School of Chemistry - Theses
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ItemExtrusion-Insertion Reactions for Organic Synthesis: a Mechanism-Based Approach to Transition-Metal Assisted Synthesis of Amides and Alkenes( 2022)The imperative to achieve highly efficient and environmentally benign chemical synthesis stimulates the endless exploration of new reactions. The isoelectronic nature of heterocumulene species has been known for a long time. Extrusion and insertion reactions are well-known elementary reactions in organometallic chemistry. However, few cases have combined these concepts to invent new procedures for use in organic synthesis. This thesis describes a mechanism-based approach to establish a series of new classes of extrusion-insertion (ExIn) reactions for use in organic synthesis. Multistage mass spectrometry (MSn) experiments, solution-phase experiments in which either NMR or GC-MS are deployed to monitor formation of products, and density functional theory (DFT) are used to underpin the subsequent synthetic method development. In Chapters 3 and 4, the role of organometallic intermediates in the palladium mediated extrusion of carbon dioxide from carboxylic acids followed by insertion of isocyanates or allenes was investigated in the gas phase via mass spectrometry experiments and DFT calculations. Subsequent condensed phase studies led to the development of one pot methods for the synthesis of amides or alkenes with moderate to good yield under mild conditions. In the case of allene insertion, a crystalline sample of the organopalladium intermediate was isolated and its structure was determined via X-ray crystallography, thereby confirming the regioselectivity of the inserted product. To extend the scope of ExIn reactions, in Chapters 5 and 6, copper(I) and silver(I) salts were used in amide synthesis via CO2 extrusion from carboxylic acids and isocyanate insertion. The initial gas-phase experiments showed the formation of an organometallic species via decarboxylation of copper or silver carboxylate ions. Only the organoargentate ion reacted with phenyl isocyanate. The silver mediated reaction between carboxylic acids and isocyanates successfully yielded amides in the condensed phase. However, mechanistic studies based on 13C labelled experiments, gas-phase studies and DFT calculations uncovered a new base-catalysed condensation mechanism as an alternative pathway, whereby the acyl group is transferred from the carboxylate to the N atom of an isocyanate molecule. In Chapter 7, desulfination was explored as an alternative to decarboxylation for the formation of the crucial organometallic intermediate. Thus, a new type of ExIn reaction using sulfinate salts and phenyl isocyanate in the presence of palladium(II) salts was explored. The initial gas phase experiments and DFT calculations were carried out on phenanthroline ligated palladium cations and revealed that the sulfinate more readily undergoes extrusion due to its flexible coordination mode. The translation of this desulfination ExIn variant from the gas phase reactions into condensed phase was not successful. This is due to the fact that a biaryl homocoupling side reaction is preferred, as supported by DFT calculations, which revealed that the barriers for this side reaction are below that of the insertion reaction.
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ItemDevelopment and Assessment of new Time-Dependent Long-Range Corrected Double-Hybrid Density Functionals for Excited States( 2021)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.
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ItemFinding the Best Ingredients of Density Functional Approximations for Ground-state Molecular Chemistry( 2021)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.