Probing Covalency in Actinide Molecules: A Computational Toolbox for Magnetic Resonance

  • Letitia Birnoschi

Student thesis: Phd

Abstract

Magnetic resonance techniques are able to accurately probe the interactions between electron spins, nuclear spins and external magnetic fields; this information is encoded in a set of effective spin Hamiltonian parameters. Of particular interest are hyperfine coupling constants (HFCCs), which display a strong dependence on unpaired electron (spin) density, thus providing important insight into chemical bonding. In particular, spin delocalisation can be used as a measure of covalency in paramagnetic actinide (An) molecules. The complexity of An bonding results from the possibility of valence electrons occupying some or all of the 5f, 6d, 7s and 7p orbitals, requiring sophisticated electronic structure techniques to be described ab initio. The non-trivial An valence space is partly a manifestation of the strong scalar relativistic and spin-orbit coupling (SOC) effects, which also need to be included in theoretical models of An chemistry. The interpretation of isotropic HFCCs as measures of spin density at the nucleus is based on a non-relativistic Schro√Ƭądinger-Pauli framework. This formalism breaks down for heavy elements, as the significant relativistic effects require a 4-component Dirac treatment. However, 4-component electronic structure approaches are unfeasible for all but the simplest systems. The computational cost can be reduced by decoupling the electronic and the positronic degrees of freedom in the Dirac Hamiltonian via a unitary transformation. Care must be taken to also apply this transformation to the hyperfine coupling operator, other- wise a picture-change error (PCE) is introduced. Our aim is to devise a computational methodology for determining relativistic, PCE-corrected HFCCs for chemical systems of arbitrary size and complexity. With this in mind, we developed Hyperion, a Python-based program that computes SF-X2C-decoupled g-values and HFCCs from active space wavefunctions, with or without SOC included a posteriori. Herein, we use Hyperion to determine HFCCs for selected atoms, as well as for two An complexes previously characterised via pulsed EPR techniques. For the latter, we further employ Hyperion results to simulate hyperfine sublevel correlation (HYSCORE) spectra, thus facilitating a direct comparison with experimental data.
Date of Award31 Dec 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorEric Mcinnes (Supervisor), Ralph Adams (Supervisor) & Nicholas Chilton (Supervisor)

Keywords

  • HYSCORE
  • relativistic quantum chemistry
  • computational chemistry
  • relativistic effects
  • EPR
  • hyperfine coupling
  • CASSCF
  • electronic structure
  • ab initio

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