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First principles conceptual models of chemical reactivity: Quantitative curly arrows and frontier orbitals

Kirsopp, Josh 2020. First principles conceptual models of chemical reactivity: Quantitative curly arrows and frontier orbitals. PhD Thesis, Cardiff University.
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Abstract

The computational expense associated with evaluating the electrostatic potential at a series of points stems from the presence of the position vector of each point in the denominator of a complicated 3D integral. A multipole expansion of the potential is significantly less computationally demanding, and yields a good approximation to the exact potential far from the charge distribution, but penetration effects lead to erroneous potentials at short range. In this work we present a new, computationally efficient method for approximating molecular electrostatics, the Reduced Orbital Potential Approximation, which combines multipole information with the full electrostatic potential arising from a simple model density, which can be evaluated at a fraction of the cost of the full density and incorporates some of the penetration correction without the need for damping functions. A new tool for the chemical interpretation of ab initio wavefunctions is also introduced which aims to establish a rigorous link between accurate computations of the potential energy surface and widely employed chemical descriptions of change during a reaction, such as frontier orbitals and \curly arrows". To achieve this, the total energy is partitioned tensorially into a global potential energy containing no quantities associated with chemical bonding and a covalency energy, for which the necessary assumptions and approximations for the use of chemically intuitive notions of reactivity can be considered valid. The scheme is applied first to canonical orbitals and shown to provide quantitative bonding information in line with classical molecular orbital diagrams, and then to localised orbitals in an attempt to recover frontier orbitals and curly-arrows. An extension to the method is also explored which enforces the assumption that core orbitals are mere spectators to reactions. The method is shown to give good results for a simple test system, and then applied to an SN2 reaction.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 23 September 2020
Last Modified: 25 May 2021 01:31
URI: http://orca.cardiff.ac.uk/id/eprint/135039

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