Hendrik G. Visser

Bio: Hendrik G. Visser is an academic researcher from University of the Free State. The author has contributed to research in topics: Denticity & Ligand. The author has an hindex of 17, co-authored 162 publications receiving 1312 citations.

Synthesis of strong red emitting Y2O3:Eu3+ phosphor by potential chemical routes: comparative investigations on the structural evolutions, photometric properties and Judd–Ofelt analysis

Abstract: In the present paper, a comparative investigation on the structural and photoluminescence properties of Y2O3:Eu3+ phosphor prepared by different wet chemical synthesis routes such as sol-lyophilisation (SL), combustion (CR), hydrothermal (HT) and microwave assisted hydrothermal combustion (MHWC), has been reported for the first time. The MHWC derived phosphor exhibited better photoluminescence than that of the samples obtained with other adopted methods. Such outcomes were due to the increased crystallinity, well defined morphology and improved compositional homogeneity in the MWHC technique. The growth of the prepared phosphors was explained in the light of chemical kinetics. Jorgensen formula and nephelauxetic ratio was used to understand the ligand behavior of Eu–O bond and to estimate the electron phonon coupling in Y2O3:Eu3+ phosphor. The spectroscopic behavior of Y2O3:Eu3+ phosphor, prepared by different routes, was determined using Judd–Ofelt theory. Thermal stability, purity and efficiency of the emitted colour were checked on the basis of different synthetic approach. An efficient synthesis method for Y2O3:Eu3+ phosphor, compatible for industrial appliances, was proposed.

phosphor by potential chemical routes: comparative investigations on the structural evolutions, photometric properties and Judd-Ofelt analysis

Abstract: In the present paper, a comparative investigation on the structural and photoluminescence properties of Y2O3:Eu3+ phosphor prepared by different wet chemical synthesis routes such as sol-lyophilisation (SL), combustion (CR), hydrothermal (HT) and microwave assisted hydrothermal combustion (MHWC), has been reported for the first time. The MHWC derived phosphor exhibited better photoluminescence than that of the samples obtained with other adopted methods. Such outcomes were due to the increased crystallinity, well defined morphology and improved compositional homogeneity in the MWHC technique. The growth of the prepared phosphors was explained in the light of chemical kinetics. Jorgensen formula and nephelauxetic ratio was used to understand the ligand behavior of Eu–O bond and to estimate the electron phonon coupling in Y2O3:Eu3+ phosphor. The spectroscopic behavior of Y2O3:Eu3+ phosphor, prepared by different routes, was determined using Judd–Ofelt theory. Thermal stability, purity and efficiency of the emitted colour were checked on the basis of different synthetic approach. An efficient synthesis method for Y2O3:Eu3+ phosphor, compatible for industrial appliances, was proposed.

Tuning the Reactivity in Classic Low-Spin d6 Rhenium(I) Tricarbonyl Radiopharmaceutical Synthon by Selective Bidentate Ligand Variation (L,L′-Bid; L,L′= N,N′, N,O, and O,O′ Donor Atom Sets) in fac-[Re(CO)3(L,L′-Bid)(MeOH)]n Complexes

Abstract: A range of fac-[Re(CO)3(L,L′-Bid)(H2O)]n (L,L′-Bid = neutral or monoanionic bidentate ligands with varied L,L′ donor atoms, N,N′, N,O, or O,O′: 1,10-phenanthroline, 2,2′-bipydine, 2-picolinate, 2-quinolinate, 2,4-dipicolinate, 2,4-diquinolinate, tribromotropolonate, and hydroxyflavonate; n = 0, +1) has been synthesized and the aqua/methanol substitution has been investigated. The complexes were characterized by UV–vis, IR and NMR spectroscopy and X-ray crystallographic studies of the compounds fac-[Re(CO)3(Phen)(H2O)]NO3·0.5Phen, fac-[Re(CO)3(2,4-dQuinH)(H2O)]·H2O, fac-[Re(CO)3(2,4-dQuinH)Py]Py, and fac-[Re(CO)3(Flav)(CH3OH)]·CH3OH are reported. A four order-of-magnitude of activation for the methanol substitution is induced as manifested by the second order rate constants with (N,N′-Bid) < (N,O-Bid) < (O,O′-Bid). Forward and reverse rate and stability constants from slow and stopped-flow UV/vis measurements (k1, M–1 s–1; k–1, s–1; K1, M–1) for bromide anions as entering nucleophile are as follows: fac-[R...

Steric vs. electronic anomaly observed from iodomethane oxidative addition to tertiary phosphine modified rhodium(I) acetylacetonato complexes following progressive phenyl replacement by cyclohexyl [PR3 = PPh3, PPh2Cy, PPhCy2 and PCy3]

Abstract: Rhodium(I) acetylacetonato complexes of the formula [Rh(acac)(CO)(PR3)] [acac = acetylacetonate, PR3 = PPh31, PCyPh22, PCy2Ph 3, PCy34] were synthesized and the iodomethane oxidative addition to these complexes were studied. Spectroscopic and low temperature (100 K) single crystal X-ray crystallographic data of the rhodium complexes (1–4) indicate a systematic increase in both steric and electronic parameters of the phosphine ligands as phenyl groups on the tertiary phosphine are progressively replaced by cyclohexyl groups in the series. Second order rate constants for the alkyl formation in the oxidative addition of iodomethane in dichloromethane at 25 °C vary with approximately one order-of-magnitude from 6.98(6) × 10−3 M−1s−1 (PCyPh22) to 55(1) × 10−3 M−1 s−1 (PCy2Ph 3) and do not follow the expected electronic pattern from 1 to 4, which indicates a flexibility of the cyclohexyl group, significantly influencing the reactivity. Activation parameters for the reactions range from 35(3) to 44(1) kJ mol−1 for ΔH≠ and −140(5) to −154(9) J K−1 mol−1 for ΔS≠, and are supporting evidence for an associative activation for the oxidative addition step.

Structure/reactivity relationships and mechanism from X-ray data and spectroscopic kinetic analysis

Abstract: The development in crystallography is nothing short of phenomenal, showing an ever increasing trend towards applications, such as in molecular biology. ‘Traditional’ small molecule chemical crystallography tends to, in many aspects, be considered as trivial due to the exponential growth in computing power and the parallel expansion in software. However, many dynamic processes (still) occur at the molecular level. Thus, the fundamental understanding of subtle nuances in structural behaviour, and the associated influence on other (kinetic) properties, is often trivialized when conclusions are simply made based on thermodynamic observations alone. This review aims to emphasize the importance of crystallography in small molecule chemistry by presenting detailed evaluations of two extended ‘case studies’, i.e. the first on structure and dynamics of the middle transition metal cyanide-oxido complexes, and the second on rhodium model homogeneous catalyst precursors. Both underline the fact of understanding the o...

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Chemistry of materials

Abstract: I am really glad to have this opportunity to write to you, specially about a subject in which I have worked for half a century. When I was your age, if somebody had told me that I would be working in chemistry of materials most of my life, I would not have believed it. At that time, chemistry of materials meant studying something about cement, steel, sand and asbestos. It was indeed dull. I never didwell in school and college exams on questions related to this subject. Much later in my life, I got greatly interested in the subject for various reasons. First, inmy study of chemistry, I was influenced by Linus Pauling who ismy academic grandfather. His book titled ‘Nature of the Chemical Bond’ which I read when I was young made a great impression. It taught me how the structure of molecules and materials is an extremely important aspect of chemistry, and how the understanding of structure and bonding provides a basis to understand chemistry and to do new chemistry. It is because of this terrific inspiration that I started studying chemistry. It was clear at the end of my undergraduate career that I wanted to be a chemist.

Piet W. N. M. Van Leeuwen. Homogeneous catalysis—understanding the art. Kluwer, Dordrecht, 2004, 407 pp; (UK). ISBN 1‐4020‐1999‐8

Abstract: Preface.- Acknowledgements.- 1: Introduction.- 1.1. Catalysis. 1.2. Homogeneous catalysis. 1.3. Historical notes on homogeneous catalysis. 1.4. Characterization of the catalyst. 1.5. Ligand effects. 1.6. Ligands according to donor atoms. 2: Elementary Steps.- 2.1. Creation of a 'vacant' site and co-ordination of the substrate. 2.2. Insertion versus migration. 2.3. beta-Elimination and de-insertion. 2.4. Oxidative addition. 2.5. Reductive elimination. 2.6. alpha-Elimination reactions. 2.7. Cycloaddition reactions involving a metal. 2.8. Activation of a substrate toward nucleophilic attack. 2.9. sigma-Bond metathesis. 2.10. Dihydrogen activation. 2.11. Activation by Lewis acids. 2.12. Carbon-to-phosphorus bond breaking. 2.13. Carbon-to-sulfur bond breaking. 2.14. Radical reactions. 3: Kinetics.- 3.1. Introduction. 3.2. Two-step reaction scheme. 3.3. Simplifications of the rate equation and the rete-determining step. 3.4. Determining the selectivity. 3.5. Collection of rate data. 3.6. Irregularities in catalysis. 4: Hydrogenation.- 4.1. Wilkinson's catalyst. 4.2. Asymmetric hydrogenation. 4.3. Overview of chiral bidentate ligands. 4.4. Monodentate ligands. 4.5. Non-linear effects. 4.6. Hydrogen transfer. 5: Isomerisation.- 5.1. Hydrogen shifts. 5.2. Asymmetric isomerisation. 5.3. Oxygen shifts. 6: Carbonylation of Methanol and Methyl Acetate.- 6.1. Acetic acid. 6.2. Process scheme Monsanto process. 6.3. Acetic anhydride. 6.4. Other systems. 7: Cobalt Catalysed Hydroformylation.- 7.1. Introduction. 7.2. Thermodynamics. 7.3. Cobalt catalysed processes. 7.4. Cobalt catalysed processes for higher alkenes. 7.5. Kuhlmann cobalt hydroformylation process. 7.6. Phosphine modified cobalt catalysts: the shell process. 7.7. Cobalt carbonyl phosphine complexes. 8: Rhodium Catalysed Hydroformylation.- 8.1. Introduction. 8.2. Triphenylphosphine asthe ligand. 8.3. Diphosphines as ligands. 8.4. Phosphites as ligands. 8.5. Diphosphites. 8.6. Asymmetric hydroformylation. 9: Alkene Oligomerisation.- 9.1. Introduction. 9.2. Shell-higher-olefins-process. 9.3. Ethene trimerisation. 9.4. Other alkene oligomerisation reactions. 10: Propene Polymerisation.- 10.1. Introduction to polymer chemistry. 10.2. Mechanistic investigations. 10.3. Analysis by 13CNMR spectroscopy. 10.4. The development of metallocene catalysts. 10.5. Agostic interactions. 10.6. The effect of dihydrogen. 10.7. Further work using propene and other alkenes. 10.8. Non-metallocene ETM catalysts. 10.9. Late transition metal catalysts. 11: Hydrocyanation of Alkenes.- 11.1. The adiponitrile process. 11.2. Ligand effects. 12: Palladium Catalysed Carbonylations of Alkenes.- 12.1. Introduction. 12.2. Polyketone. 12.3. Ligand effects on chain length. 12.4. Ethene/propene/CO terpolymers. 12.5. Stereoselective styrene/CO terpolymers. 13: Palladium Catalysed Cross-Coupling Reactions.- 13.1. Introduction. 13.2. Allylic reaction. 13.3. Heck reaction. 13.4. Cross-coupling reaction. 13.5. Heteroatom-carbon bond formation. 13.6. Suzuki reaction. 14: Epoxidation.- 14.1. Ethene and propene oxide. 14.2. Asymmetric epoxidation. 14.3. Asymmetric hydroxilation of alkenes with osmium tetroxide. 14.4. Jacobsen asymmetric ring-opening of epoxides. 14.5. Epoxidations with dioxygen. 15: Oxydation with Dioxygen.- 15.1. Introduction. 15.2. The Wacker reaction. 15.3. Wacker type reactions. 15.4. Terephthalic acid. 15.5. PPO. 16: Alkene Metathesis.- 16.1. Introduction. 16.2. The mechanism. 16.3. Reaction overview. 16.4. Well-characterised tungsten and molybdenum catalysts. 16.5. Ruthenium catalysts. 16.6. Stereochemistry. 16.7. Catalyst decomposition. 16.8. Alkynes. 16.9. Industrial applications. 17: Enantioselective Cyclopropanation.-