The development of density functional approximations (DFAs) is a very active research field, with the number of approximations continuously growing. At the same time, there is a lack of published work that gives a fair evaluation and comparison of DFAs on an equal basis. Considering the vast and important Density Functional Theory (DFT) user community, the introduction of such a variety of techniques can lead to confusion and a low acceptance rate of newly proposed methods in general. In fact, from a user's point of view, it would be beneficial to have access to only a handful of methods that distinguish themselves through their accuracy, reliability, and general robustness in benchmark studies and subsequent applications. Therefore, the primary objective of this thesis is to identify ways to the dizzying array of DFAs. Throughout this thesis, a range of topics will be discussed. They originate from the idea of making computational techniques more reliable. The findings and applications of reliable and efficient computational techniques are at the heart of this thesis. The first project of the thesis is concerned with the thorough evaluation of the double-hybrid density functionals on a large and diverse GMTKN55 database for general main-group thermochemistry, kinetics, and noncovalent interactions. Our study represents one of the largest and most thorough double hybrid studies ever conducted and can serve as an important benchmark informing method developers and users alike. We then shed light on the communication gap between method developers and users, and show that despite tremendous scientific progress in the area, people's perceptions of their favourite approaches have not changed much. The third project provides the most comprehensive database for chalcogen bonding interactions (CHAL336), with high-level wave function based interaction energies. This is followed by the first thorough evaluation of the performance of DFAs for such interactions. Fourthly, we present a proof-of-concept study with the aim of reducing the cost of double hybrid DFT calculations. In this framework, we present the suitability of Grimme's geometric counterpoise correction scheme for basis set superposition errors in double hybrids. Finally, we utilise some of the most reliable and computationally efficient DFT methods to gain insights into cyclotricatecylene-based molecular capsules that can selectively extract ions from solution. Overall, this thesis encourages a shift in the computational procedures surrounding thermochemical problems, allowing users to make more informed decisions about using DFT methodologies in computations.

This thesis presents the implementation, assessment, and applicability of the currently most accurate density functional approximations (DFAs) for the calculation of vertical excitation energies by means of linear-response time-dependent density functional theory (TD-DFT). Firstly, we demonstrate how the inclusion of a single parameter, which controls the interplay between interlectronic short- and long-regimes, improves global double-hybrid density functional approximations (DHDFAs) when it comes to long-range excitations; such as Rydberg states and charge-transfer (CT) transitions, without much loss of accuracy for local-valence ones. In this context, we define the two first long-range corrected (LC) DHDFAs optimised for excitation energies, namely, wB2PLYP and wB2GP-PLYP, being the best TD-DFT methods until then. Furthermore, we include an additional analysis of the CT problem after noticing some misconceptions by some in the developer community regarding the application of global DHDFAs as an alternative to the LC ones. In the second part, we investigate the performance of the best DHDFAs for singlet-singlet transitions in the Tamm-Dancoff Approximation (TDA) but, more importantly, we revive the applicability of DHDFAs for singlet-triplet excitations after 12 years of slumber. Herein, we demonstrate once again that our LC-DHDFAs are the best-performing methods not only for singlet but also for triplet excitations, being the most accurate and robust methods until then. Finally, inspired by the precursory work done by Schwabe and Goerigk in 2017, we extend our two previous studies by combining our methods with the Spin-Component Scaling (SCS) and Spin-Opposite Scaling (SOS) techniques, with the main difference being the inclusion of LC-DHDFAs and singlet-triplet excitations in the context of DHDFAs. We also defined the currently two best LC-DHDFAs optimised for excitation energies on the market, namely, wB88PP86 and wPBEPP86. We particularly note that the SCS and SOS variants our new functionals delivered the best TD(A)-DHDFA results to date for 1La and 1Lb transitions in Policyclic Aromatic Hydrocarbons (PAHs). Finally, SCS/SOS-wB88PP86 and SCS/SOS-wPBEPP86 have been demonstrated to be the most accurate and robust approximations for the calculation of singlet and triplet excitations regardless of the type of transition, i.e., they describe local-valence, Rydberg states, and CT transitions with the same excellent accuracy, and we highly recommend them for future applications. Note that during the peer-review of this work, new SCS/-SOS methods were also published in the very same journal. Therefore, we also included them after some minor comments from the reviewers. Nevertheless, our newly proposed methods are still the best-performing SCS/SOS-DHDFAs for the calculation of excitation energies. All the methods presented in this thesis are implemented into the ORCA5 quantum package, which is free for academics.

This thesis involves the analysis of the form, efficiency and applicability of leading density functional approximations (DFAs) for modelling ground-state molecular chemistry. Firstly, the DFT-NL methodology of including a density-based nonlocal correlation term to an exchange-correlation density functional is shown to be beneficial for many DFAs. It is also shown that the nonlocal term can be computed in one step after the self-consistent-field (SCF) procedure without loss of accuracy. The assessment in this study is based on the GMTKN55 database of general main group thermochemistry, kinetics and noncovalent interactions. Special emphasis is placed on the B97M-V, wB97X-V and wB97M-V DFT-NL, which I show to be very accurate and robust functionals. Secondly, the use of the DFT-D4 dispersion correction is investigated. This is based on the geometry, charge and electronic spin of a molecule. In addition to energetics, I investigate the approach for geometry optimisations. Several DFT-D4 corrected functionals are compared to their DFT-D3 dispersion corrected counterparts, including B97M-V, wB97X-V and wB97M-V, where the nonlocal density-based term is replaced by the DFT-D3/D4 correction. The DFT-D3 correction was developed prior to the DFT-D4 correction, and is not dependent on the charge and electronic spin of the molecule. Hence, there is emphasis on the two approaches for transition-metal systems. The DFT-D4 correction can indeed be more accurate than the DFT-D3 correction for charged and open-shell systems. Lastly, various range-separated double-hybrid functionals are assessed for energetics, utilising the GMTKN55 database. These methods calculate the exchange energy in a short-range regime composed of DFT and Fock exchange and a long-range regime composed of only Fock exchange. This is done with the aim of overcoming various problems inherent to DFAs, such as the self-interaction error, in which interactions of electrons with themselves are mistakenly calculated. This study investigates the wB2PLYP and wB2GP-PLYP semi-empirically-parameterised range-separated double hybrids, and the RSX-0DH and RSX-QIDH non-empirically-parameterised range-separated double hybrids.

Hypoxia is a critical physiological marker demonstrated in a variety of cancers, imaging it will provide valuable insight into prognosis and treatment. However, the slow kinetics of [18F]FMISO(1.2) means that there is a 2 hour delay between injection and imaging for patients. Fluorine-18 labelled Chloroethyl sulfoxides [18F]SO101 and [18F]SO201 were shown to have both faster kinetics and higher contrast compared to [18F]FMISO(1.2) in a rat model of ischemic stroke. Perceived toxicity from the nitrogen mustard analogues in [18F]SO101 and [18F]SO201 lead to structural changes resulting in [18F]SO501. [18F]SO501 demonstrated good uptake into hypoxic SK-RC-52 tumours while also clearing from muscle, giving good contrast and therefore high-quality images, however it had a low radiochemical yield of 2.5% making it impractical for routine clinical application. Structural changes were made to the base compound [18F]SO501 in order to adapt it to a different method of radiolabelling in an attempt to increase the RCY while maintaining its selectivity for hypoxic tissue. The modifications made included the introduction of a propargyl group for click chemistry, variable length PEG groups and alterations to the ester group from an ethyl ester to an isopropyl ester and in one case doing away with it entirely. Six different diaryl-sulfoxide radiolabelled compounds were synthesised for this project [18F]2.16, [18F]2.26, [18F]2.36, [18F]2.43, [18F]2.53 and [18F]2.63 from their respective diaryl-sulfoxide precursors 2.15, 2.25, 2.35, 2.42, 2.52 and 2.62. After in-vivo imaging in SK-RC-52 tumours xenografted onto BALB/c nude mice the most promising tracer [18F]2.16, which had the best tumour to muscle ratio of 1.6 at 60 minutes and 2.1 at 110min, underwent metabolism studies involving Rat S9 liver fractions.

Nanoplatelets (NPLs) are a class of nanoparticles which have garnered significant interest in the research community. Unfortunately, much of the focus has been on the usual workhorse material: cadmium selenide. Zinc selenide is a close relative of cadmium selenide, both belonging to the II-VI family of semiconductors, but little research exists on ZnSe NPLs beyond its initial synthesis. In this thesis, ZnSe NPLs are addressed from the bottom up. In Chapter 2, the formation mechanism of ZnSe NPLs and MSCs is investigated. The evolution of nanoparticles in the reaction is monitored while exploring the reaction space. It is demonstrated that the concept of surface reversibility can be used to predict the formation of NPLs and MSCs. Additionally, it is found that MSCs and NPLs compete in the reaction, and selective formation can be induced by varying selenium precursor and the ripening behaviour of the ligand. Along the way, five unreported ZnSe magic-sized clusters (MSCs) are found. Chapter 3 of the thesis is a demonstration of Mn 2+ doping into the ZnSe and ZnS NPLs. Mn 2+dopant incorporation can be confirmed via its photoluminescence and photoluminescence excitation spectra and the photostability is measured. Additionally, the unique Mn 2+ emission is used a probe to investigate the evolution of various ZnSe species. Finally, Chapter 4 is concerned with the post-synthetic shelling of ZnSe nanoplatelets. ZnSe NPLs as synthesized from literature are colloidally and photo-unstable. A common solution to this is to coat the surface of the nanocrystal with a suitable semiconductor material. By modifying the process introduced for CdSe NPLs, the synthesized ZnSe NPLs are shelled successfully via colloidal atomic layer deposition (C-ALD). This allows us to improve its photoluminescence properties and observe unique features associated with Type-II ZnSe/CdS heterostructures.

Triplet-triplet annihilation (TTA) upconversion is a process in which lower energy light can be converted into higher energy light. The primary use of this emerging technology is to increase the efficiency of solar cells via increased light harvesting which may be otherwise impossible by any other means. TTA upconversion typically uses two distinct molecules to achieve this: a "sensitizer" (for absorbing the lower energy light), and an "emitter" (to produce the higher energy light). Until recently, only small molecule were used as emitters. In this work, the first true measurements of TTA upconversion were performed with conjugated polymer emitters. In doing so, 16 new conjugated polymers were developed to study the role of steric hindrance and the electronic environment on the TTA upconversion efficiency. This study then provides insight into the key design criteria required for efficient TTA upconversion using conjugated polymer emitters.

Electrically conductive, magnetically ordered, and electrochemically active materials are important components for data processing technologies, energy conversion and storage devices, and sensors and optoelectronics. The synthesis of compounds with these properties is therefore an important scientific endeavour to meet the progressively more demanding sustainability and performance targets. Crystalline coordination polymers have recently been proposed as candidate materials to achieve these aims. Coordination polymers can exhibit synergy between electrical conductivity, magnetic ordering and electroactivity, coupled with porosity, flexibility, and tuneable architecture and functionality, not achievable using traditional inorganic materials. This thesis describes studies into anionic tetraoxolene coordination polymers constructed using ligands derived from 3,6-disubtitued-2,5-dihydroxy-1,4-benzoquinones (H2Xan, X = F, Cl, Br; anilic acids), and redox-active viologen countercations. Despite the desirable properties of tetraoxolene coordination polymers previously described in literature, the impact that viologen countercations have upon the structure and properties of tetraoxolene coordination polymers has never been explored. Chapter 2 first presents structural studies on a range of Mn(II) and Cd(II) coordination polymers synthesised using diamagnetic fluoranilate (Fan2-) and chloranilate (Clan2-) ligands and redox-active N,N'-dimethyl-4,4'-bipyridinium (MeV2+) countercations. The structural disparity observed between Fan2- and Clan2- compounds demonstrates the inherent sensitivity of reactions to supramolecular interactions between cations, metals, ligands, and solvents. It also highlights the structural unpredictability associated with the use of viologen cations, compared to redox-inactive alkylammonium countercations. Structural studies on mixed-valence Fe(III)-tetraoxolene coordination polymers containing viologen countercations are subsequently presented, showing resultant anionic tetraoxolene network topology remains difficult to predict. However localisation of the tetraoxolene radical trianion (Xan3-) state is consistently observed, caused by charge-transfer (CT) and Coulombic interactions between the pi-electron deficient viologen cations, and the pi-electron rich anionic networks. This reveals the potential for viologens to modulate the charge-state distribution in mixed-valence tetraoxolene coordination polymers. In Chapter 3, 5,6-dihydropyrazino[1,2,3,4-lmn][1,10]-phenanthrolinediium (PhenQ2+) countercations are incorporated into an isostructural family of mixed-valence 2D Fe(III)-tetraoxolene frameworks (PhenQ)[Fe2(Xan)3] (X = F, Cl, Br). As for the mixed-valence networks obtained in Chapter 2, CT interactions between PhenQ2+ cations and the anionic [Fe2(Xan)3]2- networks cause localisation of the Xan3- state and partial ligand charge-ordering. Spectroscopic studies on (PhenQ)[Fe2(Xan)3].yDMF showed the network-cation interactions impact upon the electronic structure of the material. The inclusion of PhenQ2+ gives rise to additional framework redox states and modifies the conductive and magnetic properties of the material, compared to tetraoxolene frameworks with only redox-inactive countercations. Chapter 4 investigates a family of mixed-valence Fe-fluoranilate frameworks containing MeV2+ countercations, (MeV)[Fe2(Fan)3].solvent (solvent = MeCN, DMF). X-ray diffraction studies reveal the compounds undergo a range of slight structural distortions upon changing the guest solvent. Remarkably, these guest-induced structure distortions cause partial switching of the Fe valence state, never before observed in Fe-tetraoxolene coordination polymers. The guest-induced Fe valence fluctuation in turn modifies the electronic spectra and structure of the frameworks, with resultant switching of the transport properties. In Chapter 5, the mixed-valence Fe-tetraoxolene framework (MeV)[Fe2(Clan)3] undergoes a surprising structural transformation upon lattice solvent loss, causing the isolated compound to exhibit a previously unobserved cation-network packing arrangement. This dramatic structure transformation results in changes to the electronic spectrum of the compound, as well as the electrochemical response and transport properties of the material, compared to topologically similar Fe-tetraoxolene frameworks containing redox-inactive cations. These results demonstrate the impact of structure packing and supramolecular interactions on the network electronic structure. This provides encouragement that the physicochemical properties of the compounds may be tailored by controlling these supramolecular interactions. Finally, Chapter 6 presents a summary of the results obtained in this thesis, highlights the overarching themes of this research, and puts forth possible future research directions for the field.

Process improvement and product improvement in the dairy processing industry is an ongoing dynamic process. Ultrasound processing technology for food processing is in its developmental stages, and dairy sector is a key area for potential applications. The use of low frequency and high frequency ultrasound to modify functional properties of dairy streams is being widely studied by seeking to modify the protein and lipid components of milk. The effects of ultrasound on size of lipid globules and lipid oxidation volatiles have been studied. The effects on functional properties of milk proteins such as viscosity, gelation, heat stability etc. have been documented. However, the present literature lacks information on the effects that ultrasound processing may have on the fundamental properties of dairy proteins. Therefore, it is of importance to document the effects of ultrasound processing on the molecular, structural and nutritional aspects of dairy proteins. As such, the effects of ultrasound processing on dairy proteins have been studied in the context of the amino acid composition, amyloid modifications, and bioactive peptides released. This work aims to bridge the gap in existing literature attempting to illuminate upon these fundamental aspects. Chapter 1 in this thesis introduces the premise of this research by discussing the fundamental properties ultrasound technology and presents dairy proteins as the central theme of this research. A systematic review of literature in Chapter 2 attempts to build familiarity with the effects of processing on milk proteins; and discusses the potential applications of ultrasound in the dairy industry – noting the studies done on whey are caseins. This chapter also discusses the research areas which could be addressed to answer questions related to the objectives of current PhD work. To encourage the application of ultrasound for dairy processing, it would be of significance to understand the extent of operating parameters that ensure nutritional integrity and food safety. The amino acid integrity of skim milk proteins after sonication along with the potential of ultrasound to modify secondary structural conformations of milk proteins has been demonstrated in Chapter 4. Chapter 5 expands the objectives of Chapter 4 into whole milk systems, taking into account milk lipids and the potential for oxidation of lipids on sonication. In Chapter 5 an effort has been made to better understand the underlying cause of lipid oxidation observed due to ultrasound processing of milk lipids. The observations from this study suggest that the enhanced mass transfer of oxygen due to ultrasonic shear forces makes oxygen non-limiting in lipid phase and increases auto-oxidation reactions. An assessment of amino acid integrity of full cream milk after ultrasound treatment confirmed oxidative stability of milk proteins. These findings complement the data in Chapter 4. Along with amino acid integrity, Chapter 4 also demonstrates that the secondary structural conformations of milk proteins can be modified by ultrasound. The work in Chapter 6 builds up greater understanding on such changes by studying the effects of low frequency ultrasound on isolated beta-lactoglobulin (b-lg). b-lg is a b-sheet rich protein with amyloidogenic potential. The effects of high shear and high temperature microenvironments produced by ultrasonic waves were studied on b-lg aggregation. Ultrasound induced formation of amyloid crystals in dilute b-lg solutions at neutral pH and ambient bulk temperatures. The theoretical reasoning for the aggregation phenomenon substantially adds to the understanding of protein energy landscape. Chapter 7 focusses on nutritional properties of milk proteins. It examines the effect of ultrasound pre-treatment on the release of BCM-7 bioactive peptide from milk by simulated gastro-intestinal digestion (SGID), and the effect on overall protein digestibility. It demonstrates that ultrasound processing could be used to downregulate the release of undesirable BCM-7 bioactive peptide in milk, by affecting the rate of enzymatic hydrolysis of certain intermediate peptide fragments. The findings from this body of work add to the existing knowledge for improved use of ultrasound to ensure nutritional integrity of dairy proteins and lipids, minimising oxidative damage. It also builds on current understanding of ultrasound induced protein restructuring and its potential for modification of peptide release from dairy proteins.

The study of molecular nanomagnets is important not only for fundamental scientific reasons, but also for applications in the miniaturisation of technology - the prospect of storing data on individual nanomagnets is particularly exciting. By removing unwanted interactions between dipole moments on neighbouring nanomagnets, non-dipolar toroidal moments may allow for even denser data storage. Motivated by this potential application (and by scientific curiosity for as-yet unknown benefits of toroidal moments), my thesis covers a range of topics related to the modelling of nanomagnets capable of producing toroidal magnetic states. Firstly, I provide the context for my research by introducing the fields of single-molecule magnetism, toroidal moments and molecular spintronics. Secondly, I analyse the toroidal states arising from spin-frustration in a triangular nanomagnet of three spin-1/2 centres with C3 symmetry. One breakthrough of this study, is that by including antisymmetric exchange interactions arising from weak spin-orbit coupling, non-dipolar toroidal states can be prepared in a ground-energy doublet, separated from an excited doublet of non-toroidal dipolar states. I then discuss a hypothetical experiment where the nanomagnet is placed in a tunnelling spintronics device to split the steady-state, non-equilibrium populations of toroidal states. I describe the optimal energetic regimes and device geometries to achieve the best population splitting, and show how the splitting could be monitored by measuring the extent to which a spin current's polarisation is reversed by the toroidal states. Thirdly, I extend the investigation to a spin-frustrated triangular nanomagnet with C2v symmetry, such that two sites have general spin S and the third site has spin S3 = 1/2. The resulting toroidal states are intriguing for their complete lack of reliance on spin-orbit coupling, which is typically required to produce toroidal moments in isolated molecules. Again, I characterise the nanomagnet’s spin textures and performance in a tunnelling spintronics device, describing the optimal energetic regimes. Additionally, I explore the effects of changing S and delve deeper into the mechanism by which the toroidal states’ populations are split. Fourthly, I build on a previous study by Soncini, Langley, Murray, Rajaraman and co-workers (Nat. Commun. 2017) to analyse a family of double-triangle complexes of the type {Dy3M(III)Dy3}X, where M = Cr, Mn, Fe, Co, Al and X = NO3, Cl. Based on crystallographic data and ab initio calculations, I use parameter-free models to calculate the complexes' energy spectra, powder magnetisations and direct-current magnetic susceptibilities. Confirming predicted trends and discovering new trends in the complexes' ferrotoroidic/antiferrotoroidic coupling, I describe the first toroido-structural correlations ever to be reported. I then simulate the dynamical magnetisation of these complexes, and by analysing their Zeeman spectra and various relaxation mechanisms, I suggest how to improve their magnetic hysteresis. Much work still needs to be done to explore the fundamental properties of nanomagnets with toroidal ground states, particularly after these discoveries of toroido-structural correlations, and non-dipolar toroidal moments in ground-energy manifolds of triangles with zero/weak spin-orbit coupling. Thus, I outline the next steps towards implementing toroidal moments in nanotechnology, and end this thesis with a summary of the progress made.

The three-dimensional structure of proteins and receptor binding sites requires small molecule drug candidates with complex molecular shapes, to enable greater receptor specificity and potency. A logical strategy to increase conformational complexity in small molecule drug candidates is to incorporate sp3 hybridised centres into the carbon structural framework. However, the reliance on sp2-sp2 cross coupling methodologies in medicinal and high throughput combinatorial chemistry has resulted in chemical libraries that are dominated by planar molecules. Consequently, the development of new methods to forge new C(sp3)-C bonds and rapidly access conformationally complex molecules is vital to accelerating contemporary drug discovery. This thesis aims to address this challenge in the chemical and medical sciences, by focussing on the development of novel methodologies to access C(sp3)-C bonds. To achieve this, attention is placed on two possible approaches to synthesise saturated carbon skeletons; namely, transition metal catalysed cross coupling and alkene hydrofunctionalisation (including hydrogenation) reactions enabled by visible light mediated catalysis. In chapter one, transition metal catalysed cross coupling of sp3 fragments and alkene hydrofunctionalisation is introduced and their key limitations identified. Next, strategies that have been employed to overcome these limitations are discussed. Specifically, the use of visible light photoredox catalysis has shown to be effective in affording distinct reactivity and selectivity in these reactions, compared to thermal approaches. Finally, the research focus and aims of this thesis are elaborated. Chapter 2 builds on a recent discovery by the Polyzos laboratory, which established that cyclopalladated complex consisting of 8-aminoquinoline auxiliary exhibits a long-lived excited state capable of undergoing light induced single electron transfer with alkyl halides and providing access to Pd(II)/Pd(III)/Pd(IV) redox couples. The use of this complex to achieve light induced C(sp3)-H cross coupling is investigated. Furthermore, an investigation is conducted to establish whether other cyclopalladated complexes similarly display long-lived excited states that can be harnessed for C-C cross coupling. During reaction optimisation, it was found that extensive decomposition of cyclopalladated complex occurs, following single electron transfer to aryl halides, likely due to the formation of hybrid aryl palladium(III) radical pairs. Chapter 3 describes the development of a novel C(sp3)-C cross coupling methodology using energy transfer catalysis. Specifically, the energy of excited state cyclopalladated complex consisting of 8-aminoquinoline auxiliary is transferred to polyaromatic hydrocarbons, to enable the reduction of energy demanding aryl bromides and prevent the decomposition pathways observed in chapter 2. The resulting aryl radical is trapped by ground state cyclopalladated complex with subsequent reductive elimination affording a C(sp3)-arylated product in excellent yield. In chapter 4, a novel mode of alkene activation is developed via photoredox catalysed single electron reduction. This is achieved using the multiphoton, tandem catalytic mechanism of heteroleptic iridium photocatalyst, [Ir(ppy)2dtbbpy]PF6, discovered in the Polyzos group. The reactivity of the resulting olefinic radical anion is effectively controlled to achieve a formal hydrogenation of 1,1-diarylethylenes and substituted styrenes. Mechanistic studies including luminescence quenching, deuterium labelling and radical clock experiments indicate a Birch-type reduction as the major reaction pathway. In chapter 5, an unexpected aminomethylated by-product observed in chapter 4 is optimised to establish the first photoredox catalysed aminomethylation of electron rich alkenes (1,1-diarylethylenes). During this, the ability of other heteroleptic iridium photocatalysts to engage in the tandem catalytic paradigm is investigated, to expand the reaction scope to include unprotected secondary amines. Mechanistic studies indicate that the single electron activation of both alkene and amine coupling partners, via the multiphoton tandem catalytic system, is vital for aminomethylation to occur. Chapter 6 details the experimental procedures used throughout this thesis, including spectroscopic data of all isolated compounds.

Regular monitoring of heavy metal ions concentrations in aquatic systems is essential to determine their availability to the environment and evaluate their potential risk due to their toxicity to aquatic organisms. The passive sampling based on the use of a polymer inclusion membrane (PIM) as the semipermeable membrane barrier between the sampled medium and a liquid receiving phase has been recently introduced as a promising analytical tool, not only due to the superior stability of PIMs compared to supported liquid membranes, but also because it contains a liquid receiving phase which can be analysed directly, thus simplifying its processing prior to analysis. However, passive sampling based on PIMs is relatively new and limited research has been directed towards overcoming the issues associated with the effect of environmental variables on the performance of such passive samplers. Hence, the research described in this thesis aims at further developing PIM-based passive sampling devices for Zn2+ as the model analyte. This research includes the development of a 3D-printed flow-through passive sampling device (PSD), and the study of the influence of several environmental factors (i.e., flow pattern, water matrix, and temperature) on the accumulation of Zn2+ ions, and the assessment of strategies to either minimize or eliminate their effects on the estimated time-weighted average (TWA) Zn2+ concentration.

This work focuses on the measurement and analysis of gas-phase electronic spectra of C4H2+, C4H4+, C4H5+, and C6H4+ cations. The spectra are recorded by photodissociating the cations, or their messenger tagged complexes, in a tandem mass spectrometer and monitoring the photofragment signal as a function of laser wavelength. Ultimately, structural and energetic information extracted from these spectra, complemented by quantum chemical calculations, may provide insights into the roles of cations in the chemistry of flames, plasmas, and extraterrestrial environments. Understanding the vibronic structures of the cations is also fundamental for facilitating their possible detection in remote environments. Electronic spectra of C4H2+-Ar_n (n = 1-3) and C4H2+-(N2)_n (n = 1-2) complexes are recorded over the 290-530 nm range by monitoring C4H2+ photofragments. Intense narrow bands in the visible region arise from the A2(Pi)u<--X2(Pi)g electronic transition of the diacetylene cation, HC4H+. Vibronic transitions are compared with previous results and theoretical predictions for the A2(Pi)u<--X2(Pi)g band system based on calculations at the CCSD/cc-pCVTZ and EOM-CCSD/cc-pCVTZ levels of theory. Hole burning experiments confirm that the weak, broad bands in the ultraviolet region also correspond to excitations of HC4H+. These bands are assigned to the 22(Pi)u<--X2(Pi)g electronic transition based on calculated energies and oscillator strengths, previous experiments, and spectra of isoelectronic molecules. The origin transition is observed at 29723 cm-1 (336.44 nm) for HC4H+-Ar, followed by a progression in the symmetric C-C stretch vibration, spaced by 906 cm-1. Spectral congestion observed toward shorter wavelengths is possibly due to the presence of close-lying electronic states, vibronic coupling effects, and HC4H+ being bent in the 22(Pi)u electronic state. Electronic spectra of Ar- and N2-tagged C4H4+ cations, generated from acetylene ion-molecule reactions, are recorded over the 296-550 nm range by monitoring C4H4+ and C4H3+/C4H2+ photofragments. The spectra exhibit a clearly resolved band system commencing at 511 nm, a set of weak bands around 410 nm, and an intense broad band in the UV. Band assignments are based on previous spectroscopic studies and calculations for the four lowest energy C4H4+ isomers. The visible region of the spectrum is dominated by the B2A’’<--X2A’’ electronic transition of the vinylacetylene cation (VA+) whose origin transition occurs 19558 cm-1 (511.30 nm) for VA+-Ar and at 19551 cm-1 (511.48 nm) for VA+-N2. The spectrum features a strong progression in the central C-C stretching vibration spaced by approximately 806 cm-1 and well defined vibronic transitions associated with two bending vibrational modes. The weaker 410 nm system is assigned to the D2B3<--X2B2 electronic transition of the butatriene cation (BT+). Two broad bands, following the origin transition at 24399 cm-1 (409.85 nm) in the C4H4+-Ar spectrum, arise from excitation of a symmetric C=C stretching vibration and possibly the CH2 twisting vibration. The observation of C4H4+ and C4H3+ /C4H2+ photofragments following excitation of BT+-Ar complexes over the 380-410 nm range accords with calculated dissociation pathways on the ground state C4H4+ potential energy surface. The broad band appearing at wavelengths below 320 nm is predicted to be due to electronic transitions of all four of the lowest energy C4H4 + isomers, including the VA+ and BT+ cations. The B2A’<--X2A’ electronic spectrum of the 1-butyn-3-yl cation (H3CCHCCH+) is recorded over the 245-285 nm range by photodissociating the bare cation and its Ne and Ar-tagged complexes. Origin transitions for H3CCHCCH+ and H3CCHCCH+-Ne are observed at 35936 cm-1 (278.27 nm) and 35930 cm-1 (278.32 nm), respectively. Additional bands in the spectra are assigned to vibronic transitions of H3CCHCCH+ through quantum chemical calculations and comparison of the spectra with those of similar molecules, the propargyl (C3H3+) and methyl propargyl (H3C4H2+) cations. Excitation of H3CCHCCH+ produces C2H3+ +C2H2 (protonated acetylene + acetylene) and C4H3+ +H2 (protonated diacetylene + hydrogen molecule) photofragments. The preferred formation of C2H3+ +C2H2 photofragments is correctly predicted by master equation simulations, generated from calculated dissociation pathways on the ground state C4H5+ potential energy surface. Electronic spectra of Ar- and N2-tagged C6H4+ cations, generated from acetylene ion-molecule reactions, are recorded over the 265-700 nm range by monitoring C6H4+ and C4H2+ photofragments. Bands are assigned to transitions of five isomers based on previous spectroscopic studies, calculated stationary points on the ground state C6H4+ potential energy surface, hole burning experiments, and spectral simulations generated from TD-DFT calculations. In the C6H4+-Ar spectrum, bands over the 526-700 nm range arise from the B2A’’<-- X2A’’ electronic transition of the 1-hexene-1,3-diyne cation (CH2CHCCCCH+) and the C2Bg <-- X2Au electronic transition of the trans-3-hexene-1,5-diyne cation (HCCCHCHCCH+), with origin transitions occurring at 604 and 581 nm, respectively. Over the 375-460 nm range, broad bands at 440, 425, and 423 nm are assigned to vibronic transitions within the C2A’’<-- X2A’’ band system of the 1-ethynyl-3-methylene-cyclopropa-1,2-diene cation (H2C[c –C3H]CCH+). Sharper bands at 416, 407, and 400 nm are ascribed to the B2A’’<-- X2A’’ electronic transition of the propargyl cyclopropene cation ([c –C3H2]CHCCH+). Between 265 and 375 nm, several bands appear, superimposed on a broad band that extends for several thousand wavenumbers. The 371 nm band is assigned to the B2A’’<-- X2A’’ origin transition of the 1-hexene-1,3-diyne cation (CH2CHCCCCH+), agreeing with previous assignments. In light of the TD-DFT calculations, the bands at 365 and 357 nm are tentatively reassigned to the C2B<--X2B electronic transition of the propadienylidene cyclopropene ([c –C3H2]CCCH2+) isomer. The underlying band possibly arises from the C2Bg <-- X2Au electronic transition of the trans-3-hexene-1,5-diyne (HCCCHCHCCH+) cation, which undergoes a reduction in symmetry upon photoexcitation. Formation of C4H2+ photofragments at wavelengths below 426 nm (2.91 eV) agrees with calculated C6H4+ dissociation pathways on the ground state C6H4+ potential energy surface.