Structural Characterisation of Agrochemicals and Their Metal Complexes Using Ion Mobility-Mass Spectrometry, Spectroscopy, and DFT

Abstract

Synthetic pesticides and herbicides are ubiquitous and are vital to world food security, however, many of these molecules end up having adverse effects on non-target organisms and the environment. Such substances' reactivity and chemical properties are related to their chemical structures, so thorough analysis of structure is critical to understanding of their behaviours. This thesis will focus on understanding the fundamental structures of key agrochemicals and their reactivity. Understanding the fundamental structures and reactivity provides valuable insights into their interactions with other biological and chemical systems. Metal complexation is an important form of reaction that is generally understudied for agrochemicals. Metal cations are highly prevalent in both biological and environmental systems; therefore, while understudied, these reactions have ample opportunity to occur in nature. The challenges of studying the metal complexation capability of select agrochemicals are numerous. However, as metal complexation can dictate the biological and environmental fate of agrochemicals, it should be considered and investigated in detail. This thesis applies ion mobility-mass spectrometry (IM-MS) to separate, analyse, and characterise agrochemicals and their metal complexes, including isomers. IM-MS was applied to elucidate agrochemicals' structures, especially glyphosate, imidacloprid and chlorpyrifos. In Chapter 2, a combination of IM-MS and DFT techniques are used to try and elucidate the structures of glyphosate anion fragments where separation of the linear and cyclic isomers of the fragments was attempted. In Chapter 3, the gas phase structures of protonated, sodiated and potassiated imidacloprid and chlorpyrifos were elucidated by comparing their experimental and predicted CCS values. In Chapters 4 and 5, different IM-MS techniques were used, including the relatively novel application of infrared multiple photon dissociation (IRMPD) spectroscopy in tandem with IM-MS to elucidate the structures of monomeric and dimeric glyphosate and AMPA metal complexes. Monomeric glyphosate and AMPA metal complexes possessed many isomers where structural elucidation from comparing experimental and theoretical CCS values wasn’t sufficient. For most dimeric glyphosate and AMPA metal complexes, a single isomer was observed. Their structures were successfully characterised by comparing their experimental and predicted CCS and IR spectra. The coupling of innovative experimental techniques with a suite of advanced computational and modelling methodologies proves to be extremely powerful in predicting the molecular and metal complex structures of agrochemicals. This thesis sheds light on the gas phase structures of agrochemicals and their metal complexes that haven’t been reported before with the hopes of providing better insights into their reactivity, characteristics and behaviours. Additionally, the structural characterisation attempts in this thesis also highlighted the challenges of structural elucidation by using IM-MS and the comparison of experimental and predicted CCS values. The challenges in this thesis provide valuable lessons and strategies to circumvent similar issues that might arise when analysing the same molecules or similar systems as the ones studied here