Browsing by Subject "DFT"

Now showing 1 - 3 of 3
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    Open Access
    A computational study of acidic Ionic Liquids for cellobiose hydrolysis in ionic liquids
    (2019) Nel, Jessica Lisé; Venter, Gerhard A
    The current environmental situation, with respect to global warming and the ever– approaching depletion of fossil fuel sources, places significance on the development of green fuel and platform chemical production methods. In this context, processes that utilise biomass sources as feedstock, are of great interest. Cellulose, which is the most abundant biopolymer in nature, is a renewable low–cost carbon resource derived from harvest residues and sources like wood and straw. Glucose generation from cellulose requires a saccharide conversion, whereby the β-(1,4)-glycosidic bond linkages in the cellobiose polymer repeating units are cleaved. Problems arise in the hydrolysis of cellulose as experimental and theoretical studies have shown cellulose to have very low solubility in water and most other general molecular solvents. This results in the use of harsh pretreatments at high temperatures and pressures to extract cellulose from lignocellulosic material and strong acids catalysts (pKa < −3.2). Room temperature ionic liquids (RTILs) provide potentially environmentally friendly alternative. It has been shown that ILs can dissolve cellulose under relatively benign conditions and can possibly be adapted into a one-pot-like process of hydrolysis using acid-functionalised IL catalysts. This dissertation investigated the effect of various ionic liquids on the thermodynamics of cellobiose acid hydrolysis, as both a catalyst and as a solvent, using computational means. An appropriate thermodynamic cycle protocol, a DLPNO-CCSD(T)/ccpVTZ//TPSS/def2-TZVP [M05-2X/6-31+G** (SMD)] proton exchange cycle, was established through benchmarking for the prediction of Brønsted acid-functionalised ionic liquid pKa values in ionic liquids. The sulfonyl-functionalised acidic IL was shown to be the most acidic IL resulting in a lower protonation free energy. Solvation in ionic liquids resulted in higher protonation and barrier height free energies relative to solvation in water. The current environmental situation, with respect to global warming and the ever– approaching depletion of fossil fuel sources, places significance on the development of green fuel and platform chemical production methods. In this context, processes that utilise biomass sources as feedstock, are of great interest. Cellulose, which is the most abundant biopolymer in nature, is a renewable low–cost carbon resource derived from harvest residues and sources like wood and straw. Glucose generation from cellulose requires a saccharide conversion, whereby the β-(1,4)-glycosidic bond linkages in the cellobiose polymer repeating units are cleaved. Problems arise in the hydrolysis of cellulose as experimental and theoretical studies have shown cellulose to have very low solubility in water and most other general molecular solvents. This results in the use of harsh pretreatments at high temperatures and pressures to extract cellulose from lignocellulosic material and strong acids catalysts (pKa < −3.2). Room temperature ionic liquids (RTILs) provide potentially environmentally friendly alternative. It has been shown that ILs can dissolve cellulose under relatively benign conditions and can possibly be adapted into a one-pot-like process of hydrolysis using acid-functionalised IL catalysts. This dissertation investigated the effect of various ionic liquids on the thermodynamics of cellobiose acid hydrolysis, as both a catalyst and as a solvent, using computational means. An appropriate thermodynamic cycle protocol, a DLPNO-CCSD(T)/ccpVTZ//TPSS/def2-TZVP [M05-2X/6-31+G** (SMD)] proton exchange cycle, was established through benchmarking for the prediction of Brønsted acid-functionalised ionic liquid pKa values in ionic liquids. The sulfonyl-functionalised acidic IL was shown to be the most acidic IL resulting in a lower protonation free energy. Solvation in ionic liquids resulted in higher protonation and barrier height free energies relative to solvation in water.
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    Open Access
    Synthesis and characterisation of silver(I) halo-pyridyl compounds: a study of the prominent NCIs that govern crystal packing
    (2023) Theunissen, Tristan; Bourne, Susan; Esterhuysen, Catharine
    This study aimed to identify which non-covalent interactions (NCIs) would dominate in the solid-state structures of silver(I) halopyridyl compounds. Five novel compounds were synthesised and several metal involved NCIs were observed. The five compounds which formed a cohesive series are: [Ag2(OBn)2], [Ag2(2 Pic)2(OBn)2], [Ag2(3-Clpy)2(OBn)2], [Ag2(2-Brpy)2(OBn)2], and [Ag2(2-Ipy)2(OBn)2]. X-ray diffraction analysis of single crystals constituted the solid-state study portion of the NCIs. Their influence on the crystal structure was carried out using modern computational techniques, such as molecular electrostatic surface potential (MESP), natural bond orbital or NBO analysis, and quantum theory of atoms in molecules (QTAIM). This dissertation comprises four sections, the first of which introduces background knowledge on particular topics such as crystal engineering, non-covalent interactions, and possible applications that NCIs have within crystal engineering and supramolecular chemistry. The second and third sections discuss specific methods and techniques used to study the NCIs from a crystallographic and computational viewpoint. The final section summarises the contribution of the authors' work to the understanding and body of knowledge of NCIs. As shown in this dissertation, the ‘self-assembled' [Ag2(OBn)2(Xpy)2] compounds are closely related to their strongest non-covalent metal-involved interactions, such as AgI···π, AgI···X, and AgI···AgI, while also being influenced by weaker interactions', π···π, cooperativity effects. The importance of orbital-based charge transfers as opposed to being purely electrostatic in the argentophilicity of compounds I-V has been discussed. In this study, quantitative QTAIM, NBO, and MEPS-based analyses were carried out on the metallophilic (M+···M+) interactions. The computational studies suggest a greater role of orbital-based interactions in the strength and distance of said interactions. This hypothesis was applied to other NCIs, such as semi-coordination and metal-π NCIs, in which orbital-based charge-transfers are also shown to predominate force stabilising the compounds over pure electrostatic interactions. Compounds VI-VIII, although not included within the main series (I-V) display a variety of similar NCIs, as evident from their single-crystal structures, whose natures or characteristics are formed from the foundations of computations I-V. Prominent NCIs are AgI···π, AgI···X, and AgI···AgI, which are also influenced by weaker interactions', π···π, cooperativity effects.
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    Theoretical feasibility of CO-activation and Fischer–Tropsch chain growth on mono- and diatomic Ru complexes
    (Elsevier, 2008) Welker, Cathrin; Phala, Noko S; Moss, John R; Claeys, Michael; van Steen, Eric
    Thermodynamic analyses of different proposed reaction pathways to determine the thermodynamic feasibility of FT reactions on a mono- and diatomic Ru-complex as model catalysts were performed. Ru(CO)5 and Ru2(CO)9 were taken as starting complexes. The calculations illustrate that a minimum of two adjacent metal atoms is required for C O bond cleavage and chain growth in the Fischer–Tropsch synthesis. The CO-insertion mechanism seems to be thermodynamically most feasible reaction pathway on diatomic Ru-clusters.