Theoretical Studies on the Chemical Environment and its Effect on Molecular Properties

Abstract

This thesis details both the application and development of tools in quantum computational chemistry to better understand the role the chemical environment plays in affecting molecular properties. It does this using proton transfer as a chemical motif for exploration of these topics. Chapter 1 begins the thesis by providing an overview of the significance of environmental effects on molecular properties, highlighting the important role of proton transfer, and outlining the different techniques currently used to model these effects. Chapter 2 details the theoretical tools used throughout the thesis. This thesis makes extensive use of high-level gas-phase quantum chemical techniques, as well as techniques related to the solution of the nuclear Schrödinger equation. Such high levels of theory are implemented to ensure the reliability of conclusions for both molecules and radicals, the latter of which frequently present problems for lower-level procedures. Chapters 3 and 4 explore how protonation and deprotonation, that is, proton transfer, affect the structural and thermochemical properties of a range of prototypical alcohols, substituted methanes, and their radical derivatives. The results presented are rationalised using a combination of resonance and orbital interaction arguments, as well as through the introduction of appropriate thermochemical metrics. Chapter 5 outlines an approach for modelling the long-range electrostatic effects of an environment in terms of a power series expansion. The advantages of this approach are discussed, and the limits of applicability examined using a simple model system, the ammonium chloride proton-transfer complex, ClH:NH3. Chapter 6 develops some of the utilities required for the efficient application of the power series approach in nuclear dynamics contexts. More specifically, a method for generating accurate molecular dipole moment surfaces is presented. Finally, Chapter 7 summarises the work presented heretofore and discusses possibilities for future research.