . . . . . . 66
3 Computational Details 69
4 Actinide Oxides 71
4.1 Benchmarking the methods for actinide oxides . . . . . . . . 71
4.2 QTAIM properties of actinide oxides . . . . . . . . . . . . . 76
4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5 Actinide Compounds with N-based Ligands 83
5.1 Actinide compounds with a single N-based ligand . . . . . . 83
5.2 Actinide compounds with three N-based ligands . . . . . . . 93
5.3 Actinide compounds with a single azine-based ligand in aqueous
solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.4 Actinide compounds with three azine-based ligands in aqueous
solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
6 Actinide and Lanthanide Compounds with Polyazine Lig-
ands 113
6.1 Lanthanum-bisazinecomplexesandthecontributionfromsingle
azine components . . . . . . . . . . . . . . . . . . . . . . . . 113
6.2 Actinide-bisazine complexes and the contribution from single
azine components . . . . . . . . . . . . . . . . . . . . . . . . 120
6.3 Lanthanide-bisazine complexes and the contribution from single
azine components . . . . . . . . . . . . . . . . . . . . . . . . 137
6.4 Actinide-BTP and lanthanide-BTP compounds . . . . . . . 147
6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
7 General Conclusions 157
Appendices 159
10