Penny P. Govender

DFT studies of trans and cis influences in the homolysis of the Co-C bond in models of the alkylcobalamins.

Density functional theory (DFT) calculations (BP86/6-31+G(d,p)) and an analysis of the electron density using Baders quantum theory of atoms in molecules (QTAIM) are used to explore factors that influence the bond dissociation energy (BDE) of the Co-C bond in models for the cofactor in the coenzyme B12-dependent enzymes. An increase in the basicity of L in [L-Co(III)(corrin)-CH3](n+), L = NH3, NH2(-), and NH(2-), causes an elongation of the trans Co-C bond, but this does not necessarily cause the BDE to decrease. The bond between the metal and the N-donor of L, Co-Nα, usually becomes shorter after Co-C homolysis as the resulting five-coordinate product permits the metal ion to move toward L. This contraction increases with the basicity of L and stabilizes the five-coordinate product. The BDE is found to correlate well with two variables, the basicity of L and the difference in the Co-Nα bond length between the five-coordinate product and the six-coordinate ground state. When L is a naturally occurring amino acid or a model for its metal-coordinating side chain, the BDE is found to be moderately dependent on L and decrease with an increase in the softness of the donor atom of L. Sulfides produce a BDE < 30 kcal mol(-1), whereas neutral alcohol donors produce a stronger Co-C bond with a BDE of 34-35 kcal mol(-1). All other ligands are associated with a trans Co-C bond that is almost invariant in strength and with a BDE of 31-33 kcal mol(-1). Models of the type [H3N-Co(III)(N4)-CH3](n+), where N4 = bis(dimethylglyoxime), porphyrin, corrin, and corrole, show that the nature of the tetraaza equatorial ligand can change BDE values by over 8 kcal mol(-1); the BDE when N4 = bis(dimethylglyoxime) is significantly larger than for the other three systems, among which differences in BDE are quite small (2.4 kcal mol(-1)). The differential stabilization of the five-coordinate product by the shrinking of the Co-Nα bond (in corrin and in corrole) or its elongation (in porphyrin and in bis(dimethylglyoxime)) is an important factor in determining the BDE of these systems. Corrin has the longest and weakest Co-C bond; this, together with a significant contraction of the Co-Nα after homolysis, is likely to be the origin of its relatively low BDE.

cis influence in models of cobalt corrins by DFT and TD-DFT studies.

Time-dependent density-functional theory and density-functional theory are applied to study the cis influence of the equatorial macrocycle in vitamin B(12) derivatives. A series of dicyanocobalt corrinoids, CN-[Co(III)-corrin]-CN, where the C(10)H of the corrin ring is replaced by different substituents, X, is considered. The calculated UV-visible absorption spectra, the charge distribution obtained from a Bader QTAIM analysis of the electron density, the CN stretch frequencies of the axial cyano ligands and the electron densities at some bond critical points are compared. The main absorption bands in the UV-visible spectra depend on the electron donating or withdrawing power of X, as assessed from its Hammett σ(p) constants. For X with a stronger electron donating power than H, the other properties do not change appreciably. However, when σ(p)(X) > σ(p)(H), these properties vary linearly with the electron withdrawing power of the substituent. This helps explain the experimental observation that substitution of the axial ligand is more difficult and proceeds more slowly with the increase of the electron withdrawing power of the C(10) substituent.

Understanding the mechanism of enhanced charge separation and visible light photocatalytic activity of modified wurtzite ZnO with nanoclusters of ZnS and graphene oxide: from a hybrid density functional study

Recently, semiconductor photocatalysts have received significant interest in addressing the global energy and environmental crisis. Despite the great potential application of zinc oxide (ZnO), the utilisation efficiency of solar energy is limited only to the ultraviolet (UV) region. Therefore, our research interest is centred on the development of a model hybrid ZnO-based photocatalyst material with efficient photocatalytic performance and stability. A hybrid ternary ZnS/graphene oxide (GO)/ZnO system, where the band edges of the individual components in the heterostructure will have a step-wise structure for harvesting a broader portion of the solar spectrum, is an excellent solution. Herein, we explore the charge transfer, the improved photocatalytic mechanism, and the electronic and interfacial properties of the hybrid van der Waals (vdWs) ZnS/GO/ZnO heterostructure for the first time by carrying out comprehensive hybrid density functional theory calculations. Our results reveal the existence of vdWs interaction, enhance charge transfer, narrow the band gap and show remarkable improvement in the visible light photocatalytic activity of the heterostructures compared to pure ZnO. This enhancement is ascribed to the electron acceptor–transporter role played by the GO sheet in the interfacial layer of ZnS and ZnO. According to adhesion energy results, the monolayers are in contact and form stable heterostructures. The calculated charge density difference shows interlayer charge transfer from the ZnO(001) surface to the ZnS(110) surface through the GO sheet. Most significantly, the ZnS/GO/ZnO heterostructures exhibit a type-II band alignment, which significantly restrains the recombination of charge carriers. The band edge positions of ZnS/GO/ZnO with enough driving force for electron and hole transfer are well aligned for the feasibility of splitting water into H2 and O2, as well as for photodegrading pollutants in the water system. Therefore, the vdWs ZnS/GO/ZnO heterostructures appear as a new type of photocatalyst material for solar energy application. This study illustrates the usefulness of using low cost GO as an interfacial electron acceptor–donor to boost the photocatalytic performance of ZnO-based photocatalysts. The findings in this study provide a theoretical basis for developing highly efficient ZnO-based photocatalysts, as well as attract broad interest in VdWs heterostructure research in photocatalysis.

The Synthesis of a Corrole Analogue of Aquacobalamin (Vitamin B12a) and Its Ligand Substitution Reactions

The synthesis of a Co(III) corrole, [10-(2-[[4-(1H-imidazol-1-ylmethyl)benzoyl]amino]phenyl)-5,15-diphenylcorrolato]cobalt(III), DPTC-Co, bearing a tail motif terminating in an imidazole ligand that coordinates Co(III), is described. The corrole therefore places Co(III) in a similar environment to that in aquacobalamin (vitamin B12a, H2OCbl(+)) but with a different equatorial ligand. In coordinating solvents, DPTC-Co is a mixture of five- and six-coordinate species, with a solvent molecule occupying the axial coordination site trans to the proximal imidazole ligand. In an 80:20 MeOH/H2O solution, allowed to age for about 1 h, the predominant species is the six-coordinate aqua species [H2O-DPTC-Co]. It is monomeric at least up to concentrations of 60 μM. The coordinated H2O has a pKa = 9.76(6). Under the same conditions H2OCbl(+) has a pKa = 7.40(2). Equilibrium constants for the substitution of coordinated H2O by exogenous ligands are reported as log K values for neutral N-, P-, and S-donor ligands, and CN(-), NO2(-), N3(-), SCN(-), I(-), and Cys in 80:20 MeOH/H2O solution at low ionic strength. The log K values for [H2O-DPTC-Co] correlate reasonably well with those for H2OCbl(+); therefore, Co(III) displays a similar behavior toward these ligands irrespective of whether the equatorial ligand is a corrole or a corrin. Pyridine is an exception; it is poorly coordinated by H2OCbl(+) because of the sterically hindered coordination site of the corrin. With few exceptions, [H2O-DPTC-Co] has a higher affinity for neutral ligands than H2OCbl(+), but the converse is true for anionic ligands. Density functional theory (DFT) models (BP86/TZVP) show that the Co-ligand bonds tend to be longer in corrin than in corrole complexes, explaining the higher affinity of the latter for neutral ligands. It is argued that the residual charge at the metal center (+2 in corrin, 0 in corrole) increases the affinity of H2OCbl(+) for anionic ligands through an electrostatic attraction. The topological properties of the electron density in the DFT-modeled compounds are used to explore the nature of the bonding between the metal and the ligands.

Phenolic Compounds in Water: Sources, Reactivity, Toxicity and Treatment Methods

Phenolic compounds exist in water bodies due to the discharge of polluted wastewater from industrial, agricultural and domestic activities into water bodies. They also occur as a result of natural phenomena. These compounds are known to be toxic and inflict both severe and long‐lasting effects on both humans and animals. They act as carcino‐ gens and cause damage to the red blood cells and the liver, even at low concentrations. Interaction of these compounds with microorganisms, inorganic and other organic com‐ pounds in water can produce substituted compounds or other moieties, which may be as toxic as the original phenolic compounds. This chapter dwells on the sources and reactivity of phenolic compounds in water, their toxic effects on humans, and methods of their removal from water. Specific emphasis is placed on the techniques of their removal from water with attention on both conventional and advanced methods. Among these methods are ozonation, adsorption, extraction, photocatalytic degradation, biological, electro‐Fenton, adsorption and ion exchange and membrane‐based separation.

Role of MoS2 and WS2 monolayers on photocatalytic hydrogen production and the pollutant degradation of monoclinic BiVO4: a first-principles study

The global dependence on exhaustible fossil fuel resources has made the search for an alternative renewable and sustainable fuel more urgent. Photocatalysis has gained increasing consideration as a promising technology to solve problems associated with solar energy conversion. Fabricated m-BiVO4-based heterostructures have shown improved photocatalytic activity for hydrogen evolution and pollutant degradation; however, a deeper understanding of the photocatalytic mechanism and the role of the monolayers is still lacking. Moreover, no theoretical studies have been carried out on MS2/m-BiVO4(010) heterostructures. In the present study, the roles of MoS2 and WS2 monolayers loaded onto a m-BiVO4 surface for active photocatalytic hydrogen production and pollutant degradation are explored using first-principle studies. Herein, hybrid density functional calculations and a long-range dispersion correction method were used to investigate the charge transfer, electronic properties, photocatalytic activity and mechanism of the MS2/m-BiVO4(010) heterostructures. The results showed a narrow band gap, built-in potential and a type-II band alignment for the MS2/m-BiVO4(010) heterostructures compared to pure m-BiVO4, which favour the separation and transfer of charge carriers and visible-light-driven activity. The MoS2/m-BiVO4 heterostructure showed a suitable band edge for hydrogen production and pollutant degradation compared to the WS2/m-BiVO4 heterostructure. This improvement was attributed to the role of the MoS2 monolayer as an electron donor, the many reactive sites on the MoS2 surface and the enhanced electron/hole pair separation of charge carriers at the MoS2/m-BiVO4(010) interface. Considering that the MS2 monolayer coupled with m-BiVO4 can restrain the electron–hole recombination rate without lattice distortion indicates that the heterostructure approach is better than the doping approach. Based on the analysis of the electronic properties, the MS2/m-BiVO4(010) heterostructures were shown to fit within the acceptable band gap and built-in potential range. The proposed theoretical design paves a way for the effective and large-scale fabrication of m-BiVO4-based photocatalyst for solar energy conversion and environmental remediation applications.

Recent advances in titanium dioxide/graphene photocatalyst materials as potentials of energy generation

The properties of titanium dioxide (

The effects of two–dimensional TiSe2 on the thermoelectric, electronic and optical response of Yb14MnSb11/AlSb9Yb11 heterostructures – A theoretical study

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