Scielo RSS <![CDATA[South African Journal of Chemistry]]> http://www.scielo.org.za/rss.php?pid=0379-435020110001&lang=en vol. 64 num. lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.za/img/en/fbpelogp.gif http://www.scielo.org.za <![CDATA[<b>Simulating the agostic interaction in electron-deficient (16-e) group (VI) ML6 complexes: [M(CO)5(C(Me)OMe)] (2+) (M = Cr, Mo, and W) as models</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100001&lng=en&nrm=iso&tlng=en A large number of theoretical studies have focused on understanding the molecular features of the agostic interaction in various kinds of molecular environments. However, there is a lack of electronic structure information about the agostic interaction in electron-deficient group (VI) ML6 organometallic complexes. In this simulation study, a unique case of an intramolecular stabilizing interaction has been discovered and evaluated. A geometric analysis revealed that beta-(C-H) and alpha-(C-C) can occupy the seventh and eighth coordination sites in the title Fischer carbene complexes as agostic interactions, which allows classifying the carbene as a η3 ligand in these cases. This theory was supported by the relative energies of the conformers and an NBO analysis. Both C2-C1 (σ) and C2-H1 (σ) were found to interact with the antibonding orbital of M-C6 (σ*), therefore these interactions are classified as σ→σ*. These two simultaneous interactions have significant impact on the carbene characteristics; the structure, the atomic charges, infrared stretching vibrations (C-H, C-C, and C-O), and the ¹H and the 13C-NMR chemical shifts. From a fundamental organic-organometallic chemistry point of view, this is a new addition to the orbital interaction theory and to group (VI) chemistry. <![CDATA[<b>Using PXRD to investigate the crystallization of highly concentrated emulsions of NH<sub>4</sub>NO<sub>3</sub></b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100002&lng=en&nrm=iso&tlng=en The process of crystallization of highly concentrated emulsions of ammonium nitrate can be studied using powder X-ray diffraction. The dispersed particles comprise a supercooled aqueous solution of the ammonium nitrate salt and are dispersed in a paraffin-based oil. This results in a thermodynamically unstable system that 'ages' with time resulting in changes in rheological properties and its phase composition where the collapse of the supercooled aqueous solution forms the crystallized salt. The crystallization processes of these emulsions are kinetically slow and can take up to a few months to crystallize completely. The general approach to this type of analysis is to determine the change in crystalline diffraction peak intensities relative to the halo due to the amorphous content. However, there are a number of problems associated with this method which are addressed by using Rietveld refinement methods which can take into account factors such as preferred orientation, crystallite size variations and mixtures of solid phases. The study showed that the ammonium nitrate emulsions kept at room temperature slowly crystallize predominantly to the room temperature solid ammonium nitrate phase IV. However, depending on the formulations used some samples showed crystallization to the high temperature ammonium nitrate phase II before changing to phase IV. The crystallization change could be modelled by the well-known JMAK kinetic relationship. <![CDATA[<b>Galvanostatic electrodeposition of Ni-Co alloys in DMSO under a magnetic field</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100003&lng=en&nrm=iso&tlng=en This paper focuses on the galvanostatic magneto-electrodeposition of Ni-Co alloys in dimethyl sulphoxide (DMSO) in the presence and absence of a permanent parallel magnetic field (PPMF) to the cathode surface. It was found that the mass deposition was enhanced in the presence of PPMF (9 T) compared with the deposition without PPMF. The percentage enhancement potential was elevated (ξ5% = 23.11, ξ2% = 10.65, ξ05% = 4.85) with current densities of 5,2 and 0.5 mA cm-2, respectively, in the presence of PPMF (9 T). Atomic force microscopy (AFM) showed that the roughness of the Ni-Co alloy films was reduced from 56.187 to 31.716 nm (at 0.2 mA cm-2) and 97.541 to 52.644 nm (at 0.5 mA cm-2) with applied PPMF (9 T) compared with that without the PPMF. The deposited layers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). <![CDATA[<b>Discrimination between O-H...N and O-H...O=C complexes of 3-methyl-4-pyrimidone and methanol. A matrix-isolation FT-IR and theoretical DFT/B3LYP investigation</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100004&lng=en&nrm=iso&tlng=en FT-IR matrix-isolated spectra for 3-methyl-4-pyrimidone and its H-bonded complexes with methanol in Ar were studied with the aim of discriminating between O-H...N and O-H...O=C complexes. Theoretical calculations were carried out using the DFT/B3LYP/6-31 + G(d) methodology in an attempt to predict the preferred interaction site of the 3-methyl-4-pyrimidone molecule with proton donors. The observed frequency decrease of the v(C=O) mode of 3-methyl-4-pyrimidone and the appearance of a broad v(OH...O) band in the spectrum of the complex with methanol suggest that H-bonding with methanol occurs at the carbonyl group. Computed binding energies of the hydrogen-bonded complexes (ΔEc) and computed intermolecular distances (r(O...H)) confirm that the O-H...O=C complex is preferred with methanol. However, for H-bonding with stronger acids such as HCl, the computational data suggest that the H-bonding occurs at the N1 ring atom of 3-methyl-4-pyrimidone. <![CDATA[<b>Some factors influencing distribution coefficients of trace amounts of M<sup>2</sup>+ ions during the crystallization of Mn(CH<sub>3</sub>COO)<sub>2</sub></b>•<b>4H<sub>2</sub>O</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100005&lng=en&nrm=iso&tlng=en Distribution coefficients, D2/1 (Henderson - Kracek, Khlopin) of trace amounts of Ni2+,Mg2+,Cu2+,Co2+,Zn2+,Cd2+,Ca2+, and Sr²+ have been determined during the crystallization of Mn(CH3COO)2∙4H2O. Their dependencies on several properties of co-crystallizing acetate hydrates M(CH3COO)2∙nH2O as well as co-crystallizing M²+ ions have been analyzed. The D2/1 values fall in the range from 0.011 (D Cu) to 0.28 (D Co) and indicate the possibility of effective removal of trace amounts of most investigated ions during the crystallization of Mn(CH3COO)2∙4H2O at 25 °C. They generally do not depend directly on most investigated factors, but those of Mg²+,Ni²+ and Co²+ ions, which form acetate tetrahydrates like Mn(CH3COO)2∙4H2O, closely depend on electronegativity of the elements as well as solubility and η parameter and may be estimated on the basis of the mentioned properties using simple formulae. <![CDATA[<b>Chemistry students' competence throughout their BSc course in some problem-solving strategies</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100006&lng=en&nrm=iso&tlng=en The main objective of the study was to test chemistry students' competence, in the first, second and final years of their BSc course, in some basic problem-solving strategies. Five strategies were tested: clarification and clear representation of problems; focusing sharply on the goal; identification and use of relevant principles; use of equations for calculations and deductions and use of a step-by-step procedure. The study method used was the analysis of students' solutions to questions that were carefully designed to test competence in problem-solving strategies. The study showed that performance was poor in all the problem-solving strategies tested and that there was no improvement in performance as students progressed from year to year. About a half of the students tested (average performance in all of the 11 questions used for testing) had difficulty in answering the questions. Possible reasons for students' difficulties are identified and suggestions are made for rectifying the difficulties. It is suggested that difficulties with the use of cognitive strategies are often not due to students' inability to understand and use them but to insufficient emphasis being placed on them in their courses. Since an increase in competence in cognitive strategies and cognitive skills canbe expected to lead to more efficient learning and problem-solving, not only in education courses but also throughout their lives, there is a need for training students in them until they become automatic and spontaneous mental operations. Such training should be integrated, throughout any course, with the teaching of content knowledge. <![CDATA[<b>Computer simulation studies of trishomocubane heptapeptide of the type Ac-Ala<sub>3</sub>-Tris-Ala<sub>3</sub>-NHMe</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100007&lng=en&nrm=iso&tlng=en As part of an extension on the cage peptide chemistry, the present work involves an assessment of the conformational profile of trishomocubane heptapeptide of the type Ac-Ala3-Tris-Ala3-NHMe using molecular dynamics (MD) simulations. All MD protocols were explored within the framework of a molecular mechanics approach using the PARM94 force field parameters modified in-house to mimic the implicit and explicit solvent conditions. The 50 ns MD trajectories revealed a tendency of the trishomocubane polypeptide to adopt bent conformations in vacuo, MEOH and TIP3P solvent models, consistent with previous studies undertaken in our laboratory. The aim of this paper is to exemplify the tendency of the highly constrained cage residues to promote reverse-turn characteristics in the polypeptide chains, which could play a pivotal role in the design of new cage peptidomimetics. <![CDATA[<b>A low temperature synthetic route to nanocrystalline TiN</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100008&lng=en&nrm=iso&tlng=en A simple chemical synthetic route has been developed to prepare nanocrystalline titanium nitride (TiN) in an autoclave, by the reaction of metallic Ti with NaNH2 at low temperature of 500-600 °C. The samples were characterized by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectra. The possible reaction mechanism of this process is also discussed. This method may be extended to the synthesis of other metal nitrides. <![CDATA[<b>Preparation of CN<sub>x</sub>/carbon nanotube intramolecular junctions by switching gas sources in continuous chemical vapour deposition</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100009&lng=en&nrm=iso&tlng=en Nitrogen-doped carbon nanotubes (CNx)/carbon nanotube intramolecular junctions were prepared by a simple continuous chemical vapour deposition (CVD) method at 650 °C. The catalyst was obtained by calcination of a layered double hydroxide precursor containing Fe and Mg. By switching between hexane and ethylenediamine repeatedly in the CVD process, multiple intramolecular junctions composed of CNx with a bamboo-like structure and empty hollow carbon nanotubes were observed, and such different structures at both sides of the junctions indicated some interesting properties and offered potential applications for future nanodevices. <![CDATA[<b>Electrical and optical properties of nanosized perovskite-type La<sub>0.5</sub>Ca<sub>0.5</sub>MO<sub>3</sub> (M=Co,Ni) prepared using a sol-gel method</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100010&lng=en&nrm=iso&tlng=en In this paper the uniform nanopowders of La0.5Ca0.5CoO3-δ and La0.5Ca0.5NiO3-δ (LCCO and LCNO) were synthesized from nitrates of the constituent metal ion, citric acid (CA), and ethylene glycol (EG) by the combined citrate-ethylene glycol (EG) method. These nanopowders were also characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), SEM-EDX analysis and Brunauer-Emmett-Teller (BET) method XRD results show that single perovskite phase was completely formed after calcination at 750 °C. Fourier transform infrared (FTIR) spectra were measured for the xerogels and powder samples after calcination. In addition, the TEM images show that the average particle size of nanoparticles is approximately 25-32 nm in diameter. The ultraviolet (UV) spectrum was used to calculate the absorption coefficient (a) as a function of photon energy (hv) and the obtained results indicate that the optical band gap energy (Eg) for LCNO sample is smaller than that of LCCO. Finally, the temperature dependence of the specific resistivity of LCCO and LCNO sintered at 1100 °C was compared and revealed that the resistivity of both compounds decreases exponentially by increasing the temperature. <![CDATA[<b>Simultaneous determination of metals in coal with low-resolution continuum source atomic absorption spectrometer and filter furnace atomizer</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100011&lng=en&nrm=iso&tlng=en The setup including low-resolution spectrometer with the charge-coupled device (CCD) detector, continuum radiation source and filter furnace (FF) atomizer was employed for direct simultaneous determination of Al, Fe, Mg, Cu and Mn in coal slurry. In the FF, sample vapour entered absorption volume by filtering through heated graphite. Absorption spectrum within 200-400 nm was repeatedly recorded during the atomization period with spectral resolution 0.3 nm. The output of the CCD elements was measured within each spectrum, atomic absorption at specific wavelengths measured and corrected with respect to the linearization algorithm, and integrated. Calibration was performed using carbon slurry impregnated by the analyte metals as well as with the analytes added to the slurries as multi-element solutions. The comparison showed preference of the second method, which provides for 60 % of measurements the results within 10 % deviation range from the certified reference data independent of concentration of the analyte. Low-resolution spectral instrument with fast CCD detection makes possible simultaneous detection of transient absorption signals for several elements. The use of continuum light source makes it possible to determine broad range of concentrations without slurry dilution. <![CDATA[<b>Synthesis of new benzocoumaryl oxadiazolyls as strong blue-green fluorescent brighteners</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100012&lng=en&nrm=iso&tlng=en The benzocoumarin-3-ethylcarboxylate 2 on treatment with hydrazine hydrate at room temperature afforded benzo-coumarin-3-carbohydrazide 3. The compound 3 served as key intermediate in the synthesis of the title compounds. Thus, benzocoumarin-1,3,4-oxadiazolyls 6a-e were obtained in two ways, i.e. one by direct cyclization of benzocoumarin-3-carbohydrazide 3 with substituted benzoic acids in POCl3 and the other by cyclization of Schiff bases of compounds 5a-e in the presence of bromine/acetic acid. The structures of the novel benzocoumaryl oxadiazolyls 6a-e were confirmed by spectral analysis. The benzocoumarin-1,3,4-oxadiazolyls 6a-e exhibited strong blue and green fluorescent properties. The Stoke's shifts range from 43 to 165 nm. The absorption and fluorescence maxima of the benzocoumaryl oxadiazolyls showed good bathochromic shifts. <![CDATA[<b>Solventless synthesis of quinoline derivatives: Acceleration of Friedländer reaction by supported heteropoly acids</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100013&lng=en&nrm=iso&tlng=en Different Keggin type heteropoly acids (HPAs) and supported ones on solids with different nature and textural properties were used in the Friedländer reaction in order to obtain quinoline derivatives. This conversion has been preceded by tungstophosphoric acid supported on silica, KSF and activated carbon as optimized catalysts in high yields and short reaction times. The general applicability of this method is demonstrated by using various substrates including ketones, β-ketoesters and β-diketones. For most substrates the reaction worked well. These catalysts were found to be reusable and considerable catalytic activity could still be achieved after the fourth run. <![CDATA[<b>A greener method towards the synthesis of 1,3-diarylimidazolium tetrafluoroborates</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100014&lng=en&nrm=iso&tlng=en A new strategic method for the synthesis of 1,3-diarylimidazolium tetrafluoroborate salts is illustrated herein. A solvent-free approach was employed in the synthesis of the diimines which are precursors in the preparation of the imidazolium salts. The reaction proceeds faster, cleaner and in a better yield than previously reported methods. <![CDATA[<b>Chloride-binding effect of blast furnace slag in cement pastes containing added chlorides</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100015&lng=en&nrm=iso&tlng=en Corrosion of rebar in concrete is commonly associated with, and to a large degree influenced by, the free chloride concentration in the pore water. It is standard industry practice to add various mineral admixtures such as pulverised fuel ash (PFA), or fly ash, ground granulated blast furnace slag (GGBS) and silica fume (SF), to concrete mixtures to increase the corrosion resistance of the reinforcement in the matrix and its subsequent design life span. Various investigations have reported on the effect of mineral admixtures and additions on chloride binding in cementitious matrices, and the current study contributes further to knowledge in this field. Unlike previous investigations, this study attempted to make a clear distinction between the contributions of the two components in a blended cement consisting of ordinary Portland cement (OPC) and ground blast furnace slag (BFS). These contributions of each component have been quantified. Relationships between the total amount of chloride bound, the level of BFS additions, and the levels of initial chloride content present in the matrix were determined. It was found that the OPC/BFS blended cement with partial BFS replacements of up to 50 % displayed a lower binding capacity than that of the OPC on its own. This observation was derived based on the assumption that the OPC and slag reactions were treated as competing and equivalent and did not take any potential time delays into account, nor the degree of cement hydration. The chloride-binding efficiency by the BFS is dependent on both the BFS partial replacement addition level as well as the initial amount of chloride present in the matrix. It is shown that both the OPC and BFS contribute to chloride binding in cement pastes, depending on the amount ofBFS that replace the OPC component in the matrix. <![CDATA[<b>Nucleation, melting behaviour and mechanical properties of poly(L-lactic acid) affected by the addition of N, N'-bis(benzoyl) suberic acid dihydrazide</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100016&lng=en&nrm=iso&tlng=en A new category of nucleating agent for poly(L-lactic acid) (PLLA) was developed. An organic nucleating agent; N,N'-bis(benzoyl) suberic acid dihydrazide (NA) was synthesized from benzoyl hydrazine and suberoyl chloride which was deprived from suberic acid via acylation. The nucleation, melting behaviour and mechanical properties of the PLLA containing NA were investigated. NA showed excellent nucleating effects on PLLA crystallization. Compared to the neat PLLA, with the addition of 0.8 % NA, the crystallization temperature (To) increase from 105.88 °C to 125.57 °C and the crystallization enthalpy( AHc) increase from 1.379 J g-1 to 31.63 J g-1 at a cooling rate of 1 °C min-1 from melt. In the presence of NA, the melting behaviour of PLLA was affected significantly, and a double-melting peak resulted during melting-recrystallization. The tensile strengths and tensile modulus increase with the increase of NA content and possesses a maximum value for the critical NA loading of 0.8 %. However, the elongations at break of the samples decreased significantly. <![CDATA[<b>Highly efficient method for solvent-free synthesis of diarylmethane and triarylmethane from benzylic alcohols using P<sub>2</sub>O<sub>5</sub>/Al<sub>2</sub>O<sub>3</sub> or P<sub>2</sub>O<sub>5</sub>/SiO<sub>2</sub> at room temperature</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100017&lng=en&nrm=iso&tlng=en A highly efficient procedure for the synthesis of triarylmethane and diarylmethane via benzylation of aromatic hydrocarbons from benzyl alcohols using supported P2O5 on SiO2 and/or Al2O3 under solvent-free conditions is described. Excellent yields of triarylmethane and diarylmethane were obtained using P2O5-SiO2 (50 % W/W) and/or P2O5-Al2O3 (50 % W/W) at room temperature. The reusability of both supported P2O5 on SiO2 and Al2O3 were examined. Both supported reagents show favorable activities in first and second runs, however, a decline in reactivity was observed in following attempts. The reaction is scalable to >0.03 mole amounts. <![CDATA[<b>[H<sub>2</sub>-cryptand 222]<sup>2+</sup>(Br<sub>3</sub><sup>-</sup>)<sub>2</sub> as a tribromide-type catalyst for the trimethylsilylation/tetrahydropyranylation of alcohols</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100018&lng=en&nrm=iso&tlng=en A stable organic tribromide, [H2-cryptand 222]2+(Br3-)2 was utilized as an active catalyst for the trimethylsilylation/ tetrahydropyranylation of alcohols. The method is general for the preparation of OH-protected aliphatic (acyclic and cyclic), aromatic, primary, secondary and tertiary alcohols. <![CDATA[<b>Speciation of Co(II), Ni(II) and Cu(II) complexes with L-glutamic acid in Dioxan-Water mixtures</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100019&lng=en&nrm=iso&tlng=en Chemical speciation of Co(II), Ni(II) and Cu(II) complexes of L-glutamic acid in the presence of Dioxan-Water mixtures at an ionic strength of 0.16 mol L-1 at 303 K was studied pH- metrically. Glu is taken as a model compound for amino acid residues and dioxan is used to mimic the low dielectric constant at the active site cavities in bioactive molecules like enzymes and proteins. The active forms of the ligand were LH3+, LH2, LH- and L2-. The models containing different numbers of species were refined using the computer program, MINIQUAD75. The predominant species detected were ML2H2,ML2H-,ML2(2-), MLH+ and ML. The best fit chemical models were arrived at based on statistical parameters. The trend in variation of complex stability constants with the medium composition was explained on the basis of changes in the dielectric constant of the solution. Effect of errors in the ingredients on the stability constants was also studied. Chemical speciation is discussed based on the distribution diagrams. <![CDATA[<b>Determination of titanium dioxide in commercial sunscreens by inductively coupled plasma-optical emission spectrometry</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100020&lng=en&nrm=iso&tlng=en A variety of sunscreen products have been developed to afford the consumer protection against some of the deleterious effects, for example erythema, caused by solar ultraviolet radiation. The requirement that suncare products offer broad-spectrum protection has resulted in the use of inorganic oxides, such as micronized titanium dioxide (TiO2), in their formulation. However, there are now concerns about the photocatalytic effects of the TiO2 in these products and its potential phototoxicity to the skin through the generation of reactive oxygen species such as hydroxyl and superoxide anion radicals. It is important, therefore, that the amounts of TiO2 in suncare products be closely monitored and maintained within stipulated ranges. A simple, fast and reliable analytical method for the determination of TiO2 in commercial sunscreen products by inductively coupled plasma-optical emission spectrometry (ICP-OES) was developed and validated. The limits of detection and quantitation were found to be 0.018 and 0.062mL-1, respectively. The average percentage recovery of TiO2 was 102.32 ± 2.87 % with a RSD of 2.81 %. The method was applied to determine the concentration of TiO2 in 22 commercial suncare samples of which 14 contained TiO2. The amounts of TiO2 measured in these sunscreens ranged from 0.05 to 3.21 %. To our knowledge this is the first study that reports the amounts of TiO2 in sunscreen products available on the South African market. <![CDATA[<b>Elucidation of the complex Baylis-Hillman reaction of 3-methoxy-2-nitrobenzaldehyde with methyl vinyl ketone</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100021&lng=en&nrm=iso&tlng=en DABCO-catalyzed reaction of 3-methoxy-2-nitrobenzaldehyde with methyl vinyl ketone (MVK) affords a mixture of products, comprising the 'normal' Baylis-Hillman adduct, the MVK dimer and a pair of diastereomeric bis-(MVK)Baylis-Hillman adducts. ¹H NMR spectroscopy-based kinetic studies have provided clear insights into the competing pathways and product distribution in this complex reaction. <![CDATA[<b>Overcoming conceptual difficulties in first-year chemistry students by applying concrete teaching tools</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100022&lng=en&nrm=iso&tlng=en Students entering university to study chemistry have difficulty understanding the concepts involved when attention is focused at the particulate level of matter. No-one can actually see what happens to individual molecules or atoms during any process of change and most means of explanation at the visible, or macro level are inadequate when describing behaviour at the particulate, or micro level. Structured worksheets and coloured Lego® building blocks were employed in order to facilitate understanding of the physical changes that water undergoes during changes of temperature. A sample size of 154 Foundation Programme students was used and the responses of these students investigated. A constructivist approach, enabling students to apply concrete reasoning in building their own knowledge, was evaluated. Students worked with interlocking building blocks to improve their understanding of molecular structure and behaviour. The students' academic performance improved when using these more concrete tools. This demonstrates that teaching is more effective when allowing visual and tactile senses to interact. It is therefore the purpose of this paper to substantiate the use of concrete tools, such as Lego® blocks, to help explain difficult concepts in chemistry, such as the behaviour of atoms and molecules. <![CDATA[<b>Micelle-mediated extraction and cloud point pre-concentration for the spectrophotometric determination of phenol in water samples</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100023&lng=en&nrm=iso&tlng=en In this paper, a cloud point extraction method for the determination of trace amounts of phenol by spectrophotometry is described. The method is based on the colour reaction of phenol with diazotized p-nitroanilinean alkaline media and the cloud point extraction of azo dye product using of nonionic surfactant Triton X-114. The effects of reaction and extraction parameters were studied and optimum parameters were established. The calibration graph was linear in the range of 2.0-400 ng mL-1 of phenol. Detection limit based on three times the standard deviation of the blank (3Sb) was 1.0 ng mL-1 and the relative standard deviation (RSD) for 50 ng mL-1 of phenol was 1.73 % (n = 10). The proposed method was applied for the determination of phenolin water samples. <![CDATA[<b>Synthesis and antiplasmodial activity of EG-artemisinin ethers and artemisinin-quinoline hybrids</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100024&lng=en&nrm=iso&tlng=en The aim of this study was to synthesize a series of ethylene glycol (EG) ethers and quinoline hybrids of the antimalarial drug artemisinin and to evaluate their antimalarial activity in vitro against Plasmodium falciparum strains. The ethers were synthesized in a one-step process by coupling ethylene glycol (EG) moieties of various chain lengths to carbon 10 of dihydroartemisinin, while the artemisinin-quinoline hybrids were obtained by condensation of dihydroartemisinin with different amine-functionalized quinoline moieties. For solubility reasons, part of the hybrids were converted to oxalate salts upon reaction of the free bases with oxalic acid. All the synthesized compounds were tested against chloroquine (CQ) susceptible (CQS) D10 and chloroquine resistant (CQR) Dd2 Plasmodiumfalciparum strains. The IC50 values revealed that all the ethers were active against both strains but less potent than artemether irrespective of the strain. However, they were more active than CQ against the resistant strain. Ether 8 featuring three EO units was the most active of all ethers. It showed activity similar to that of CQ against D10 and much more potency than CQ against Dd2 strain (IC50, 0.023 vs. 0.473 nM). The hybrids and their salts were also all active against both strains. Hybrid 19 which possessed an isopropyl linker and its oxalate salt 19A: were the most active against the Dd2 strain. They were more potent than CQ (IC50, 0.009 and 0.011 vs. 0.255 nM, respectively). <![CDATA[<b>Dissolution and quantification of tantalum-containing compounds: Comparison with niobium</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100025&lng=en&nrm=iso&tlng=en Dissolution and quantification of different tantalum compounds was undertaken as part of the development of local processes for the beneficiation and separation of tantalum and niobium from different mineral ores. Dissolution of Ta metal powder, TAN-1 CRM, TaF5, TaCl5 and Ta2O5 was undertaken with different acid, alkaline fluxes as well as with microwave digestion while quantification was performed using ICP-OES analysis. The success of the different dissolution methods was evaluated on percentage recovery basis, the results discussed and finally compared with those obtained from the corresponding niobium compounds. Quantification results obtained from this study clearly indicated that the percentage recovery of tantalum depended on i) the type of tantalum source and ii) the dissolution process of the different tantalum compounds. Excellent recoveries were obtained with water soluble TaF5 and TaCl5 with 101(1) and 100(2) % respectively. Low tantalum recovery was obtained for Ta metal and Ta2O5 with microwave digestion in the presence of strong acids (4.1(8) and 9.7(8) %, respectively in the presence of H2SO4) and acidic fluxes (1.5(5) % with K2S2O7). Improved recoveries were obtained using basic fluxes for both the metal and the oxide. Fluxing the metal with KOH resulted in a 85(3) % Ta recovery while a maximum of 68(4) % Ta recovery was obtained for Ta2O5. Tantalum recoveries of between 75(10) and 90(6) % were obtained for TAN-1 CRM (Ta present as Ta2O5) and 95(6) % for Ta2O5 with Li2B4O7 as fluxing agent on the removal of the excess of boric acid prior to analysis. A stability study indicated constant recovery in abasic medium while a decrease of up to 10 % in tantalum recovery was obtained in an acidic environment. A comparison with the niobium results obtained in a previous study highlighted the different acid/base properties of the two oxides as well as a possible alternative dissolution/separation step for the two elements from the mineral ore. <![CDATA[<b>Coordination of tridentate Schiff base derivatives of 4-aminoantipyrine to rhenium (V)</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100026&lng=en&nrm=iso&tlng=en The potentially tridentate Schiff base derivatives H2nap and Hoap were synthesized by the condensation reaction of 4-amino-2,3-dimethyl-1-phenyl-5-pyrazoline (4-aminoantipyrine)with 2-aminobenzaldehyde and salicylaldehyde, respectively. The reaction of H2nap with [ReOBr3(PPh3)2] and cis-[ReO2I(PPh3)2](1) gives rise to the products [Re(nap)Br2(PPh3)]Br (2) and [ReO(OEt)(Hnap)(PPh3)]I (3), respectively. In 2 the ligand nap is coordinated as a tridentate imido-imino-ketone, while in 3 Hnap is bonded as an amido-imino-ketone. The reaction of Hoap with [ReO2I(PPh3)2] affords the product [ReO(OMe)(oap)(PPh3)]I (4). Spectroscopic data and the X-ray crystal structures of compounds 2-4 are reported. <![CDATA[<b>Students' competence in some problem solving skills throughout their B.Sc. course</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100027&lng=en&nrm=iso&tlng=en The main objective of the study was to test students' competence, throughout all their years (first, second and final years) of a B.Sc. course, in five important types of problem solving skills: information processing skills; skills concerning equations; graphical skills; three-dimensional visualization skills and inverse proportion reasoning skills. The study method used was the analysis of students' solutions to carefully designed questions. Students' performance was found to be poor for most of the skills tested. For example, more than half of all the students tested could not deduce information organized in equations, transform quantitative information in statements into equations and use inverse-proportion reasoning to do a calculation; and about a quarter of the students could not combine two equations and also could not visualize three-dimensionally the drawing of a cube. The study also showed that there wasn't much improvement in cognitive skills as students progressed from year to year. This suggests insufficient emphasis being placed on training students in cognitive skills in their courses. Since the development of students' competence in cognitive abilities should be an important objective of education courses, there is a need for explicitly identifying important cognitive skills and strategies and training students in them. Such training should be integrated with the teaching of content knowledge. <![CDATA[<b>Redox potentials of ligands and complexes - a DFT approach</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100028&lng=en&nrm=iso&tlng=en A review of the limited literature concerned with theoretical ways to predict experimentally measured redox potentials of ligands and complexes is presented. Electrochemical and related DFT studies involving series of para-substituted nitrobenzenes and β-diketone bidentate ligands are discussed. New studies involving ferrocenes and bimetallic complexes (containing both rhodium and iron) are additionally reported. Correlations of redox potentials with calculated descriptors; electron affinity (EA), group electronegativity (xR), electrophilicity index (ω), LUMO energy (E LUMO) and HOMO energy (E HOMO) - obtained from calculated electronic energies of neutral, anionic and cationic molecules, are compared. Observed E0,Epa or Epc gave excellent correlations in the linear relationships between Epc and E LUMO (R² > 0.99), and Epa and E HOMO (R² > 0.92). Close correlation with the HOMO-1 energy was also found with the ferrocene-based second oxidation in the Rh complex. <![CDATA[<b>Part II: <i>Ab initio</i> and NMR investigations into the barrier to internal rotation of various oxo- and thio- analogues of 2-Oxo-2<i>H</i>-chromen-7-yl dimethylcarbamates</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100029&lng=en&nrm=iso&tlng=en A range of 2-oxo-2H-chromen-7-yl dimethylcarbamates were synthesized as described in part I of this publication, containing either an oxygen or sulphur a to the carbonyl or thiocarbonyl group of the amide moiety. Variable temperature and exchange spectroscopy NMR was performed on these compounds and the barrier to free amide rotation was calculated. Each of these compounds were also modelled ab initio and the gas phase barrier to rotation calculated. These three sets of data were compared and the influence of the a-heteroatom on rotation for amides and thioamides evaluated. <![CDATA[<b>Synthesis, characterization and application of nano lepidocrocite and magnetite in the degradation of carbon tetrachloride</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100030&lng=en&nrm=iso&tlng=en Degradation of halogenated organic compounds using nanoparticles is one of the innovative technologies in environmental remediation. In the present study, the synthesis, characterization and application of lepidocrocite (FeOOH) and magnetite (Fe3O4) nanoparticles for the remediation of water contaminated with carbon tetrachloride (CCl4) has been investigated. The synthesized nanoparticles were characterized by using advanced instrumental techniques such as field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The synthesized FeOOH and Fe3O4 nanoparticles were found to have an average crystallite size of 10.94 nm and 18.10 nm, respectively. Batch degradation experiments for the degradation of CCl4 were carried out separately using both of the above synthesized nanoparticles. The degradation reactions were found to be first-order while using Fe3O4 and second-order while using FeOOH nanoparticles. Moreover, the reactivity of freshly synthesized Fe3O4 nanoparticles was found to be much higher than that of FeOOH nanoparticles. <![CDATA[<b>Expedient access to an <i>N</i>-phenylpyrrolidin-2-yl heterocycle <i>via</i> a base-induced intramolecular <i>aza</i>-Michael reaction</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100031&lng=en&nrm=iso&tlng=en Ethyl 2-(1-phenylpiperidin-2-yl) acetate was formed in a spontaneous cyclization from (E)-ethyl 7-oxohept-2-enoate whereas ethyl [1-(2-bromophenyl)-2-pyrrolidinyl acetate could be synthesized in good overall yield when employing a stoichiometric amount of base to facilitate the intramolecular aza-Michael reaction. <![CDATA[<b>High school Physical Sciences teachers' competence in some basic cognitive skills</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100032&lng=en&nrm=iso&tlng=en The successful implementation of the national high school Physical Sciences curriculum in South Africa, which places strong emphasis on critical thinking and reasoning abilities of students, would need teachers who are competent in cognitive skills and strategies. The main objectives of this study were to test South African high school Physical Sciences teachers' competence in the cognitive skills and strategies needed for studying Physical Sciences effectively and also to identify possible reasons for their difficulties and suggest methods for overcoming them. The study method used was the analysis of teachers' answers to questions that were carefully designed to test competence in explanation skills, mathematical skills, graphical skills, three-dimensional visualization skills, information-processing skills and reasoning skills. Seventy-three teachers from about 50 Dinaledi schools in the North West and Kwazulu-Natal provinces were tested. Teachers' competence was found to be poor in most of the skills tested. About 40 % (average performance in all 14 test questions) of them had difficulty in answering the questions. Teachers' lack of competence in cognitive skills and strategies would be an important limiting factor in the successful implementation of the Physical Sciences curriculum. An urgent need therefore exists for training teachers to increase their competence in the cognitive skills and strategies that are needed for studying science effectively. <![CDATA[<b>Kinetics of oxidation of aliphatic alcohols by potassium dichromate in aqueous and micellar media</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100033&lng=en&nrm=iso&tlng=en The kinetics of oxidation of four aliphatic alcohols in acidic aqueous and micellar media were investigated. The reaction was found to be first-order with respect to both alcohol and oxidant. Pseudo-first-order kinetics were found to be perfectly applicable with ethanol, 1-propanol and 2-propanol while deviation was observed at intermediate stages of the reaction with methanol. The pseudo-first-order rate constants were found to be independent of concentration of the oxidant. The presence of TX-100 enhanced the rate of the reaction for all alcohols. Negative salt effects were observed with addition of KCl to the reaction mixture. A suitable mechanism for the reaction was suggested which agrees with the experimental findings. <![CDATA[<b>DSC of milk fats from various animals with high levels of medium-chain, unsaturated and polyunsaturated fatty acids</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100034&lng=en&nrm=iso&tlng=en Mammals of different species provide milk lipids with a wide variety of fatty acid composition yet with common stereospecific features. This allows the investigation of crystallographic properties of milk lipids that cannot be achieved by interesterified lipids due to the random stereospecific distribution that is obtained. The milk fats of elephant and white rhinoceros contain high amounts of 8:0, 10:0 and 12:0 which form triglyceride species that melt between 8 and 22 °C. The crystallographic behaviour of the milk lipids from blesbok and blue wildebeest differ from the other ruminant lipids, and that of horse and vervet monkey differ from the other non-ruminant lipids. It seems that a low content of 18:0 and 18:1, and a high content of saturated short- to medium-length fatty acids prevent the formation of the high and low temperature melting isotherms, between 35 and 42 °C, and between -45 and -10 °C, respectively, which are normally observed for milk fats. <![CDATA[<b>Potentiometric determination of free chloride in cement paste - An alternative method for low-budget laboratories</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100035&lng=en&nrm=iso&tlng=en Corrosion of rebar in concrete is commonly associated with, and to a large degree influenced by, the free chloride concentration in the pore water. The amount of chloride in concrete is important because chloride can promote corrosion of steel reinforcement when moisture and oxygen are present. A potentiometric procedure that makes use of direct measurement with a chloride ion selective electrode has been developed to analyze free chloride in the pore water extracted from cement paste.16 The accuracy and reliability of this analytical technique has been checked against a certified reference material, Merck sodium chloride solution. Confidence levels (CL095), of 0.03 and relative standard deviations of 0.2 % for chloride were determined for ordinary Portland cement (OPC) chloride binding capacity. <![CDATA[<b>Characterization of South African coal for metals, inorganic and organic sulfur compounds</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100036&lng=en&nrm=iso&tlng=en This work characterized South African coal for metals, organic and inorganic sulfur compounds. Microwave assisted extraction (MAE) followed by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) were used for extraction, identification and quantification of inorganic, organic, total sulfur forms as well as selected metals related to the presence of sulfur in coal. The total sulfur content was also determined in coal by direct elemental analysis for carbon, hydrogen, nitrogen and sulfur. Thiophene organic sulfur compounds in coal were extracted with ultrasonication system followed by identification and quantification using gas chromatography and flame ionization detector. The ultrasonication extraction system was optimized for the extraction organic solvent, the extraction time and influence of sample concentration. Microwave assisted extraction followed by ICP-OES gave an average of 8758, 4500, 3600 and 600 mg kg-1 for total sulfur, pyrite sulfur, organic sulfur and sulfate sulfur, respectively. Pyrite and organic sulfur forms were found to compete for the largest amounts in coal samples. The total sulfur content in South African coal was found to be less than 20 000 mg kg-1 which suggests that the coal may be of low sulfur content compared to others in the world. The mean percentages of carbon, hydrogen, nitrogen and sulfur in coal samples was 66.20, 2.98, 1.15 and 1.11 confirming that it is of bituminous rank. The concentration levels of selected metals were found in this decreasing order; Na>Ca>Fe>Mg>K>Ba>Mn>Cr>Pb>Zn>Cu>As>Co>Sb>Hg. Thiophene organic sulfur compounds were best extracted with dichloromethane with five sequential extractions of 15 min each. Recovery of extraction from spiked concentration ranged from 40 % to 70 % depending on the individual compound. More volatile organic sulfur compounds showed lowest recovery especially at low spiked concentrations. 2-methyl thiophene was the only compound identified in all coal samples. The concentration of thiophene sulfur compounds ranged from about 4-16 mg kg-1. Dibenzothiophene was found with the highest concentration (15.5 mg kg-1) while 2-ethyl thiophene was found with the lowest concentration. <![CDATA[<b>Structure of coordination complexes: The synergy between NMR spectroscopy and computational chemistry</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100037&lng=en&nrm=iso&tlng=en Illustrative examples of how NMR spectroscopy and computational chemistry data can be used in synergy to gain information on structure, coordination mode, bonding, symmetry and isomeric distribution of transition metal complexes, is presented. Isomer distribution and the most stable structures in a series of Ti(/3-diketonato)2Cl2 and Ti(β-diketonato)2(biphen) complexes as determined by density functional theory (DFT) methods and the application of the Boltzmann equation, are in agreement with crystal structures and variable temperature NMR results. Secondly, the DFT determined coordination mode of the 4-amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole, (bpt-NH2) which has the appropriate chemical geometry to behave as anionic or neutral bidentate chelating group to form a 5- or 6-membered complex, is shown to be in agreement with ¹Ή NMR shifts for [Rh(bpy)2(bpt-NH)]2+, [Rh(phen)2(bpt-NH)]2+, [Rh(bpt-NH)(cod)] and [Ir(bpt-NH)(cod)] (cod = 1,5-cyclooctadiene, phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine). The oxidative addition of CH3I to [Rh(/3-diketonato)(CO)(PPh3)] complexes consist of three reaction steps and involves isomers of two different RhIII-alkyl and two different RhIII-acyl species. For this reaction experimental ¹Ή NMR techniques complement the stereochemistry of reaction intermediates and products as calculated by density functional theory. NMR properties, in agreement with computational results, proved to be useful to access the nature of the κ3 to κ2 distortion in coinage metal-ethylene complexes supported by tris(pyrazolyl)borates. The last example showed that NMR, X-ray crystal and computational results showed C2 symmetry for a series of metal(II) complexes coordinated to a 16-membered pentaaza macrocycle. <![CDATA[<b>Twenty golden years of battery R&D at CSIR, 1974-1994</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100038&lng=en&nrm=iso&tlng=en This paper is a personal reflection of 20 years of sodium- and lithium-based battery R&D in South Africa between 1974 and 1994. The impact of the innovative materials science and engineering research conducted during this period is highlighted by the later successful implementation of batteries in powering practical systems, as well as in technology licensing. <![CDATA[<b>Synthesis of 9<i>H</i>-indeno [1, 2-<i>b</i>] pyrazine and 11<i>H</i>-indeno [1, 2-<i>b</i>] quinoxaline derivatives in one-step reaction from 2-bromo-4-chloro-1-indanone</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100039&lng=en&nrm=iso&tlng=en The reaction of 2-bromo-4-chloro-1-indanone with 2,3-diaminomaleonitrile, benzene-1,2-diamine and 4-methylbenzene-1,2-diamine in glacial acetic acid gave 8-chloro-9H-indeno[1,2-b]pyrazine-2,3-dicarbonitrile, 1-chloro-11H-indeno[1,2-b]quinoxa-line and 1-chloro-7-methyl-11H-indeno[1,2-b]quinoxaline, respectively, in good yield. <![CDATA[<b>The crystal structures of two novel cadmium-picolinic acid complexes in relation to the solution species</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100040&lng=en&nrm=iso&tlng=en The crystal structures of two novel cadmium-picolinic acid complexes grown in aqueous solutions at selected pH values are reported. The structures are compared to expected solution species under the same conditions. The crystal structure of complex 1 exhibits a seven coordinate structure which contains a protonated picolinic acid ligand thatbonds bidentately to the cadmium ion via both carboxylate oxygens. Two nitrates coordinate to the metal centre within the same plane, one monodentately and the other bidentately, and two water molecules are found in the axial positions. The structure of complex 2 shows cadmium bonded to three of N,O-bidentate picolinic acid ligands to produce a distorted octahedron. <![CDATA[<b>Microwave irradiation <i>versus</i> conventional heating in the synthesis of <i>N</i>-methyl-2-pyrrolidone catalyzed by highly active copper powder</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0379-43502011000100041&lng=en&nrm=iso&tlng=en A convenient one-pot synthesis of N-methyl-2-pyrrolidone is achieved either by conventional heating or microwave irradiation. The intermediate N-methyl-4-hydroxybutyramide formed from the exothermic reaction of 7-butyrolactone with aqueous methylamine undergoes an intramolecular condensation reaction catalyzed by highly active copper powder to form the pyrrolidone in good yield.